Index

Component information

    General

    Few designers spend time pondering the traits of resistors, capacitors, or other simple passive components. Determine the necessary nominal value, tolerance, and temperature coefficient for each instance in your circuit, and you're pretty much done. Consider the nonelectrical variables like package and pricing, and you're a certifiable good corporate citizen. When it comes to the active components, there are also many other things to consider.

    Resistors

    Resistors are passive electronic components used extensively on the circuit boards ofelectronic equipment. Resistors are usually used to limit current, attenuate signals, dissipate power (heating) or to terminate signal lines. Resistors are usually color coded with stripes to reveal their resistance value (in ohms) as well as their manufacturing tolerance.

    Most importans characteristics of resistor are the resistance, tolerance of resistance and the power handling capacity. Resistors are generally available from the fractions of ohms upto several megaohms (higher value special components are also available).

    Most small general purpose resistors have power handling capacity ofaround 0.25W. Most resistors used to be this type, and most electronics designs expect this kind of resistor unless the power rating is mentiones. In typical circuits you can nowadays see resistors with power handling of 0.125W up to 1W. The typical voltage rating of such "normal" resistorsis normally 125..250V. Also special power resistors are available, generallywith power rating from few watt up to 50-100W. Higherst power power resistorsare generally built to metal case which is designed to be connected to a heatsink.

    The resistors are manufactured with some tolerance. For example a typical resistor can have a 5% tolerance, which means that the resistance value can be 5% higher or 5% lower than what the color code indicated. There are special accurate resistors also available, for example resistors with 1% or better accuracy.

    There are many differetn resistor types which are characterized by the material they are made of and how they are constructed. Here are some details of different resistor types:

    • Carbon film resistor: cheap general purpose resistor, works quite well also on high frequencies, resistance is somewhat dependent on the voltage over resistor (does not generally have effect in pratice)
    • Composite resistor: Usually some medium power resistors are built in this way. Has low inductance, large capacitance, poor temperature stability, noisy and not very good long time stability. Composite resistor can handle well short overload surges.
    • Metal film resistor: good temperature stability, good long time stability, cannot handle overloads well
    • Metal oxide resistor: mostly similar features as metal film resistor but better surge handling capcity, higer temperature rating them metal film resistor, low voltage dependity, low noise, better for RF than wire wound resistor but usually worse temperature stability
    • Thick film resistor: Similar properties as metal fim resistor but can handle surges better and can withstand high temperatures
    • Thin film resistor: good long time stability, good temperature statiblity, good voltage dependity rating, low noise, not good for RF, low surge handling capacity
    • Wirewound resistors: Wire would rsistors are built just by winding a thin wire (made of material that has considerable resistance) over some insulating material (so that different wire sounds do not touch each other). Wirewound resistors are the quietest resistor type having only thermal noise. The downside of them is that because the wire in resistor makes a coil with quite many turns, they usually have considerably high inductance (not suitable for high frequency operation). Wire wound construction is mainly for high power resistors and high power potentiometers. Wire wound construction is also used to make very accurate resistors for measuring circuit.

    In some applications the temperature dependance of resistor value is important. Metal oxide thick film resistors typically have temperature coefficients of +/-200ppm/C and can be run at up to 125C, which implies a resistance change of up to 20,000ppm, or 2% on that temperature range. Regular metal film resistors, at +/-100ppm/C, are a bit better. A carbon film resistor has typically a negative temperature coefficient ofaround 500ppm/C.

    Resistors have a limited voltage handling capacity. If there is higher voltage that the rated votlage over the resistor, it can fail and arch-over. The voltage rating of typical 0.25W general purposes resistors is typically in 200-300V range. If higher voltage handling is needed, then depending on application a special high voltage resistor or several lower voltage resistors are wired in series. There are high voltage resistors that can handle high voltages up to many kilovolts.

    In some applications resistors are used like a fuse (for example in some power supplies and telecom applications). In those applications theresistor burns up when it is overloaded. In this type of application non-flammable resistor are used to avoid the flames and risk of fire. If the application calls for non-flammable resistor (usually haswhite case), do not replace it with any other type. Sometimes special resistors designed to be used as fuses are called fusible resistors.

    Sometimes you can see a zero ohm components on the circuit board. Zero "0" ohms means that there is no resistance. In theory, it means total conductance. In practice, it can be a piece of wire. Zero ohm resistors are typically used as jumpers on boards. Sometimes those zero-ohm resistors can act like fuse links or as local RF stand off chokes with a by pass cap on the other side.

    Potentiometers are adjustable resistors. The resitance value of potentiometer is adjusted by moving the potentiometer control. The potentiometer control is usually rotating control, but can be sometimes linear slider. Potentiometers are usually available with aroud 6 mm axles where user can attach knobs. In USA those potentiometer control diameter is typically 1/4 inched (6.35 mm) and in Europe it is generally 6 mm. Potentiometers are used often as variable resistor or where only a portion of an output voltage from a signal source is needed (for example audio volume controls). A potentiometer generally has 3 terminals. 2 of the terminals are connected to the opposite ends of a resistive element. The 3rd terminal (usually, is physically in-between the other 2 terminals) is called the wiper. The wiper is a contact that slides along the resistive element. Most potentiometers are constructed so that they are controlled with a rotatable shaft, but there are also potentiometer where the control movement is linear movement (used for example in audio mixer sliders). The potentiometers with rotatable shaft in them typically turn their whole range at around 270 degrees or turning, but there are also other constructions with different turns rations and even special multi-turn potentiometers (for example 10 turns range). The potentiometer response from control resistance to resistance value does not need to be linear. There are for example potentiometers with logarithmic, semilogarithmic and inverse-logaritmic response available (many of them used in audio circuits adjusting audio signal attenuation or amplification). In addition to the normal potentiometers, there are also similar components called "trimmers". Those work in the same way as potentimeters, but are generaly made adjustable with a crewdriver and not designd for continuous adjustation (limited mechanical workind life). Trimmers are generally put to circuit boards in places where there is need for factory adjustments (for example calibration) and repair adjustments. Overview of different potentiometer types:

    • Potentiometer: Designed for frequent changes, for example a volume control or other user control
    • Trimmer: Designed for less then frequent changes (occasional), for example a trim pot on a PCB to finalize a circuit resistance
    • Rheostat: A three terminal potentiometer which only uses two terminals.
    Overview of different potentiometer contructions / materials:
    • Carbon composition: This is the most commonly used material, cheap to make, usually medium power handling capacity, the secular change of the resistance as a result of exposure to high temperature or humidity is rather great (but very uniform over the entire resistor), typical available resistance 50ohm - 50Mohm. In the manufacturing a coating of carbon material is applied to a substrate and cured.
    • Resistive wire (Wirewound): Used to make medium or high power potentiometer, properties similar to wirewound resistors, available resistance range is typically quite low resistance values, typical available resistance range 10ohm - 100Kohm. The potentiometer is contructed by winding resistor wire to base material. Please note that the resistance of wirewound potentiometer is changed in discrete steps as the wiper moves from one wire to another.
    • Conductive Plastic: Humidity can have large effect on resistance, low power handling capacity, low noise, typical available resistance range 100ohm - 5Mohm. A conductive plastic material is generally molded into the desired resistive element shape.
    • Cermet (a mixture of ceramic and metal): Quite immune to humidity, works well for high frequency, low and high power constructions available, typical available resistance range 10ohm - 5Mohm.
    • Thin film: A thin coating of resistive metal is evaporated or sputtered onto a substrate. The available resistance range of the element can be limited.

    In carbon composite, conductive plastic, and cermet potentiometers the resolution is essentially infinite, however contactresistance between the wiper and the cermet element must be taken into consideration.

    The subject of resistor types comes up quite often in audio circuit design. Some people will recommend only using carbon composition resistors, others will tell you that metal film resistors are better. Who is correct? Well, the answer depends on what your design goals are. From a noise aspect, there are several things to take into consideration. Resistor noise is made up of three main types: thermal, contact, and shot noise. Thermal noise is mainly dependent on temperature, bandwidth, and resistance, while shot noise is dependent on bandwidth and average DC current, and contact noise is dependent upon average DC current, bandwidth, material geometry and type. Wirewound resistors are the quietest, having only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.

    The thermal noise of a resistor is equal to: Vt = SQRT(4kTBR)
    where: Vt = the rms noise voltage, k = Boltzmann's constant, T = temperature(Kelvin), B = noise bandwidth, R = resistance
    Since the characteristics of thermal noise have a Gaussian probability density function, and the noise of the two separate sources is uncorrelated white noise, the total noise power is equal to the sum of the individual noise powers.

    In general, the thermal noise of any connection of passive elements is equal to the thermal noise that would result from the real part of the equivalent total impedance. If we are dealing with pure resistances, the thermal noise is equal to the thermal noise produced by an equivalent resistance. Therefore, the thermal noise of a 1K carbon resistor is the same as a 1K metal film; it is independent of material. The only way to reduce this noise is to reduce the resistance value.

    Contact noise is dependent on both average DC current and resistor material/size. The predominant noise in carbon comp, carbon film, metal oxide, and metal film is composed of contact noise, which can be very large at low frequencies because it has a 1/f frequency characteristic. Wirewound resistors do not have this noise, only resistors made of carbon particles or films. This noise is directly proportional to both the current flowing in the resistance and a constant that depends upon the material the resistor is made of. If no current (AC or DC) flows in the resistor, the noise is equal to the thermal noise. The contact noise increases as the current is increased. The material and geometry of the resistor can greatly affect the contact noise.

    Shot noise is dependent upon current, so the more average DC current through a resistor, the more noise you get. In order to reduce this type of noise, you must keep the DC current to a minimum

    In general, for low-noise design:

    • Keep resistance values low, because thermal noise is directly proportional to resistance value.
    • Wirewound resistors are the best choice for noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.
    • Use the largest practical wattage resistors (unless you are using wirewound resistors) because contact noise is decreased in a larger geometry material.
    • Keep the DC and AC currents to a minimum because contact noise is proportional to current.
    • Don't forget that potentiometers are also resistive elements, and are almost always carbon composition, and generally are large values. Those can be a major source of noise.

    Since high-quality metal film resistors and wire wound resistors are more expensive than cheaper carbon films, there are also costs that needs to be considered in the selection of best resistor type for the application.

    Please note that resistors have a maximum voltage rating that needs to be looked at on applications where high voltages are present or can be present. For example many 0.25-0.5W resistors and some 1 watt resistors are only rated for 250-350V. Be sure to get a resistor rated for the appropriate voltage that is used in your circuit.

    Integrated circuits

    Integrated circuits are miniaturized electronic devices in which a number of active and passive circuit elements are located on or within a continuous body of material to perform the function of a complete circuit. Integrated circuits have a distinctive physical circuit layout, which is first produced in the form of a large scale drawing and later reduced and reproduced in a solid medium by high precision electro chemical processes. The term "integrated circuit" is often used interchangeably with such terms as microchip, silicon chip, semiconductor chip, and micro-electronic device.

      Power supply ICs

      Voltage regulators are very commonly used ICs in small power supplies. There are many easy to use regulators in the market, meaning tha you can usually just put one to circuit without too much design work. But when you use voltage regulator, remeber the decoupling capacitors (on both input and output), because many regulators will oscillate if they don't have enough outputcapacitance.

      Popular general purpose ICs

      Operational amplifiers

      Op-amps are generic term for a generic device that has a differentialinput of nearly infinite gain. The circuits using them rarely depend ona specific type. Use any basic circuit and any good op-amp. The operational amplifier is the work horse of the analog world. It is found in applications ranging from cellular phones to laptop computers to smoke detectors. Operational amplifiers are the child of the analogue signal processing age. Ironically, perhaps, today's emphasis on digital systems shifts such computational duties from continuous-time to clocked-circuit operation, but systems engineers require more op amps than ever before to bridge the analogue-to-digital divide.

      The classical ideal operational amplifier is characterized by infinite input impedances (zero input currents), zero output impedance (ideal voltage source output), and an infinite open-loop voltage amplification factor, A. OpAmps with very large A factors (100-120 dB being quite normal) and a surrounding negative-feedback circuit (a circuit path between the output terminal and the inverting input terminal) will lead to the simplification in circuit design known as the virtual-ground principle: The output voltage remains finite, whereas the input voltage difference is suppressed by the feedback mechanism to virtually zero! The output voltage of any real opamp will of course be restricted by the limited supply voltages of the opamp.

      Two main factors now challenge semiconductor-device and end-user equipment designers alike: the trend toward single-supply operation and the explosive growth in mobile devices. Each of these factors adds its own requirements, but both share the ever-lower power-consumption requirements that contemporary designs demand. Single-supply operation now dominates op-amp applications for several reasons. First, it's convenient; you no longer have to design and accommodate multiple power supplies. Just lowering the supply voltage from the traditional ?15V to, say, 5V helps you to conserve energy and minimise power dissipation.

      Certain applications, such as audio, demand low-noise performance. The amplifier itself generates internal, or amplifier, noise. The designer must account for the effects of amplifier noise, because the wrong instrumentation amplifier can make amplifier noise dominant. The most important parameter in low-noise design is the source impedance. Low source impedance dictates selection of a low-voltage-noise amplifier. High source impedance dictates that you select a low-current-noise amplifier. And medium source impedance means that the amplifier selection is a compromise between voltage- and current-noise performance. JFET is usually a better choice than CMOS for low-noise performance in the 20-Hz to 20-kHz frequency range.

      There are two different classes of operation amplifiers in use today: voltage-feedback amplifiers (VFA) and current-feedback amplifiers (CFA). The name, operational amplifier, was given to voltage-feedback amplifiers (VFA) when they were the only op amps in existence. These new (they were new in the late .40s) amplifiers could be programmed with external components to perform various math operations on a signal; thus, they were nicknamed opamps. Current-feedback amplifiers (CFA) have been around approximately twenty years, but their popularity has only increased in the last several years. Two factors limiting the popularity of CFAs is their application difficult and lack of precision.

      The VFA is familiar component, and there are several variations of internally compensated VFAs that can be used with little applications work. Because of its long history, the VFA comes in many varieties and packages, so there are VFAs applicable to almost any job. VFA bandwidth is limited, so it can't function as well at high signal frequencies as the CFA can. The VFA has some other redeeming virtues, such as excellent precision, that makes it the desirable amplifier in low frequency applications.

      Fortunately, precision is not required in most high frequency applications where amplification or filtering of a signal is predominant, so CFAs are suitable to high frequency applications. The lack of precision coupled with the application difficulties prevents the CFA from replacing the VFA in many traditional opamp application.The CFA circuit configuration was selected for high frequency amplification because it has current-controlled gain and a current-dominant input. Being a current device, the CFA does not have the Miller-effect (resulting from stray capacitance) problem that the VFA has. The input structure of the CFA sacrifices precision for bandwidth, but CFAs achieve usable bandwidths ten times the usable VFA bandwidth.

      The input impedance of a VFA and CFA differ dramatically because their circuit configurations are very different. The VFA input circuit is a long-tailed pair, and this configuration gives the advantages that both input impedances match. Also, the input signal to VFA looks into an emitter-follower circuit that has high input impedance. So far, the implicit assumption is that the VFA is made with a bipolar semiconductor process. Applications requiring very high input impedances often use a FET process.The CFA has a radically different input structure that causes it to have mismatched input impedances. The noninverting input lead of the CFA is the input of a buffer that has very high input impedance. The inverting input lead is the output of a buffer that has very low impedance. There is no possibility that these two input impedances can be matched. The CFA is practically limited to the bipolar process because that process offers the highest speed (FET process is not attractive today).

      Stability is important in opamp circuits. The op amp contains many poles, and if it is not internally compensated, it requires external compensation. The op amp always has at least one dominant pole, and the most phase margin that an op amp has is 45?. Phase margins beyond 60? are a waste of op amp bandwidth. Wiring the op amp to a printed circuit board always introduces components formed from stray capacitance and inductance. Stray capacitance causes stability to increase or decrease dependingon its location. Stray capacitance from the input or output lead to ground induces instability, while the same stray capacitance in parallel with the feedback resistor increases stability. Stray inductance becomes dominant at very high frequencies. The CFA stability is not constrained by the closed-loop gain, thus a stable operating point can be found for any gain, and the CFA is not limited by the gain-bandwidth constraint. If the optimum feedback resistor value is not given for a specific gain, one must test to find the optimum feedback resistor value. Stray capacitance from any node to ground adversely affects the CFA performance. Stray capacitance of just a couple of pico Farads from any node to ground causes 3 dB or more of peaking in the frequency response. Stray capacitance across the CFA feedback resistor, quite unlike that across the VFA feedback resistor, always causes some form of instability. CFAs are applied at very high frequencies, so the printed circuit board inductance associated with the trace length and pins adds another variable to the stability equation. The wiring of CFAs is critical, so stay with the layout recommended by the manufacturer whenever possible.

      A quite often mentioned opamp type in hobby circuits is 741. 741 has been around for awhile (introduced 1968), it is still made because it is such a good,reliable general purpose device. It's an ancient op-amp with quirks that canbe frustrating unless you know how its internals work (those relate to internal compensation). If you are designing anything new demanding yourself, I recommend to use something better (there is wide choise of those). The LM324 quad and the LM358 dual op amps are cheaper, more compact,just as crummy and work as single supply op amps. For not so demanding simple application 741 is still useable workhorse, and it is widely available also in well shielded cases. For military applications, and the occasional harsh industrialenvironment, but even there, using the 741 is generally a cop-out.These days there are always better amplifiers for any specific job.

      Some well known operational amplifiers:

      • ?A702 is the first solid-state monolithic op-amp introduced 1963. It was manufactured by Fairchild Semiconductors.
      • ?A709 is first widely successfull opamp (introduced 1965), +/- 15 Volt DC power supply
      • LM101 first opamp with 'short-circuit' protection, and simplified frequency compensation (external capacitor across selected connection pins), increased gain (up to 160K)
      • ?A741 was internally an internally compensated op-amp introduced 1968, short-circuit protected output, this device is still made because it is such a good, reliable general purpose device
      • RC4558 in 1974, characteristics similar to the ?A741
      • LM324 quad op amp from National Semiconductor in 1974, similar in characteristics in comparison with the ?A741 in speed and input current, low-power consumption, single-power-supply requirement, four opamps in one case, this is still made and widely used
      • CA3130 was the first FET input op amp (1974), can be supplied by a +5 to +15vdc single supply system
      • TL084 op amp in 1976, a quad JFET input op amp, low bias current and high speed
      Most of the mentioned op-amps have of course been replaced over time, keeping the same model number, with cleaner and low-noise types.We now enjoy a variety of op amps that will provide the user essentially with anything s/he needs, such as high common-mode rejection, low-input current frequency compensation, cmos, and short-circuit protection. All a designer has to do is expressing his needs and is then supplied with the correct type. Op-Amps are continually being improved, especially in the low-noise areas.

      Comparators

      Comparators compare two voltage levels and provideo digital 1/0 output depending on the input voltage levels. Comparators have an op-amp front end and a digital back end that operates like a gate. The comparator output stage may be an open collector transistor, so it often connects to the logic supply through a pullup resistor. Regardless of the input voltage, the output voltage is saturated at either power-supply rail because the analog front end amplifies input voltages with an almost infinite gain. Manufacturers employ digital semiconductor techniques to make the output circuits in a comparator; thus, the comparator comes out of saturation very quickly. The response time of a comparator is so important that it is a data-sheet specification.

      In some cases you can use an opamp as a comparator. If you leave the feedback resistor out of an op-amp circuit, it operates like a comparator, but you shouldn't use op amps to perform comparator functions except under limited conditions. Opamp manufacturers employ analog semiconductor techniques to make the output circuit of an op amp. The opamp designers assume that the output never saturates, hence, the response time of an op amp driven into saturation is uncontrolled.

      • Adding hysteresis to comparators - Comparators have very high open-loop gain, and, without some type of positive feedback, they have no noise immunity. This column adds hysteresis to comparators to eliminate multiple switching on the output.    Rate this link
      • Designing with comparators - Comparators have an op-amp front end and a digital back end that operates like a gate. The comparator output stage may be an open collector transistor, so it often connects to the logic supply through a pullup resistor. Regardless of the input voltage, the output voltage is saturated at either power-supply rail because the analog front end amplifies input voltages with an almost infinite gain. If you leave the feedback resistor out of an op-amp circuit, it operates like a comparator, but you shouldn't use op amps to perform comparator functions except under limited conditions.    Rate this link

      Other analogue ICs

      • Analog ICs for 3V Systems - Single 3V operation is available for many op amps, comparators, and microprocessor supervisors, and for some RS-232 interface ICs.    Rate this link
      • How did analog ICs get that good? - building blocks available on a typical IC fabrication process are really not very good in absolute terms because the key transistor parameters such as transconductance, input threshold voltage and output impedance vary by at least plus or minus 20% and are not as good as can be produced in discrete form but with correct desing it is possible to make very high performance analogue ICs    Rate this link
      • Reinventing The Role Of Analog/Mixed-Signal - not long ago, analog and mixed-signal functionality were treated as though they were an afterthought in the system design process but now markets move towards mixed-signal technology which combines analog and digital functionality    Rate this link
      • Selecting the Right CMOS Analog Switch - First developed about 25 years ago, integrated analog switches often form the interface between analog signals and a digital controller. This tutorial presents the theoretical basis for analog switches and describes some common applications for standard types.    Rate this link

      Digital to analogue converters

      • Take the rough edges out of video-filter design - Incorrectly processed image-frequency information can distort displays generated from digital-video sources. Oversampling and well-implemented video-DAC-output filters can save the day, but improperly designed filters can make matters worse. Before you design your next digital-video system, take some time to investigate video-reconstruction-filter design and trade-offs in oversampling.    Rate this link

      Voltage references

      • A quick guide to voltage references - A review of reference topologies and a quick look at the various ways that manufacturers specify references will help you pick the best part for your next design.    Rate this link
      • Selecting Voltage References - Voltage references are simple devices, but making the right choice for a given application can be a chore if you don't take an orderly approach. This article simplifies the task with a review of the available reference types and a discussion of the specifications manufacturers use to describe them.    Rate this link

    Capacitors

    A capacitor is simply two charged plates placed close together with a dielectric (non-conducting)material sandwiched between the plates. When a charge is applied to one plate, it repelscharges on the opposite plate, until an equilibrium is established. For direct current, the capacitorcharges up with a time constant that depends on the capacitance value and the impedance throughwhich the current flows into the capacitor. Once the capacitor is fully charged, no more current flows. This means that the capacitor is an effective block for direct current. For alternating current (like audiosignals), the response is more complicated. The charge that develops on the capacitor depends on howfast the current is changing. It takes time for the charge to build up, and that time results in a frequency dependent delay (or phase shift) in the output signal.

    Capacitor device is often used to store charge in an electrical circuit. A capacitor functions much like a battery, but charges and discharges much more efficiently. A basic requirement for all electronic circuits is the inclusion of bypass, or decoupling, capacitors. These devices reside across the positive supply to ground, as close as possible to the supply pin of the active device. You may get away with excluding these capacitors in low-frequency circuits, but many low-frequency active devices have high-frequency entities inside the active devices. Digital devices are not the only chips that require bypass capacitors. Analog circuits also benefit from including bypass capacitors but in another way. Although bypass capacitors in digital systems control fast rising- and falling-time glitches from the device, bypass capacitors in analog systems help reduce power-supply noise at the analog device. Typically, analog devices have built-in, preventive power-supply filtering or line-rejection capability. These noise-rejection mechanisms effectively reduce low-frequency power-supply noise, but this scenario is not the case at higher frequencies. Typically, manufacturers include suggested bypass-capacitor values in their data sheets, but you can also determine the proper value on your own.

    A basic capacitor is made up of two conductors separated by an insulator, or dielectric. The dielectric can be made of paper, plastic, mica, ceramic, glass, a vacuum or nearly any other nonconductive material. Capacitor electron storing ability (called capacitance) is measured in Farads. One Farad is actually a huge amount of charge (6,280,000,000,000,000,000 electrons to be exact), so we usually rate capacitors in microfarads (uF = 0.000,001F) and picofarads (pF = 0.000,000,000,001F ). Capacitors are also graded by their breakdown (i.e., smoke) voltage.

    There are very many different capacitor types. You have to realize that not all capacitors are equal. A 1uF ceramic definitely is NOT the same thing as a 1uF tantalum. You choose the device according to the application.

    Two 'parasitic' effects of capactitors are 'effective series resistance' (ESR) and series inductance. High ESR will cause power loss in higher-frequency applications (caps will get hot) especially in switching power supplies. High ESR also limits the effective filtering (your power supplies end up with more ripple). Except for very high frequency (multi-megahertz)applications a high inductance isn't quite so critical.

    The rated DC voltage is also very important. Usually it is a good idea to select capacitors rated at least 1.5 times or twice the maximum voltage you think they'll ever see. Temperature ratings also exist.

    The most common capacitor types are ones built using standard capacitor plates + insulator and then there are electrolytic capacitors. Typical capacitors consists of some form of metal plates and suitable insulation material in between those plates. This insulation can be some form of plastic, paper, mica, ceramic material, glass or air (some physical separation between layers). Those metal plates used in capacitors are usually thin metal foils. This type of capacitors have usually very good propertied otherwise, but the available capacitance is usually quite small (usually goes from pF to few microfarads). This kind of capacitors can take easily DC at both polaritied and AC without problems. This type of capacitors are available with various voltage ratings from few tens of volts up to few kilovolts as ready made components. For special application same technique can be used for very high voltage capacitors.

    Here is overview of most common capacitor types:

    • Ceramic: Fairly cheap but not available in really high capacitances - 2uF-10uF are about the max for any practical devices. Extremely low ESR. Surface mount devices have essentially no series inductance and are commonly used to bypass high-frequency noise away from digital IC's. Not polarized.
    • Electrolytic: Cheapest capactitance per dollar, but high ESR. Mostly used for 'bulk' power supply. Typical values 1uF-5000+uF. Polarized. Fairly durable, but will literally explode if reverse-biased. Tolerances of +-10% and +-20% are not uncommon. A tolerance of +80% and -20% is common for capacitors used in power supplies.
    • Tantalum: The 'cadallac' of capacitors. Very low ESR (not as low as ceramic, though), very high capacitance values available, but expensive (10x electrolytic). Usually used where one might use electrolytics. Polarized.
    • Polyester: Kinda expensive, not very high capacitance values, ESR not too bad. Polyester capacitors have very very stable temperature characteristics (capacitance change is very small as temperature changes). Used where stable capacitance is important like oscillators and timers. NOT polarized.
    There's others, of course, such as 'X' caps made to connect directly across mains AC power supplies that literally 'heal' themselves after an overvoltage. There are also so called 'Y' capacitors which are used in mains filters where they are connected between ground and live+neutral connectors. Y-capacitors have special safety regulations related to them.

    When selecting capacitor there is always a among size, dielectric, and value. Use the better grades of ceramic capacitors to obtain low impedance at high frequencies. Mica has the highest frequency response, but mica capacitors have the least volumetric efficiency. Aluminum and tantalum-electrolytic capacitors pack lots of capacitance to a small package but are useless at high frequencies (greater than 1 and 10 MHz, respectively).

    Electrolytic capacitors are constructed using a metal electrodes put into some form of electrolytic liquid. This kind of capacitorcan give high capacitances (from microfarads to tens of thousands of microfarads). Fixed Aluminum Electrolytic consists of two conducting electrodes, with the anode having an aluminum metal oxide film acting as the dielectric material or insulating medium. The typical voltage rating of electrolytic capacitor varies from few volts to few hundred volts. The biggest disadvantage if electrolytic capacitors is that they are polarity sensitive: you are only allowed to charge them only on one way. The capacitors have the positive / negative terminals marked. The capacitor must be put in the right way to the circuit (putting it wrong way will cause serious damage to the capacitor).For power supply smoothing capacitor applications, where large capacitances are needed, aluminium electrolytic capacitors arethe most common choise. There is aging thign to consider with electrolytic capacitors. Electrolytic capacitors contain a wet electrolyte that gradually dries with time and leads to an increased ESR (EquivalentSeries Resistance) and reduced capacitance, and this causes increased voltage ripple and possible timing problems in analogue circuitry. The of drying depends on the capacitor temperature (hotter they run, faster they will fail). A good example of electrolytics drying out is the appearance of a strong visual ripple or image tearing on old video monitors. The fix in this case is to change the main PSU electrolytics.

    Tantalum capacitors have a good capacitance versus size ratio but start to behave like an inductor at very low frequency due to high-parasitic inductance called equivalent series inductance (ESL). Standard tantalum capacitors have quite a high equivalent series resistor (ESR). Tantalum capacitors lose their intrinsic capacitance value when working at frequencies in excess of tens of kHz. Tantalum capacitors are polarity sensitve like all electrolytics and can be damaged easily if you apply wrong polarity voltage to the capacitor.

    Fixed Ceramic Monolithic capacitors are constructed by co-firing alternate layers of metal (electrodes) with ceramic (dielectric) materials. Used in filters, timing elements and HF coupling. Ceramic capacitors offer smaller global impedance (ratio 100:1) when compared to standard tantalum capacitors. The intrinsic capacitance value is kept more constant with variations in frequency compared to tantalum devices. In filtering applications the ripple voltage generated by ceramic capacitors is lower than those generated by tantalums. There is a de-rating specification that designers must apply to ceramic capacitors. The manufacturer measures the specified capacitor value at 0 VDC. When polarized, the capacitance value decreases according to the polarization applied to the capacitor (5 percent to 30 percent losses for X7R). It is trongly recommended if you are using using X5R/X7R to take the nominal voltage needed and multiply it by a factor of two when selecting the capacitor (tantalum 1 μF used on a 3.3-V device can be replaced by a ceramic X5R/X7R 1 μ/6.3 V). There are various ceramic family members that offer different characteristics.

    • Y5V, Y5U, Z5V and Z5U (class III grade) have very high dielectric constants and allow manufacturers to offer price-competitive, high-capacitor value in small packages (up to over 100 uF). Y5V, Y5U, X5R and X7R cannot be used in distortion sensitive applications because of their high dielectric absorption. They cannot be practically used in tuning applications because they are not practical as tight tolerance components.
    • X7R and X5R are materials suitable for more severe temperature atmospheres. They offer practically the the same dielectric constant as class III grade types. Values for X7R can be found up to 47 μF/6 V. X5R and X7R cannot be used in distortion sensitive applications because of their high dielectric absorption. They cannot be practically used in tuning applications because they are not practical as tight tolerance components.
    • NPO (strontium titanate?class I grade) ceramic capacitors are the most stable with respect to temperature, frequency, aging and tolerance. Typically, NPO ceramic components are used in tuning (filters, etc.) applications where aging, tight tolerance, stability versus temperature, and voltage are a must. However, because the constant dielectric is very low the capacitors tend to be big and there is limitation on what are the maximum values available.

    Film capacitors can be made at a very tight tolerance, while exhibiting a very stable behavior over frequency, temperature and aging, without showing any voltage de-rating. They are available in high-voltages, making them well suited for applications like filter. Film capacitors also offer the ability to sustain voltage surges well above their nominal voltage for a small amount of time, without loosing any of their properties. This makes them ideal for use with the line transformer that must deal with the high telephone line voltages (anywhere between 250 to 630 V) and in mains voltage circuits. Fixed Film Snubber capacitors are generally used a part of a suppression network to reduce spikes and EMI in power supply designs. You should keep in mind that the film capacitor family is extremely sensitive to the soldering process and that care must be taken during manufacturing, specifically for surface mount device (SMD) types of film capacitors. The manufacturers typically give strict rules concerning the temperature and time of soldering.

    High Voltage capacitors are the ones that have a working voltage exceeding 500V. They are found in high voltage power supplies, X-ray devices, pulse applications and voltage multipliers.

    Fixed AC (Incl. Motor Start) capacitors can be found in motors in conjunction with the windings. Usually, required where motor starting and/or running torques must be relatively high in relation to running torque.

    For power signal wire and power plane decoupling in digital electronics ceramic and tantalum capacitors are considered as the best solutions.

    Low Equivalent Series Resistance (ESR) refers to capacitors that have a low equivalent internal AC series resistance. Used in power supplies, high-current pulse circuitry, RF/microwave elements.

    For RF applications ceramic capacitors are common. Ceramics do not suit for all applications, because most of ceramics have strange effects, like changing capacitance with bias voltage.

    In audio applications type of insulation material does make a difference. For audio applications IIRC, ceramic, paper, mica, electrolytic and tantalum are all considered inferior by high-end hifi people. The plastic-film kind (especially polystyrene) are the preferred dielectric in very high quality audio applications.

    The newest type of capacitors are supercapacitors. A new category of capacitors called supercapacitors offer the high power delivery of capacitors and the high energy storage capacitry approaching batteries (nowadays energy densities only a small fraction of batteries). Supercapacitors are a cross between traditional capacitors and batteries, supercapacitors uniquely combine battery-like energy storage with capacitor-like power discharge in a small package. Supercapacitors hold more electricity than capacitors and transfer and recharge faster than batteries. Supercapacitors store energy as electricity like normal capacitors (not using chemistry like batteries). Supercapacitors are expected to be the next evolution of energy and power storage devices. They are replacing conventional batteries and capacitors, or being used to complement them. The main component in supercapacitors is activated carbon and organic electrolyte. Supercapacitors are manufactured using precisely controlled physical reaction that results in a fantastic carbon sponge filled with molecular-size pores, making almost every atom of carbon part of an available surface to store or release a charge. There are also very high capacitance capacitors made using gold based technology (Panasonic Gold Capacitors is a small 0.33F capacitor) and using aerogel (1 Farad capacitors). High capacity supercapacitors are generally low voltage devices (for example 2.5V or few volts voltage ratings).

    Capacitors used in bypass applications are implemented as shunt elements and serve to carry RF energy from a specific point in the circuit to ground. Proper selection of a bypass capacitor will provide a very low impedance path to ground. Satisfying capacitive bypass application requirements entails careful analysis of various frequency dependent capacitor parameters such as series resonant frequency (FSR), equivalent series resistance (ESR), and the magnitude of the impedance. The ESR and impedance should always be evaluated at the operating frequency.Nowadays a lot of talked about capacitor feature is ESR. ESR is an abbreviation for Equivalent Series Resistance, the characteristic representing the sum of resistive (ohmic) losses within a capacitor. The ESR rating of a capacitor is a rating of quality. A theoretically perfect capacitor would be loss less and have an ESR of zero (=no in-phase AC resistance). ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a capacitor (thus the name Equivalent Series Resistance). This resister can cause circuits to fail that look just fine on paper and is often the failure mode of capacitors. While ESR is undesirable, all capacitors exhibit it to some degree. Materials and construction techniques used to produce the capacitor all contribute to the component's ESR value. ESR is a frequency dependent characteristic, so comparison between component types should be referenced to same frequency. Industry standard reference for ESR is 100kHz at +25?C. Power dissipation within the capacitor, and the effectiveness of the capacitor's noise suppression characteristics will be related directly to the ESR value.

    Another important thing to keep in mind is ESL. ESL (Equivalent Series Inductance) is pretty much caused by the inductance of the electrodes and leads. The ESL of a capacitor sets the limiting factor of how well (or fast) a capacitor can de-couple noise off a power buss. The ESL of a capacitor also sets the resonate-point of a capacitor. Because the inductance appears in series with the capacitor, they form a tank circuit which is tuned to some frequency.

    When selecting a capacitor, designers should be sure to carefully consult the capacitor's data sheet before selecting the part. Failure to do so could result in a significant modification of the characteristics of the capacitor, or even in the destruction of the capacitor. Ultimately, the price and package size will be major factors in the designer's final choice.

      Electrolytic capacitors

      Name electrolytic capacitor refers to capacitors where the dielectric is formed by an electrolytic process. Wet electrolytic capacitors have an actual moist electrolyte, while dry or solid electrolytic capacitors don't. Most electrolytic capacitors have dielectric that is made up of a thin layer of oxide formed on a aluminum or tantalum foil conductor.Aluminium electrolytic is the term used by capacitor manufacturersfor electrolytic capacitors constructed with aluminium electrodes. This is the most commonly used type, and most often then peopletalke about "electrolytics" they mean aluminium electrolytic capacitors. Tantalum electrolyticis the term used by capacitor manufacturersfor electrolytic capacitors constructed withtantalum electrodes. The largest advantage of electrolytic capacitor is that they can fit large ampunts of electricity (large capacitance) to a very small size component.Electrolytic capacitors have several undesirable properties. They are inherently polar devices, meaning that the anode of the capacitor must be more positive than the cathode (There are also special true bipolar electrolytic capacitors available). Most electrolytic capacitors can withstand small and brief amounts of reverse voltages, but this is not recommended. The main concern is internal heat and gas generation. You need to pay attention to correctly hooking a polarized capacitor like electrolytics. If you "push" a polarized capacitor hard enough, it is possible to begin "electrolyzing" the moist electrolyte. Modern electrolytic capacitors usually have a pressure relief vent to prevent catastrophic failure of the aluminum can. Be warned that large value capacitors may explode if abused very badly. Leakage currents are higher ESR's are higher and operating voltages and failure rates are higher than non-electrolytic capacitors. Electrolytic capacitors have low self-resonance frequencies and are unsuitable for high frequency work. Electrolytic capacitor tolerances are normally high. The one factor that outweighs all these undesirable properties is the very high volumetric density that electrolytic capacitors exhibit. This means that you get lots of capacity in small size package. Several metals, such as tantalum, aluminum, niobium, zirconium and zinc, can be coated with an oxide film by electrochemical means. These metal oxides are remarkable dielectrics under the proper conditions. However, the metal-metal oxide interface is rectifying. That is, in one direction it is a good insulator, and in the other direction it is a conductor. This is why capacitors are polar. Non-polar electrolytic capacitors are made by using two oxidized films back-to-back. Please note that with electrolytic capacitors the operation voltage can have effect on the capacitance. Some electrolytic capacitors can show reduced capacitance values when operated very much below their designed operating DC voltage.

      • Capacitor Reforming - or : How to avoid the Big Bang!    Rate this link
      • Electrolytic Capacitors - What is an electrolytic capacitor?    Rate this link
      • Electrolytic Capacitors - Electrolytic capacitors are major components of any power converter in use today. Proper understanding of their characteristics allows designers to better utilize them while optimizing their designs. This design note will shed some light on the main features of electrolytic capacitors.    Rate this link
      • Guidelines For Using Aluminum Electrolytic Capacitors - When using Aluminum Electrolytic Capacitors, please observe the following points to ensure optimum capacitor performance and long life.    Rate this link
      • Reforming Electrolytic Capacitors - Aluminum electrolytic capacitors can develop internal short-circuits over time. When an aluminum electrolytic capacitor is first manufactured, the internal materials are not ready for use. The capacitor must first go through a process called formation to activate and condition the capacitor. This process generally involves charging the capacitor at very low currents until the rated voltage is reached. The aluminum oxide layer is maintained every time the capacitor is used. The capacitor uses a tiny amount of current called leakage current to maintain its oxide layer. Over time, the aluminum oxide layer can partially or completely dissolve. If a capacitor with a partially or fully dissolved oxide layer is placed into operation, failure can occur. Reforming is a process of rebuilding the oxide insulation layer back to original specification. The easiest way to reform a capacitor is to charge the capacitor to its rated voltage over 12-24 hours. This allows the oxide layer to build up in a uniform manner. This is a very simple process with the instructions given in this page.    Rate this link
      • UltraCap technology - Basically, an UltraCap is an electrochemical double layer capacitor consisting of two electrodes, which are immersed into an electrolyte. The high energy content of UltraCaps in comparison to aluminum electrolytic capacitors originates in the activated carbon electrode material, which has an extremely high specific surface area of about 2000 m2/g and the extremely short distance between the opposite charges of the capacitors, which is of the order of a few nanometers (2 ... 5 nm). Since the dielectric is extremely thin ? it only consists of the phase boundary between electrode and electrolyte ? capacitance of a few thousand Farads can be realized in devices as small as a soda can.    Rate this link

      Capacitor markings

      There is difference how different capacitors can be marked.Large capacitor have usually the value printed plainly on them, such as 10 uF (Ten Micro Farads).Many mall disk types along with plastic film types often have just 2 or three numbers on them. First, most will have three numbers, but sometimes there are just two numbers. These are read as Pico-Farads. An example: 47 printed on a small disk can be assumed to be 47 Pico-Farads (or 47 puff as some like to say). Here is short introduction to markings you might see on circuit digrams:

      • 1 F = 1 Farad
      • 1 mF = 1 milli Farad = 1/1,000th of Farad or .001 Farads
      • 1 uF = 1 micro Farad = 1/1,000,000 of Farad or 0.000 001 Farads (10-6 )
      • 1 nF = 1 nano Farad = 1/1,000,000,000 of Farad or 0.000 000 001 Farads (10-9)
      • 1 pF = 1 pico = 1/1,000,000,000,000 of Farad or 0.000 000 000 001 Farads (10-12)
      Sometimes you might see combination markings like 1n5, where decimal dot is marked with letter. Here 1n5 means 1.5 nF. In the same way 2p2 means 2.2 pF. This is a common practice by some manufactures and the reason for thisis quite simple. By putting the "letter" in place of the "Tiny Decimal Point"it eliminates the chance of missing it on a poorly photo-copied or printedcopy of a schematic.

    Coils

    An typical inductor is simply a coil of wire, which can be wrapped around either air or metal cores. As current flows into an inductor, a magnetic field is created around the coil. When the current stops, the magnetic field collapses, generating an induced current flow in the coil. Low frequency currents flow easily into the inductor, but as the alternating current frequency increases, the impedance of the inductor increases. The inductor introduces a phase shift to AC signal going through it. Inductors allow direct current to flow, but as the frequency of oscillation increases, so does the inductor's impedance. A coil (of any sort) is an inductor

    Inductors behave to electricity as mass does to a mechanical system. Inductors resist change in current flow, just as masses resists change in physical movement. Stand in front of a moving car and try to stop it: its mass keeps it going. In the same way, if you suddenly try to stop the current flowing in an inductor - the inductor will resist the change in current. The same way the mass of the car resisted the mechanical stopping, so will the inductance of the coil resist the stopping of the electrical movement - the current flow.

    An inductor is an energy storage device. A coil/inductor can be as simple as a single loop of wire or consist of many turns of wire wound around a special core. Energy is stored in the form of a magnetic field in or around the inductor. By placing multiple turns of wire around a loop, we concentrate the magnetic field into a smaller space where it can be more useful. When you apply a voltage across an inductor, a current starts to flow. It does not instantly rise to some level, but rather increases gradually over time. The relationship of voltage to current vs. time gives rise to a property called inductance. The higher the inductance, the longer it takes for a given voltage to produce a given current.

    Whenever there is a moving or changing magnetic field in the presence of an inductor, that change attempts to generate a current in the inductor. An externally applied current produces an increasing magnetic field which in turn produces a current opposing that applied externally, hence the inability to create an instantaneous current change in an inductor. This property makes inductors useful as filters in power supplies.

    In most practical circuits inductive devices, operating in d.c. circuitry, which are switched on and off should have a diode or other suitable protection component connected across their coils to catch the inductive fly back.

    Most simple coils are air-core coils. They consists just winded copper wire. Air-core coils canproduce stable inductance over wide range of DC bias currents and work up to very high frequencies. The biggest downside od air-core coils is that very many turns are needed to produce large inductances. Other downside is that they produce somewhat large magnetic fields around them.

    Larger inductance coils can be produced by using suitable magnetic material core. With this approach large inductances are possible. Many types of cores are commonly used in inductors. magnetic material in coil core tends to concentrate the inductor's magnetic field inside the core and increases the effective inductance. While a magnetic core can providegreater inductance in a given volume, there are also drawbacks. A magnetic core can contain only a limited magnetic field. The limitations of the cored coils are the usually limited operating frequency range and possibility of core saturation because of excessive AC current or large DC current. All those characteristics depend on core material characteristics ans coil design and coil core type. Toroid inductors minimize the magnetic field around the coil.

    Inductors are often used in filter circuits and switch mode power supplies. Inductors have a bad reputation as filter components in some applications. They not only transmit EMI, but they act as antennas for receiving EMI as well (unless they are made wo a very good magnetic core).

    The markings of inductors can vary. Molded inductors usually follow the same coloring scheme as resistors except the units are usually microhenries. A brown-black-red inductor is most likely a 1000 uH. Sometimes a silver or gold band is used as a decimal point. So a red-gold-violet inductor would be a 2.7 uH. On some inductors you can see a wide silver or gold band before the first value band and a thin tolerance band at the end. Some inductors have their values marked to them and some have no markings in them. If you are unsure of some inductor value, then it is a good idea to measure it with an inductance meter (usually inductance scale on RLC meter).

    Inductors are useful components, but generally they are less common and harder to get than most other basic components (like resistors, capacitors, transistors etc.). In some applications the function of an inductor is replaced with an elec