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The Design of Car Audio Power Amplifier

This article is for those who wants to make their own car amplifier. The basics of calculation will be discussed below. If you have understand it you will be able to make car amplifier yourself.


There are many designs of good amplifier published,  solid state (SS) or tube designs. But few have written the design of car power amplifier

Actually the difficulty of designing the car power amplifier does not lies with the audio power amplifier, but it is more to providing the switching power supply.

As we knows, the output power of any audio power amplifier is approached by formula :

P = Vpp2/(8*Rl)

where Vpp= peak to peak supply voltage, Rl is the speaker impedance load. For car voltage of 12Vdc, if we connect it to 4 Ohm speakers we will only have power of 144/32 = 4,5 Watt. Bridging the amplifier will double the power, but will never be more than 40 W.

If we want to make more powerful amplifier, lets say 170 watt at 4 ohm speaker load, we will need supply voltage of 74Vpp, or +/- 37 Vdc. The way to have this voltage from car supply of 12VDC is to make DC-DC converter.

In this article, I will discussed the car power amplifier in 3 steps :

1.      The design of audio power amplifier

2.      The design of DC-DC converter

3.     Miscellenous  tips for making car power amplifier.



In fig1 we can see that audio power amplifier can be splitted into 3 main functions, that is:

-         First stage / input stage

-         Second stage / voltage amplifier stage

-         Third stage / output stage


First stage is the stage that receives the input audio signal and  Negative Feedback  (NFB) signal from the output of the amp. Feedback is the back signal used to stabilized the audio amplifier, like the gain factor. For first stage built by discrete transistors, both signals is fed to basis of the transistor, like in fig1. Both basis of the transistors is the Non- Inverting input and Inverting Input, like those in the op-amp.

Second stage is the stage that responsibles for the Voltage Gain in the power amplifier.

Third stage is the Current Gain.

We can explain those stages in a simple way like this : Input signal, like from car radio or CD player have low voltage, about 1Vpp with few milliampere current. To produce power of 170 Watt at 4 ohm speaker load, than the signal has to have maginitude of 28Vpp and current of 6.5A (from the equation of P=I2*R = V2/R)

 The first stage receives this signal in the non-inverting input and the inverting input receives NFB signal to make sure the voltage gain that the amplifier produces has a constant number, lets say 28 x. The output signal from the first stage has not reach 28Vpp, it tends to have the magnitude similiar to the input voltage. Second stage amplifies the voltage that the first stage generates. Second stage will amplifies the voltage to produce a signal that is enlarge 28x  for the amplifier to have a 28Vpp signal from 1Vpp signal, but this 28Vpp signal still have small current , only a few mA and cannot drive the speaker load. The third stage amplifies the current from few mA to 6.5 A.

Offcourse the explenation for three stages above is not that simple in the real amplifier. We should take the nature's law for a transistor gain, that is G=RC/RE. This principles must be applied in each transistor in those 3 amplifier stages.


First stage designs have main component, that is Constant Current Source (CCS) which can be seen in fig2. One of the basic of electronic law that works on every circuit is that the voltage drop of Basis and Emitor (Vbe) equals the drop voltage of one dioda = 0.67V. It can be seen in  fig2 that the voltage drop of 2 dioda IN4148 = 2 x 0.67V = 1,34V. We can see in RE and Q1, then V=0,67 is substracted by  Vbe of Q1 and the other 0,67V  will be the drop of RE. So we will have a Constant Current Source of 0,67/RE. In fig2 the Ic is = 4,4mA. CCS first stage varies between 1-4mA.

In fig1 first stage, each component will be explained like this:

-         R1 is the impedance of the audio amplifier, the range is 10 Kohm – 47Kohm

-         C1 is the highpass filter from the equation :  Fhp = 1/(2 x pi x R1 x C1)

-         RED1 and RED2 is between 50-150 ohm

-         RM1 and RM2 is picked up so the voltage drop will be  50mV – 150mV

-         Q3 and Q4 is the Current Mirror that ensures the current in RM1 and RM2 will have the same magnitude.

-         RF and CF will be discussed later.

Before we discuss Second Stage and Third stage, first we will discuss the amplifying effect of a transistor. In fig3a we will see a circuit of Common Emitor Mode (CEM). This circuit will amplifies the voltage. In fig3b we see a Common Colector Mode (CCM). This circuit is the current amplifier without voltage amplifier. So if we want to amplifies voltage we use CEM circuit and to amplifies current we use CCM circuit.


The Second stage responsibles for all voltage gain (Maximum Voltage Swing) in an audio power amplifier. This is why the Second stage is generally known as VAS or  Voltage Amplifier Stage. This stage consist of a voltage amplifier/CEM transistor(Q5 in fig1) in the bottom, Constant Current Source in the top, and a bias control circuit in the middle. Second stage CCS has current magnitude between 4-8mA

In the second stage there is an important capacitor for an audio power amplifier , that is  Miller Capacitor (CC in fig1). CC defines the pole of the frequency response for an audio amplifier and the magnitude usually in small order (severalpF).

Bias control circuit consist of a transistor, resistor and a VR like in  fig5. This circuit uses a transistor that is placed in the heatsink, because the transistor have good heat compensation factor (for bipolar transistors).  For the amplifier that uses mosfet transistor for the final device, the bias circuit only needs potentio or dioda only because mosfets have different heat characteristic than bipolar transistors. The bias voltage magnitude depends on the type of the third stage used, which will be discussed later.


Third stage / Output Stage is the current amplifier. Third stage and the bias circuit will defines whether an amplifier works in class A, class AB or class B.

It can be said that almost 90 % of car audio power amplifier works in class B. Operation in class B does not mean that the sound produced is not good or corrupted. With good design, we will have good audio results, both from class A or class B. The choice of class B in car audio power amplifier is  conected to efficiency and the heat generated. Heat generated is a very important factor, because if not considered carefully, it will lead to amplifier breakdown.

Many configurations of the output stage can be seen in fig4. Each configuration has different optimum bias voltage. It depends on how many Vbe's that have to be passed.  Example : In fig4(a)  the signal has to pass 4 Vbe's, which is Vbe Q1, Q3, Q4 and Q2. So the optimum bias = 4 x 0.67V = 2.8V.

Both 3 stages that we have discussed above, if we connect the together will be a circuit that can be seen in  fig5. Parts of this circuit can be explained like this:

-         The value of Negative Feedback (NFB) resistor is determined by determining the gain factor with the equation :  Gain = 1+(R10/R8) = 1+10k/500 = 21 x. The value of R10 = value of R1 to balance input. R20 and C7 are the pole and slope compensator.

-         C2 limits the DC gain factor, value ranging from 47-220 uF, usually using a nonpolar capacitor.

-         R21, R22 and C11 will stabilize CCS. Here we use CCS with 2 transistor system,but the equation used still the same, that is  Ic = 0,67/RE .

-         The output of differential pair tapped from collector of T10 and send to VAS which is built by T12 and T4. This configuration is called Darlington VAS and the value of R8 is standard.

-         C3 is the Miller capacitor with value of 100pF.

-         C5 is called Speed Up Capacitor. Several designs do not use this capacitor

-         R18, C6,L1 and R19 are output power stabilisator. If there is any oscilation occur in the audio power amplifier, the first tobe effected is R18 besides the final transistors.

Car Power amplifier usually loaded by low impedance speakers, usually 4 ohms and can reach  ½ ohm on bridge mode. Here we know the term “High Current Amplifier”. The difference is the number of final transistors, or in fig5 it is the number of pairs of T7 and T8. As a rule of thumb, the number of transistor needed first has tobe calculated by equations above, and then we determine the number of final transistor needed with assumption that 1 transistor can handle 50 Watt output. A pair of bipolar transistor can handle 100 Watt. The power is raised by parrarelling several output transistors, so the currrent flowing will be larger. For large number of final transistors, we change the predriver stage with darlington configuration.

Several designs uses symetrical design, like those used in  AXL and Crescendo schematic. this design is developed from the basic principal above, but the signal handling for + and - part is handled by complementary circuits. 

I have an example about another kind of power amplifier, that is a non-feedback amplifier. You can view the principles of the "millenium power amplifier" in the . This amplifier has a certain gain factor in first and second stage, while the third stage is only current amplifier.


For building car power amplifier, we need symmetrical power supply (+, 0, -) by building  DC-DC converter. The converter system discussed below will be the SMPS(Switch Mode Power Supply) type PWM (Pulse Width Modulation). This system will deliver stable output voltage, regardless of the input voltage (usually the car electrical system will range in 9-15Vdc).

To explain the  SMPS type PWM, it can be analogued by the next example. Look at  fig6. There is a voltage pulse V1 on-off with 50% wide. These pulses if passed  through suitable  L and C filter will be transformated into straight voltage of V2 which is V2 = ½ V1. (noticed the marked area below pulsed V1 is the same total  area of the marked straight V2 ). With the same logic, if the pulse width of V1 is narrowed, we will have a lower V2 and if we enlarge the width of V1 pulse, we will have higher V2. Some may ask, how can we get 30VDC from the car's 12VDC? The answer is simple. If we get the V1 voltage to 60VDC, then in the 50% duty cycle, we will get 30VDC straight. This is the part where the power switching transformer takes control, to make the 60VDC from 12VDC, and then chopped by the PWM.  This is the princip of PWM. (Like the principal of class D digital power amplifier). In this design, we use regulating PWM IC's, like TL494, TL594, SG3524, SG3525. These IC's will compare the output of DC-DC converter with a reference voltage. If the output of DC-DC converter is smaller than reference voltage, then the IC will enlarge the pulse width so the voltage will raise equally to to reach determined voltage. So as if the output of DC-DC converter is higher than the reference voltage, the IC will narrow the pulse width so the output voltage will be lowered to the determined voltage.

 Generally SMPS used in car audio amplifier is the push-pull system with switching frequency between 20-70Khz. In push pull sytem like in fig7, Q1 and Q2 gives alternating switched current pulses so the transformator will be objected to maximum flux swing change without saturating the core.

In this design we will use PWM IC with SG3524 from SGS Thompson. Specifications can be seen in  SGS Thompson's website. Fig8 shows the configuration of 16 pins on this IC. To make is simpler, lets design a SMPS by explaining the function of each pin.

For the stereo power amplifier in  fig5, we will need a  SMPS 12Vdc input and summetrical output of  +/- 37Vdc with 8A rating.

1.   First we make the Remote Turn On circuit , which is connected from the car radio / CD player. The circuit can be seen in fig9a. This circuit will turn on the SMPS by giving 12Vdc to pin 12, pin 13 and pin 15.

2.     The SMPS switching frequency is determined 50Khz. For this, the clock inside IC SG3524 is adjusted  2 x 50 Khz = 100Khz. This clock is built up by pin 7(Ct) and pin 6(Rt). The approach can be done with  equation Fclk = 1 /(Rt x Ct). Here we use Ct = 1nF and Rt = 10Kohm like in fig9b

3.      Pin 2(Non Inv In). In pin 2 we put stable reverence output for the SMPS. Here we use reference voltage of ½ from reference pin 16.

4.      Pin 1(Inv In) is the output voltage detector . Pin 1 is connected to the optoisolator type 4N35 like in  fig9b. Optoisolator is an important component in making this SMPS so we can have  Floating Secondary Ground which will prevent noises (especially whine/storing) if the power amplifier is placed in car. The value of  zener diode is 2 x 37V = 74V. If it is difficult to have zener voltage of 74 V, then we can series several zener values until we have total of 74 V.

5.      Pin  (4) and  pin(5) are not used and connected to ground, pin(8) and pin(10) connected directly with ground.

6.       Pin no 9(Comp) determines slope and pole of feedback from the whole SMPS system. In this design we use only 1 capacitor of 100nF.

7.      Pin no 16(Vref) gives reverence voltage of 5,1 Vdc . This pin is placed with 10nF as a voltage stabilisator.

8.      The output ripple (Vr) of the SMPS is determined by equation :

       Vr = 8 x 10-6 x I / Co. With I = 8A and Vr = 0,029V we will have Co of 2.200uF    in +37Vdc ->-37Vdc rail or  4400uF each in +37Vdc_0  and 4.400uF in 0_-37Vdc.

9.      For output filter capacitor of 2.200uF, we will need approximately  4x 2.200uF or 8.800uF in the SMPS's input 12Vdc . The larger the value of this capacitor, more energy stored for the SMPS.

10.  Output filter inductor Lo is determine by : Lo = 0,5 x Vout/ (I x F). With Vout = 2 x 37V = 74V, I = 8A dan F = 50Khz, we will have Lo = 0,092mH or Lo = 0,046mH on each supply rail + and – 37Vdc.

11.  Pin 11 and pin 14 are output pins that will drive the primary winding switching mosfets. Inside IC SG3524 both pins have already opereated in mode push-pull. The circuit for driving power mosfets can be seen in  fig9b. The number of power mosfet used is 3 in each transformator primary. So total there is 6 power mosfets type BUZ11.

12.  Transformator(trafo) for SMPS is selfwould from ferrite toroidal core (like donuts) like in fig10. It is very important that for SMPS frequency above 20Khz, we cannot use iron core transformator like we use in homes. The ferite core transformator will have black color like in the speaker magnets, but do not have magnetizing force. The basic of equation for switching power supply with  12Vdc input is:

  (1)  Np = 1,37 x 105 / (F x Ae), where Np= primary number of turns, F =  switching frequency, Ae = X x Y = window area of ferrite in cm2. Look at fig10. To make it easy to wound the transformator, we will have to choose the toroid core with minimal diameter of 2,5 cm and window area minimal of 0.75cm2.This is necessary for the easyness of self handwound. Remember that in push-pull system there is 2 primary windings.

(2) Ns/Np = Vo/8,8, where Ns = secondary number of turns, Vo = secondary output voltage

(3) Ap = 0,004 x Vo x Io, where Ap = window area of primary wire in  mm2, Vo = output voltage, Io = output current.

(4) As = 0,13 x Io, where As = window area of secondary wire in mm2.

Example :  If we use toroidal ferrite core with window area of  Ae = 1 cm2. then from equation no. 1 we will have number of primary turn Np = 1,37 x 105 / (50Khz x 1 cm2) = 2,74 turns. In practice, number of minimal primary turns is 4 so the primary will cover the whole toroidal core. So we use 4 turns for Q1 and 4 turns for Q2.

From equation (2) we have that Ns/Np = 37/8.8 = 4,2. From here we can calculate that the number of secondary windings is = Np x Np/Ns = 4 x 4,2 = 16,8 or 17 windings. Like the primary, in secondary we use 2 x 17 turns, that is 17 turns for  +37V –> 0 and 17 turns for 0-> -37V

Equation (3) is used tp determine the number of primary winding wires. We have  Ap = 0,004 x 74 x 8 = 2,36mm2. If we use a 1mm diameter magnet wire, we will have window area of 0,785mm2 so we will need  3 wire magnets for each primary windings

Equation (4) is used to determine the number of wire needed for secondary windings. We have As = 0,13 x 8 = 1mm2 So if we use wire magnet with diameter of 0,8mm(window area = 0, 5mm2), then we will need 2 wires with diameter 0,8mm for each secondary windings. 

13.  The secondary output voltage is rectified by full bridge configuration like in fig11. Bridging diode must be the type of fast rectifier, usually looks like transistor TO220 with plate heatsink. For SMPS we cannot use ordinary 50/60Hz rectifier diode. For this design we use diode type  BYW29-150, which have rating  of 8A, 150V. We can also use other diodes like with prefixes FE…,MUR..., as long as it is a fast rectifier diode with minimal specification like above.


Car power amplifier has specific accesories like preamp gain circuit, an inverting channel so that the power is bridgeable. These functions usually done with opamps. The circuit can be seen in fig12a and the supply circuit can be seen in fig12b. The circuit is placed before the audio amplifier circuit.

The transformator is handwound on toroidal ferrite core. The output filter inductor can be made with ferrite core material or MPP core material. It can be made with 1.2mm wire magnet, handwound and measured until we have 0,046mH

Handwound the transformator core can be done as follow (fig13b):

-         First we wound the secondary winding of 4 wires of 0.8mm magnet wires at once with 17 numbers of turn. The turn can be made in any direction as long as we consistent with the direction of the wound. If we have finished wounding it, the toroidal core will look like fig13a. We named the wires with wireA,B,C, and D.  If we start the wound on top of the core, the end will be at the bottom of the core. Make sure each wire edges with AVOmeter. Connect start edge of wire A and B to point S1 and the end edge of wire A and B to point G.  The start edge of wire C and D is connected to point G and the end edge of wire C and D is connected to point S2.  Point G will be the secondary ground of the power amplifier and point S1 and S2 will be connected to bridging diode of  BYW29.

-         After we finished with secondary winding, we start to wound primary winding. Edges of primary wires is placed diagonally to the edges of the secondary wires like in fig13c. Like winding the secondary wires, we wound 6 wires of 1mm diameter at once. Name them wire A,B,C,D,E,and F.  Connect the start edge of wire A,B,C to point P1 and the end edge of wire A,B,C to point P+. Connect the start point of wire D,E,F to point P+ and the end edge of wire D,E,F to point P2 (fig13d

        If you have finished winding the primary and the secondary, the whole transformator will have the same wire directions like in fig12e. Connect point P+ to the +12VDC of the car battery, point P1 to the drain of power mosfets Q1 and point P2 to the drain of the power mosfets Q2.

It is important to remember that all tracks in PCB layer that is connected to the power transformer has to have sufficient width due to large current will be involved. Also it is better if we soldered those tracks to have more current transfer.

After finishing winding the transformator, place all the rest of the component and finish assembly of the SMPS. You can test it by connect it with 12VDC input from the battery. Don't forget to connect the remote turn on with 12VDC. There should be output voltage of  +37V, 0 and –37V without any large current draw in the 12VDC line. Check for any mistakes, if the output voltage do not present or if the SMPS draws large current from 12VDC input.

In the assembly process of car audio power amplifier, we have to pay attention in mounting all transistors to the heatsink. We must use sufficient heatsink surface so the heat won't damage the amplifier. Use mica isolator and white silicon pasta to make sure the heat transfer. Firmly tighten all the bolts to press all the transistors. Car amplifier works in vigorous environment like in the trunk of a car. Placing an extra fan always a good idea in making car power amplifier.

After we connect the SMPS to the audio amplifier, we are ready to test the car power amplifier. First trim the bias potentiometer fully left side to have minimum bias. Turn on the SMPS and look for the current draw in 12VDC line with ampmeter. The ampmeter indicator will raise for a moment to fill all the capacitors. After a few moment, the ampmeter indicator must turn back to minimum indication of ampere. If not, there is some problem. Then we trim the bias to optimal point. Usually for car stereo power amplifier total quiscent ampere will not exceed 2A of 12VDC line.  



From Wikipedia, the free encyclopedia

A resistor is a two-terminal electronic component designed to oppose an electric current by producing a voltage drop between its terminals in proportion to the current, that is, in accordance with Ohm's law: V = IR. The resistance R is equal to the voltage drop V across the resistor divided by the current I through the resistor.

Resistors are characterized primarily by their resistance and the power they can dissipate. Other characteristics include temperature coefficient, noise, and inductance. Practical resistors can be made of resistive wire, and various compounds and films, and they can be integrated into hybrid and printed circuits. Size, and position of leads are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power. Variable resistors, adjustable by changing the position of a tapping on the resistive element, and resistors with a movable tap ("potentiometers"), either adjustable by the user of equipment or contained within, are also used.

Resistors are used as part of electrical networks and electronic circuits.

There are special types of resistor whose resistance varies with various quantities, most of which have names, and articles, of their own: the resistance of thermistors varies greatly with temperature, whether external or due to dissipation, so they can be used for temperature or current sensing; metal oxide varistors drop to a very low resistance when a high voltage is applied, making them suitable for over-voltage protection; the resistance of a strain gauge varies with mechanical load; the resistance of photoresistors varies with illumination; the resistance of a Quantum Tunnelling Composite can vary by a factor of 1012 with mechanical pressure applied; and so on.


The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Ohm. The most commonly used multiples and submultiples in electrical and electronic usage are the milliohm, ohm, kilohm, and megohm.


Carbon composition

Carbon composition resistors consist of a solid cylindrical resistive element with embedded wire leadouts or metal end caps to which the leadout wires are attached, which is protected with paint or plastic.

The resistive element is made from a mixture of finely ground (powdered) carbon and an insulating material (usually ceramic). A resin holds the mixture together. The resistance is determined by the ratio of the fill material (the powdered ceramic) and the carbon. Higher concentrations of carbon, a weak conductor, result in lower resistance. Carbon composition resistors were commonly used in the 1960s and earlier, but are not so popular for general use now as other types have better specifications, such as tolerance, voltage dependence, and stress (carbon composition resistors will change value when stressed with over-voltages).

Carbon film

A carbon film is deposited on an insulating substrate, and a spiral cut in it to create a long, narrow resistive path. Varying shapes, coupled with the resistivity of carbon, (ranging from 9 to 40 µΩm) can provide a variety of resistances. Carbon film resistors feature a power rating range of 1/6 W to 5 W at 70°C. Resistances available range from 1 ohm to 10M ohm. The carbon film resistor can operate between temperatures of -55°C to 155°C. It has 200 to 600 volts maximum working voltage range.[2]

Thick and thin film

Thick film resistors became popular during the 1970s, and most SMD resistors today are of this type. The principal difference between thin film and thick film resistors is not the actual thickness of the film, but rather how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors).

Thin film resistors are made by sputtering the resistive material onto an insulating substrate Sputtering is a method used in vacuum deposition. The film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards: i.e. the surface is coated with a photo-sensitive material, then covered by a film, irradiated with ultraviolet light, and then the exposed photo-sensitive coating, and underlying thin film, are etched away.

Because the time during which the sputtering is performed can be controlled, the thickness of the thin film can be accurately controlled. The type of material is also usually different consisting of one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and/or bismuth iridate (Bi2Ir2O7).

The resistance of both thin and thick film resistors after manufacture is not highly accurate; they are usually trimmed to an accurate value by abrasive or laser trimming. Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, or 1%, and with temperature coefficients of 5 to 25 ppm/K.

Thick film resistors may use the same conductive ceramics, but they are mixed with sintered (powdered) glass and some kind of liquid so that the composite can be screen-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C.

Thick film resistors, when first manufactured, had tolerances of 5%, but in the last few decades standard tolerances have improved to 2% or 1%. Temperature coefficients of thick film resistors are high, typically ±200 or ±250 ppm/K; a 40 kelvin (70°F) temperature change can change the resistance by 1%.

Thin film resistors are usually far more expensive than thick film resistors, although, for example, SMD thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when bought in full size reel quantities, are about twice the cost of 1%, 250 ppm/K thick film resistors.

Metal film

A common type of axial resistor today is referred to as a metal-film resistor. MELF (Metal Electrode Leadless Face) resistors often use the same technology, but are a cylindrically shaped resistor designed for surface mounting. [Note that other types of resistors, e.g. carbon composition, are also available in "MELF" packages].

Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though that is one such technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix through the coating rather than by etching. [This is similar to the way carbon resistors are made.] The result is a reasonable tolerance (0.5, 1, or 2%) and a temperature coefficient of (usually) 25 or 50 ppm/K.


Wirewound resistors are commonly made by winding a metal wire around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. The wire leads are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heatsink to dissipate the heat; the rated power is dependent on being used with a suitable heatsink, e.g., a 50 W power rated resistor will overheat at around one fifth of the power dissipation if not used with a heatsink.

Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although inductance can be minimized by winding the wire in sections with alternately reversed direction.

Foil resistor

The primary resistance element of foil resistors is a special alloy foil several micrometres thick. Foil resistors have had the best precision and stability ever since they were introduced in 1962 by Felix Zandman. One of the important parameters influencing stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further improved over the years.[3] One range of ultra-precision foil resistors offers a TCR of 0.14ppm/°C, tolerance ±0.005%, long-term stability 25ppm/year, 50ppm/3 years (further improved 5-fold by hermetic sealing), stability under load 0.03%/2000 hours, thermal EMF 0.1μvolt/°C, noise -42dB, voltage coefficient 0.1ppm/V, inductance 0.08μH, capacitance 0.5pF[1].

Current sensing resistor

A current sensing resistor (CSR) is essentially a normal resistor optimized to monitor the current flowing through a circuit by a very simple application of Ohm's law: the voltage drop across the resistor, which is measured, is equal to the product of the current through it by the resistance[2]. The resistance of the CSR is selected so as to produce a voltage drop small enough not to interfere with circuit operation, but large enough to be measurable to the required accuracy. Typical values range from 0.004 to 0.1 ohm. These resistors are designed with tight tolerance, low temperature coefficient, and low thermal EMF for maximum accuracy. For the most exacting cases 4-terminal resistors are manufactured, with one pair of terminals providing the path for the current to be measured, and the other for connecting to the measurement circuit; this is common practice with precision standard resistances used at measurement standards laboratories. 4-terminal construction minimizes lead resistance, TCR of copper terminals, and TCR of solder joints.

Construction materials and configuration style are important parameters to consider. Materials include carbon composition, carbon film, ceramic composition, metal alloy, metal film, thick film (chip), thin film (chip) and wire-wound. Configuration styles include single resistor, resistor network, and resistor chip array. Single current sensing resistors have a single resistance value. Current sensing resistor networks feature a pin layout with multiple resistors connected in series. Current sensing resistor chip arrays connect multiple resistors in parallel.

Specified properties of current sensing resistors may include resistance, number of terminals, tolerance, power rating, continuous operating voltage, temperature coefficient parameter (TCP), and operating temperature. Single current resistors may have either two or four terminals. Resistance range is measured in ohms. Tolerance is specified as percentage. Power rating is the maximum power level that a current sensing resistor can dissipate. The continuous operating voltage is measured in volts. The temperature coefficient parameter (TCP) measures the rate at which the nominal resistance value changes as a function of temperature. Typically, TCP is expressed as parts-per-million per degree Celsius (ppm/C). Operating temperature is an important environmental parameter to consider, especially for current sensing resistors that are exposed to flow, reflow, or wave soldering.

Grid resistor

In heavy-duty industrial high-current applications, a grid resistor is a large convection-cooled lattice of stamped metal alloy strips connected in rows between two electrodes. Such industrial grade resistors can be as large as a refrigerator; some designs can handle over 500 amperes of current, with a range of resistances extending lower than 0.04 Ohms. They are used in applications such as dynamic braking and load banking for locomotives and trams, neutral grounding for industrial AC distribution, control loads for cranes and heavy equipment, load testing of generators and harmonic filtering for electric substations.[4][5][6]

The term "grid resistor" is sometimes used to describe a resistor of any type connected to the control grid of a vacuum tube. This is not, however, a resistor technology.

Strain gauges

The strain gauge, invented by Edward E. Simmons and Arthur C. Ruge in 1938, is a type of resistor that changes value with applied strain. A single resistor may be used, or a pair (half bridge), or four resistors connected in a Wheatstone bridge configuration. The strain resistor is bonded with adhesive to an object that will be subjected to mechanical strain. With the strain gauge and a filter, amplifier, and analog/digital converter, the strain on an object can be measured.

Negative resistors

A device exhibiting negative resistance over part of its characteristic curve can be made with active circuit components.

Other types

Power dissipation

The power dissipated by a resistor is the voltage across the resistor multiplied by the current through the resistor:

P = I^2 R = I V = \frac{V^2}{R}

All three equations are equivalent. The first is derived from Joule's law, and the other two are derived from that by Ohm's Law.

The total amount of heat energy released is the integral of the power over time:

W = \int_{t_1}^{t_2} v(t) i(t)\, dt.

If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance, and may be damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials.

Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use. Air circulation and proximity to a circuit board, ambient temperature, and other factors can reduce acceptable dissipation very significantly. Rated power dissipation may be given for an ambient temperature of 25°C in free air. Inside an equipment case at 60°C, rated dissipation will be significantly less; if we are dissipating a bit less than the maximum figure given by the manufacturer we may still be outside the safe operating area, and courting premature failure.

Series and parallel resistors

Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):

A diagram of several resistors, side by side, both leads of each connected to the same wires
\frac{1}{R_\mathrm{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \cdots +  \frac{1}{R_n}

The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations. For two resistors,

R_\mathrm{eq} = R_1 \| R_2 = {R_1 R_2 \over R_1 + R_2}

The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:

A diagram of several resistors, connected end to end, with the same amount of current going through each
R_\mathrm{eq} = R_1  + R_2 + \cdots + R_n

A resistor network that is a combination of parallel and series can sometimes be broken up into smaller parts that are either one or the other. For instance,

A diagram of three resistors, two in parallel, which are in series with the other
R_\mathrm{eq} = \left( R_1 \| R_2 \right) + R_3 = {R_1 R_2 \over R_1 + R_2} + R_3

However, many resistor networks cannot be split up in this way. Consider a cube, each edge of which has been replaced by a resistor. For example, determining the resistance between two opposite vertices requires matrix methods for the general case. However, if all twelve resistors are equal, the corner-to-corner resistance is 56 of any one of them.

The practical application to resistors is that a resistance of any non-standard value can be obtained by connecting standard values in series or in parallel.

Resistor standards

Production resistors

There are various standards specifying properties of resistors for use in equipment:

  • BS 1852
  • EIA-RS-279
  • MIL-PRF-26
  • MIL-PRF-39007
  • MIL-PRF-55342
  • MIL-PRF-914
  • MIL-R-11
  • MIL-R-39008
  • MIL-R-39017

There are other United States military procurement MIL-R- standards.

Resistance standards

Resistors of extremely high precision are manufactured as substandards of resistance for calibration and laboratory use. They may have 4 terminals, using one pair to carry an operating current, and the other pair to measure the voltage drop; this minimizes temperature coefficients and thermal EMFs. The data sheet for a resistance standard used for calibration is here.

Resistance decade boxes

A resistance decade box is a box containing resistors of many values and two (or four) terminals, with a mechanical switch that allows a resistance of any value allowed by the box to be dialed. Usually the resistance is accurate to high precision, ranging from laboratory/calibration grade accurate to within 20 parts per million, to field grade at 1%. Inexpensive boxes with lesser accuracy are also available. All types offer a convenient way of selecting and quickly changing a resistance in laboratory, experimental and development work without having to stock and seek individual resistors of the required value. The range of resistance provided, the maximum resolution, and the accuracy characterize the box. For example, one box offers resistances from 0 to 24 megohms, maximum resolution 0.1 ohm, accuracy 0.1%.

Resistor marking

Most axial resistors use a pattern of colored stripes to indicate resistance. Surface-mount resistors are marked numerically. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.

The value of a resistor can be measured with an ohmmeter, which may be one function of a multimeter.

Four-band axial resistors

Four-band identification is the most commonly used color-coding scheme on all resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.

For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000Ω at ±2% tolerance accuracy. 560,000Ω changes to 560 kΩ ±2% (as a kilo- is 103).

Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below.

Color 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient
Black 0 0 ×100

Brown 1 1 ×101 ±1% (F) 100 ppm
Red 2 2 ×102 ±2% (G) 50 ppm
Orange 3 3 ×103
15 ppm
Yellow 4 4 ×104
25 ppm
Green 5 5 ×105 ±0.5% (D)
Blue 6 6 ×106 ±0.25% (C)
Violet 7 7 ×107 ±0.1% (B)
Gray 8 8 ×108 ±0.05% (A)
White 9 9 ×109


×10-1 ±5% (J)

×10-2 ±10% (K)

±20% (M)

Preferred values

Early resistors were made in more or less arbitrary round numbers; a series might have 100, 125, 150, 200, 300, etc. Resistors as manufactured are subject to a certain percentage tolerance, and it makes sense to manufacture values that correlate with the tolerance, so that the actual value of a resistor overlaps slightly with its neighbors. Wider spacing leaves gaps; narrower spacing increases manufacturing and inventory costs to provide resistors that are more or less interchangeable.

A logical scheme is to produce resistors in a range of values which increase in a geometrical progression, so that each value is greater than its predecessor by a fixed multiplier, chosen to match the tolerance of the range. For example, for a tolerance of ±20% it makes sense to have each resistor about 1.5 times its predecessor, covering a decade in 6 values. In practice the factor used is 1.4678, giving values of 1.47, 2.15, 3.16, 4.64, 6.81, 10 for the 1-10 decade (a decade is a range increasing by a factor of 10; 0.1-1 and 10-100 are other examples); these are rounded in practice to 1.5, 2.2, 3.3, 4.7, 6.8, 10; followed, of course by 15, 22, 33, … and preceded by … 0.47, 0.68, 1. This scheme has been adopted as the E6 range of the IEC 60063 preferred number series. There are also E12, E24, E48, E96 and E192 ranges for components of ever tighter tolerance, with 12, 24, 96, and 192 different values within each decade. The actual values used are in the IEC 60063 lists of preferred numbers.

A resistor of 100-ohms±20% would be expected to have a value between 80 and 120 ohms; its E6 neighbors are 68 (54-82) and 150 (120-180) ohms. A sensible spacing. E6 is used for ±20% components; E12 for ±10%; E24 for ±5%; E48 for ±2%, E96 for ±1%; E192 for ±0.5% or better. Resistors are manufactured in values from a few milliohms to about a gigaohm in IEC60063 ranges appropriate for their tolerance.

5-band axial resistors

5-band identification is used for higher precision (lower tolerance) resistors (1%, 0.5%, 0.25%, 0.1%), to specify a third significant digit. The first three bands represent the significant digits, the fourth is the multiplier, and the fifth is the tolerance. Five-band resistors with a gold or silver 4th band are sometimes encountered, generally on older or specialized resistors. The 4th band is the tolerance and the 5th the temperature coefficient.

SMT resistors

Surface mounted resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance Surface Mount Technology (SMT) resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten (the number of zeroes). For example:

"334" = 33 × 10,000 ohms = 330 kilohms
"222" = 22 × 100 ohms = 2.2 kilohms
"473" = 47 × 1,000 ohms = 47 kilohms
"105" = 10 × 100,000 ohms = 1 megohm

Resistances less than 100 ohms are written: 100, 220, 470. The final zero represents ten to the power zero, which is 1. For example:

"100" = 10 × 1 ohm = 10 ohms
"220" = 22 × 1 ohm = 22 ohms

Sometimes these values are marked as "10" or "22" to prevent a mistake.

Resistances less than 10 ohms have 'R' to indicate the position of the decimal point (radix point). For example:

"4R7" = 4.7 ohms
"0R22" = 0.22 ohms
"0R01" = 0.01 ohms

Precision resistors are marked with a four-digit code, in which the first three digits are the significant figures and the fourth is the power of ten. For example:

"1001" = 100 × 10 ohms = 1 kilohm
"4992" = 499 × 100 ohms = 49.9 kilohm
"1000" = 100 × 1 ohm = 100 ohms

"000" and "0000" sometimes appear as values on surface-mount zero-ohm links, since these have (approximately) zero resistance.

Industrial type designation

Format: [two letters][resistance value (three digit)][tolerance code(numerical - one digit)]

Power Rating at 70 °C
Type No. Power
BB 1/8 RC05 RCR05
CB ¼ RC07 RCR07
EB ½ RC20 RCR20
GB 1 RC32 RCR32
HB 2 RC42 RCR42
GM 3 - -
HM 4 - -
Tolerance Code
Industrial type designation Tolerance MIL Designation
5 ±5% J
2 ±20% M
1 ±10% K
- ±2% G
- ±1% F
- ±0.5% D
- ±0.25% C
- ±0.1% B

The operational temperature range distinguishes commercial grade, industrial grade and military grade components.

  • Commercial grade: 0 °C to 70 °C
  • Industrial grade: −40 °C to 85 °C (sometimes −25 °C to 85 °C)
  • Military grade: −55 °C to 125 °C (sometimes -65 °C to 275 °C)
  • Standard Grade -5°C to 60°C

Electrical noise

In precision circuits it is often necessary to minimize electronic noise. As dissipative elements, even ideal resistors will naturally produce a fluctuating "noise" voltage across their terminals. This Johnson–Nyquist noise is a fundamental noise source which depends only upon the temperature and resistance of the resistor, and is predicted by the fluctuation–dissipation theorem. For example, the gain in a simple (non-) inverting amplifier is set using a voltage divider. Noise considerations dictate that the smallest practical resistance should be used, since the Johnson–Nyquist noise voltage scales with resistance, and any resistor noise in the voltage divider will be impressed upon the amplifier's output.

In addition to this intrinsic noise, practical resistors frequently exhibit other, "non-fundamental", sources of noise, usually called "excess noise." Thick-film and carbon composition resistors generate more noise than other types at low frequencies; wire-wound and thin-film resistors, though much more expensive, are often utilized for their better noise characteristics.

Precautions required in high-precision applications

Various effects become important in high-precision applications. Small voltage differentials may appear on the resistors due to thermoelectric effect if their ends are not kept at the same temperature. The voltages appear in the junctions of the resistor leads with the circuit board and with the resistor body. Common metal film resistors show such effect at magnitude of about 20 µV/°C. Some carbon composition resistors can go as high as 400 µV/°C, and specially constructed resistors can go as low as 0.05 µV/°C. In applications where thermoelectric effects may become important, care has to be taken (for example) to mount the resistors horizontally to avoid temperature gradients and to mind the air flow over the board.[8]

Failure modes

Like every part, resistors can fail in normal use. Thermal and mechanical stress, humidity, etc., can play a part. Carbon composition resistors and metal film resistors typically fail as open circuits. Carbon-film resistors may decrease or increase in resistance.[9] Carbon film and composition resistors can open if running close to their maximum dissipation. This is also possible but less likely with metal film and wirewound resistors. If not enclosed, wirewound resistors can corrode. Carbon composition resistors are prone to drifting over time and are easily damaged by excessive heat in soldering (the binder evaporates). Variable resistors become electrically noisy as they wear.

All resistors can be destroyed, usually by going open-circuit, if subjected to excessive current due to failure of other components or accident.

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