NOTE: I relied heavily upon R. G. Keen’s article, The Secret Life of Pots, to learn much of the material presented here.

Variable Resistors

Let’s say the day arrives when you need a 5K variable resistor in your AMZ Mosfet Booster for the gain control. But all you have is a 20K linear pot. “Hah!” you say to yourself. “No problem. I have a 7.5K resistor and if I put it parallel to my pot then I will have a 5.5K variable resistor.” And you will be correct. Your pot will vary the resistance from 0Ω to 5.5KΩ.

But something else quite wonderful will also happen. The feel of your pot will be different. Instead of the gain bunching up so much at the end of the rotation, the gain increase will be spread out a little. This is caused by the nonlinear way in which parallel resistors combine their resistance.

On resistors: in parallel there is an explanation of the resistance for two parallel resistors, valued R₁ and R₂:

In this case, one of the resistors is variable. Let’s say its the first one and that its resistance is x R_p, where x is the fraction of rotation. Then the resistance of the variable resistor in parallel with a fixed resistor is

which depends nonlinearly on x. To summarize the effect for various resistor and pot values, it is helpful to express the fraction of resistance as a function of the amount of rotation:

In this expression, the scale of the resistors does not matter. Only the ratio of the two resistor values deterimines the shape of the pattern. As R₂ gets smaller relative to R_p, the resistance increases faster at the beginning of the rotation and slower at the end.

So a fixed resistor parallel to a variable resistor evens out a gain control that has most of its increase at the end of its rotation. This is an example of nonlinear taper. Unfortunately, there is no way to flip the taper around so that the resistance increases slowly at first and then accelerates. You can reverse the terminals so that the resistance decreases slowly at first, but you cannot turn this concave path into a convex path.

Another popular approach to the gain bunching problem is to put a variable resistor in series with a fixed resistor. On resistors: in series, we explain that the resistance of two resistors in series is the sum of the individual restistances. Using a 2K variable resistor in series with a 3.3K resistor combines to make a variable resistor that goes from 3.3K to 5.3K. If that is a good approximation to where there is action in the gain control then focussing the variable resistance in that range will help make the gain pot seem more responsive.

You can also see a resistor in series with a pot in the way some builders wire their Fuzz Face gain control. Much of the gain for this pot, when it is linear, also appears late in the pot rotation.

Pots

We just described how the effective taper of linear variable resistors bows up when they are placed in parallel with a fixed resistor. The taper of pots can also be changed using fixed resistors, but it works differently because pots are voltage dividers. It becomes possible to obtain voltage (not resistance) patterns that approximate both audio/log and reverse audio/log pot behavior.

The first thought might be to put a resistor across lugs 1 and 3 of the pot. This merely creates a current divider because the resistor and the pot are in parallel. Voltage differences are unaffected (according to Kirchoff’s voltage law).

Instead, one places a fixed resistor across lugs 1 and 2 or lugs 2 and 3. First, we’ll look at putting a resistor across lugs 1 and 2 of a linear taper pot. In the Taper section of resistors: pots, we explain how the action of a voltage divider causes a linear pot to give

where V_OUT is the voltage at lug 2, V_IN is the supply voltage, and x is the fraction of pot rotation. The voltage changes proportionately with the throw of the pot. If we put a resistor R₂ across lugs 1 and 2, then the resistance across lugs 1 and 2, originally x R_p, is replaced by the resistance of x R_p and R₂ in parallel:

Putting this into the voltage divider formula with the (1 – x) R_p resistance between lugs 2 and 3 gives the voltage at lug 2 as

To summarize this relationship, we rewrite the fraction of voltage attenuation as a function of pot rotation and the ratio of the resistances and create the plot on the right

This is the sort of voltage attenuation that an audio pot delivers. At the start of the rotation, voltage increases quite slowly. But after the half way point, voltage accelerates until it is increasing quite quickly. When R₂/R_p = 1/16, the taper is closest to an audio taper, where the attenuation at the half way point is around 10%.

If we had placed R2 across lugs 2 and 3 instead, similar logic leads to the voltage attenuation expression

This taper is shown on the right.

You can take all of this one step further and put taper resistors across both pairs of lugs. Joe Davisson provides an on-line calculator for customizing your linear pot this way. Follow the “Linear Pot” and “Tapered Pot” links on his Electronics Math Helper page.

I am going to leave things there for now. I plan to add an explanation about using these mods under the conditions described by R.G. on his page, The Secret Life of Pots:

Unfortunately there’s a gotcha in there. It’s true that the voltage division ratio of this rig is arbitrarily close to that of a log taper pot. However, neither the load seen by whatever drives Vin or the source resistance as seen by the input of whatever is connected to Vout is close to what would exist for a real log pot of value R. In fact, the load on Vin varies from 1/(1+1/b)*R up to R. That means that if we’re trying to do a log taper with b = 1/4, the load on Vin will be as much as 0.2* R. This may be OK, but you have to keep it in mind.

Read his page to understand what this means.

Appearance

Pots are resistor voltage dividers in which the component resistances can be varied by rotating the shaft. The sum of the resistances is constant and equal to the value of the pot. Pots appear in such controls as volume (or level), tone, distortion/overdrive, compression, and for blending/mixing. Pots are also often wired to work as variable resistors.

The photo above is taken from the on-line catalog of Small Bear Electronics. This is the pot that I have learned to use most often. It is made by Taiwan Alpha. On the shaft are a washer and a nut for fastening the pot to an enclosure. On the left you may also notice a small tab sticking up from the case. This can prevent the pot twisting when the shaft is rotated. Lots of people break this off with a pair of pliers because they find it more trouble than it is worth. On the front you see 3 solder lugs. These lugs are for a panel mount pot. There are also pots with lugs that go directly into the holes on a PCB.

This is a 16mm pot, which means that the diameter of the case is roughly 16mm. I use this size more than any other. I will talk more about this below in the Ratings section.

There is another kind of pot, called a trim pot or a trimmer, that also appears often in stompbox builds. These are small potentiometers that are mounted on the circuit board and are only accessible inside the stompbox enclosure. They are typically used to set a fixed resistance that cannot be deteremined before the circuit is made. For example, some types of transistors have characteristics that vary so widely that resistors connected to them must be individually adjusted.

Somewhere on the case, on the bottom in this picture, the characteristics of the pot are stamped or printed. The printed resistance, 5KΩ in this case, refers to the fixed resistance across the outer solder lugs of the pot. The letter B refers to the taper (or path of resistance) as the shaft is turned. To explain this, I have to describe the conventional way to hookup a pot. And for that description, it is useful to have some symbols and a schematic.

Symbols

Just as there are two symbols for resistors, there are two symbols for pots. Both are resistors with an arrow pointing into the middle. In addition, you may see numbers or letters on the three terminals and another arrow pointing along the resistor. Sometimes this arrow is accompanied by the letters CW, or CCW, or something similar. Not everyone is clear in their schematics about their hookups for pots.

These schematic symbols follow what appears to be a fairly common, but certainly not universal, convention on Aron’s forum that coincides with familiar ways

1. to put a voltage divider into a schematic and
2. to picture the structure of a pot.

On the left is a schematic for the voltage divider discussed on the resistors: series page. On the right is a diagram for the inside of a pot looking down from above the shaft, the way the pot would be actually used.

The lugs marked 1 and 3 are connected to the ends of a resistive strip and a wiper connected to lug 2 rotates from lug 1 around to lug 3 as the shaft is turned clockwise (CW). When the pot is “turned down” (fully counter-clockwise or CCW), lug 2 is connected directly to lug 1 without any resistive strip between them. On the other hand, when the pot is “turned up” (fully clockwise), lug 2 is connected directly to lug 3 with the complete resistive strip between them.

So, looking at the schematic symbols, the voltage divider schematic, and the pot diagram, the convention is that

• lug 1 is the CCW lug or the bottom lead of R2,
• lug 2 is the wiper or Vr junction
• lug 3 is the CW lug or the top lead of R1, and
• the arrow indicates a clockwise rotation from lug 1 towards lug 3.

If you look at the image of the bottom of the pot above full size, you will see 1, 2, and 3 stamped into the bottom of the case just as described here. Another convention replaces 1 with A, 2 with W, and 3 with B.

If you hook up a volume pot the way the voltage divider is drawn in the schematic, with the voltage source connected to lug 3, ground connected to lug 1, and the output connected to lug 2, then the volume knob works as you would expect. With the knob fully CCW, there will be no output because the output is connected to ground. With the knob fully CW, you will get the maximum output and there will be a smooth transition between these extremes as the knob is turned. The taper of a pot determines how the volume changes as you turn the knob.

To see an example, look at the schematic for the Red Llama clone from the layouts page. On the far left you will see a pot labelled “VOLUME” hooked up with this configuration. If you look at the top of the schematic, you will also see a pot labelled “GAIN” that is hooked up differently. This pot is configured as a variable resistor.

Variable Resistor

Pots can also be wired as variable resistors. This is often the case for gain or overdrive pots. Also, a pot is sometimes used in place of a fixed resistor when the exact resistance needed is not known in advance. In such a case, a much smaller pot called a trimpot is often used. Common sizes are 1/4″ and 3/8″ square which fit neatly on a pcb where they are set once, or at least infrequently.

To use a trimpot as a variable resistor, simply short lugs 1 and 2 or lugs 2 and 3, and treat lugs 1 and 3 as the leads of the resistor. The resistive track between the wiper and the shorted lug is irrelevent because there is a wire with no resistance connecting the two lugs.

If you short lugs 1 and 2, then the resistance decreases as you rotate the shaft CW. Conversely, if you short lugs 2 and 3, then the resistance increases as you rotate the shaft CW. To see an example, look at the schematic for the BSIAB2 on the layouts page. At the top of the schematic, slightly to the right, is a 100K pot called “TRIM” serving as a trimpot. You will also see a volume pot labelled “LEVEL” with the volume hook up described above. That pot has the value designation 100K-A. The letter A denotes the suggested taper, which is the next topic.

Taper

So, given that we are going to hook up our pot with lug 1 to ground, lug 2 to output, and lug 3 to input, let’s talk about pot taper. Suppose that the resistive strip inside the pot is uniform in such a way that the resistance along the strip from the wiper to a lug is proportional to the length of the strip between them. If we call the resistance along the entire length of the strip R_p and if we say the wiper (lug 2) is x of the distance along the strip, then the resistance between lug 1 and lug 2 is

and the resistance between lug 2 and lug 3 is

Using these resistance values, the voltage divider forumula tells us the relationship between V_IN and V_OUT:

In words, the percentage attenuation equals the percentage of pot rotation. There is a one-for-one linear relationship. Such a pot has a linear taper and usually, but not always, this is designated with the letter B, as on the bottom of the pot case pictured above.

A linear taper is not ideal for volume control. It turns out that we hear a linear taper as increasing volume too quickly early in the pot rotation and very little in the last half of the rotation. So there is another taper, called audio or log, designed to work better for a volume application. The resistance increases slowly at first and then accelerates. This new pattern can be created by making the resistance strip actually taper over its length. The audio or log taper is usually (but not always) designated by the letter A.

Generally, trimpots have a linear taper. I suppose this is because a linear taper is cheaper to make than other tapers and trimpots are typically used for setting a resistance once, after a circuit has been built, for fine tuning.

If you are not sure what type of pot you have, then follow these helpful instructions from R. G. Keen.

Values

Pots do not come in as many values are fixed resistors because pots provide a range of resistances themselves. A glance at the value of the linear pots available at Small Bear Electronics probably tells the story at most sources: 1K, 2K, 5K, 10K, 25K, 50K, 100K, 250K, 500K, 1M, and 2M. Trimpots start at lower resistances because they are used as substitutes for fixed resistors: 100, 200, 500, 1K, ….

Ratings

Like resistors, pots are rated by power in watts. This is a value that should not be exceeded. Linear 24mm Taiwan Alpha pots have a 0.5W rating, while other tapers are 0.25W. Linear 16mm Taiwan Alpha pots have a 0.25W rating, while other tapers are 0.06W. General good practice is not to go below 0.25W, but lots of people are using 16mm audio taper pots without reporting problems. As I mentioned in resistors: limiting current, one should figure out the actual power requirements and then follow R. G. Keen’s advice to double the power requirements to get an approximate rating for your components.

Further Information

R. G. Keen has written an excellent introduction in The Secret Life of Pots. He even shows how to disassemble a Taiwan Alpha pot and gives pictures of the guts, including the wiper and the resistive strip. R.G. also has some interesting historical notes.

Another useful source is Potentiometers (Beginners’ Guide to Pots) by Rod Elliott of Elliott Sound Products (ESP).

And here’s an additional note about the audio taper described above. First, I have not found a formula on the web for the audio taper so I have used one based on a rule of thumb described many places on the web: to get half the perceived loudness use one-tenth the voltage. The function with this property is

where x is the fraction of pot rotation. This agrees loosely with another rule of thumb: the resistance across lugs 1 and 2 is approximately one-tenth the total resistance of an audio pot when the shaft is turned to the half-way point.

Second, audio pots do not actually have tapers with such smooth acceleration. Look at a data sheet for a Taiwan Alpha audio pot and you will see the figure on the right.

If you need a 3K resistor but only have 1K and 2K resistors, you can make an equivalent circuit by placing a 1K resistor in series with a 2K where the 3K resistor goes.

Kirchoff’s Voltage Law

This formula for resistors in series can be derived from Ohm’s law, Kirchoff’s current law, and Kirchoff’s voltage law, or KVL for short.

Kirchoff’s Voltage Law: The sum of the electrical potential differences around a circuit must be zero.

The total potential difference across the resistors in series is the voltage supply V_S. KVL says that the potential differences across the individual resistors, V₁ and V₂, sums to V_S:

To this relationship, we add that the currents through every point of the circuit are equal:

You can think of this as KCL in action: the current flowing through a resistor equals the current flowing through its leads. So the current flowing through R1 must equal the current flowing through R2.

Combining these equations with Ohm’s law, we can predict the current, the voltages, and the effective resistance of two resistors in series. Ohm’s law and KCL predict that

KVL adds that

Rewritten, this equation tells us that

will be the current through the circuit.

This last equation can be reinterpreted as telling us the combined resistance of the resistors in series. If a single resistor were in the place of the series and we measured a voltage supply equal to V_S and a current equal to I_S then we could compute the value of that resistor using Ohm’s law as

This is the resistance of two resistors in series predicted by Ohm’s law and KCL and KVL.

Resistor Order

Notice that the order of the resistors does not matter. If R2 precedes R1 then the resistance of the series is still the sum R₁ + R₂. Ohm’s law, Kirchoff’s current law, and Kirchoff’s voltage law together predict that the voltage across any series of resistors is invariant to the ordering of the resistors within the series.

This invariance extends to a series of voltage sources and resistors. The current through the series is the same for every component by KCL. The voltage across the two terminals of the series is the sum of the component voltages no matter what the order. And Ohm’s law does not depend on order either.

This invariance applies to the LED supply example in my discussion of Ohm’s law. The 2V drop across the LED can occur before or after the current limiting resistor with the DC analysis unchanged. So the LED may precede or follow the current limiting resistor; the current is reduced by the resistor either way.

Voltage Divider

Resistors in series have another important feature and this feature does depend on the order of the resistors. R1 and R2 form a voltage divider at their junction:

where we have substituted in the expression for I_S given above. In words, V₂ is a fraction of V_S determined by R₁ and R₂. If, for example, the resistances are equal then the voltage supply is “divided” in half.

Applying the voltage divider formula to the schematic at the top of this page, gives

LTspice produces this answer, as the figure shows. The message at the bottom appears when one places the mouse over the junction marked Vr after running a DC simulation. For more information on LTspice, see this introduction to LTspice. The divided voltage is often called the reference voltage and denoted by Vr on a schematic.

Voltage dividers appear frequently in stompbox circuits. On the left is a schematic for a basic fuzz face created in LTspice (see R.G. Keen’s article The Technology of the Fuzz Face). There are three voltage dividers in this schematic: the resistor pairs R3 and R4, R6 and R7, and R5 and R8.

Two of these pairs, R6 and R7 (out) and R5 and R8 (gain), are potentiometers (or pots) set at resistance mid-points. As components distinct from fixed resistors, pots are described on resistors: pots.

Acknowledgements

Thanks to idlechatterbox for finding a typo on this page!

Resistors can be combined in parallel and in series to create new resistances. Two resistors are connected in parallel in this LTspice figure. In this combination these two resistors act like a single resistor with a resistance that depends on the two individual resistances:

where R₁ is the value of R1 and R₂ is the value of R2. This is useful when you don’t have the value resistor that you need: you may be able to construct from resistors that you do have. For example, two 1K resistors in parallel provide the resistance

and a 1K and a 3K in parallel provide

Kirchoff’s Current Law

The formula for the resistance of parallel resistors is implied by another basic electronic relationship called Kirchoff’s Current Law, or KCL for short.

Kirchoff’s Current Law: The sum of currents flowing towards a point in an electrical circuit equals to the sum of currents flowing away from that point.

In our circuit with parallel resistors, KCL says that the currents flowing through the two resistors sum to the current flowing from the power supply:

where I_S is the power supply current, I₁ is the current through R1, and I₂ is the current through R2.

As the schematic above shows, the potential across the two parallel resistors equals the voltage of the power supply. So we can also write

where V_S is the power supply voltage, V₁ is the voltage across R1, and V₂ is the voltage across R2.

If we combine these relationships with Ohm’s law, then we can figure out the currents through each resistor and the effective resistance of the resistors in parallel from V_S and the resistors’ values. Ohm’s law applies to each resistor:

where R₁ is the resistance of resistor R1 and R₂ is the resistance of resistor R2. Substituting V_S for the voltage values, these equations imply that

and substituting these values into the current equation gives

This last equation can be reinterpreted as telling us the combined resistance of the parallel resistors. If a single resistor were in the place of the parallel resistors and we measured a voltage supply equal to V_S and a current equal to I_S then we could compute the value of that resistor using Ohm’s law as

This is the resistance of two parallel resistors predicted by Ohm’s law and KCL.

This part of a circuit is sometimes called a current divider. The parallel resistors effectively divide the current of the circuit I_S into V_S/R₁ and V_S/R₂.

Resistance appears in the simplest form of a fundamental electronic relationship called Ohm’s law:

where V is the potential difference or voltage drop measured in volts, I is the current in amperes (or amps), and R is the resistance in ohms. Ohm’s law says that voltage and current vary proportionally, where the constant of proportionality is the value of a resistor.

Here is a simple circuit schematic for applying Ohm’s law: The circle represents a constant voltage source with positive and negative terminals and the triangle at the bottom represents ground. The lines connecting these components represent wires or traces on a printed circuit board (PCB). This figure comes out of LTspice, a free program for simulating such circuits. We can build an actual circuit with a breadboard, a 9V battery, and 1K resistor.

Ohm’s law predicts that the current will be the ratio of voltage to resistance:

To see how to simulate this circuit in LTspice, see this introduction to LTspice. LTspice gives the same answer.

To see actual experimental results, I plan to write an introduction to using a multimeter.

Example: Inferring Current from Voltage

An immediate application of Ohm’s law is to measuring current somewhere in a circuit. To measure current through a circuit using a multimeter, you must break a connection and complete the circuit with the two probes of the multimeter. If you do not want to break a connection on a circuit board, you may still be able to measure the current indirectly using Ohm’s law. If you know a resistance and the voltage drop across that resistance, then you can calculate current as voltage divided by resistance.

With the voltage supply disconnected, use your multimeter to read the resistance of a resistor that is conducting the current you wish to measure. Alternatively, use the color bands on the resistor to determine its approximate resistance. With the voltage supply connected, use your multimeter to read the the voltage across the resistor. The ratio of the voltage reading in volts to the resistance in ohms is the approximate current in amperes.

This is one of the principles behind R.G.’s method for measuring the gain of transistors.

Example: Internal Resistance of a Power Supply

Taken at face value, Ohm’s law implies that a power supply that supplies a constant voltage will also supply an infinite current to a zero-resistance wire connecting the supply to ground: if V is constant, as R approaches zero I must approach infinity. But Ohm’s law and the constant voltage supply are just approximations. Actual power supplies have maximum current ratings that should, or must, be respected. If, for example, you hook up (or short) the positive and negative terminals of a real battery, the battery does not instantaneously deplete itself delivering an infinite current.

A better approximation to a real power supply is a constant voltage source in series with a resistor. The resistor represents the internal resistance of the power supply. A fresh 9V battery might supply 120mA so that its initial internal resistance would be

So replacing the 1K resistor in the schematic above with a 75 ohm resistor gives a better approximation to a 9V battery than a constant voltage source does.

Example: Resistor for an LED

We can also apply Ohm’s law to designing a guitar effects pedal with a light emitting diode (LED) to show when the guitar pedal effect is by-passed. The light will be on when the effect is in the signal path and off when bypassed. Here we focus on providing the LED with the appropriate power supply when the effect is swiched on.

I will describe diodes more generally on this site sometime in the near future. For this example, I will just say that our LED conducts current like a simple wire if there is at least 2V across its leads. The effect of the LED is that the voltage of the power supply appears to drop by 2V. So we can use Ohm’s law to predict the current in a simple circuit that powers an LED like (this one).

We need to predict the current through the LED because an LED burns out if it gets too much current. Common LEDs are designed to operate at 20mA and their current should not exceed 50mA. The initial current of a shorted 9V battery typically exceeds 100mA and many 9V DC power supplies are rated for several hundred mA. So if we simply hook up an LED to a 9V power source to an LED, the LED will probably just burn out.

However, by adding a resistor in series with the LED, we reproduce the simple Ohm’s law circuit above. The modified circuit is pictured in the schematic to the right, where an LED is followed by a resistor. We only have to adjust the voltage of the power supply. The presence of an LED does not affect the current but potential difference across the resistor drops by 2V from 9V. In this situation, Ohm’s law predicts how the level of the current varies with the level of the resistor:

The larger the resistance is, the smaller the current will be.

We can choose a value R for the resistor to limit the current to 20mA given V, which is 7V = 9V – 2V for a 9V power supply. For this reason, such a resistor is called a current limiting resistor. To get a 20mA = 0.020A current, Ohm’s law says

Because the standard ±5% resistor values in this neighborhood are 330 and 360, you would choose 360 to be on the safe side. Often, builders use much higher values, 4.7K for example, because battery drain is so much lower yet the LED is still bright enough.

Power

In my description of resistors, I reported that 1/4 watt resistors are recommended for building stompboxes. R.G. Keen gave a helpful explanation on Aron’s forum here. Power in watts equals voltage in ohms times current in amperes. Using Ohm’s law, formulas for power P are

The current limiting resistor for our LED is dissipating 7V at 20mA giving a power of 140mW. A 1/4W = 250mW can handle that, but not a smaller, 1/8W resistor. Admittedly, the current in other parts of the stompbox circuit are lower but one has to do such calculations to decide what power rating to use. And R.G. recommends that good practice is to use a rating roughly twice what you predict you will need.

The fixed resistors commonly used in DIY stompboxes look like small cylinders with leads (or wires) coming out of each end. Such components are called axial leaded. Resistors can be soldered onto a circuit board oriented either way; the leads are interchangeable. The leads are bent so that they can stick into holes on the circuit board. As a result, this type of resistor is also called through-hole. These resistors have a fixed value and the color bands on the cylindrical case of a resistor tell this value. The case itself identifies the type of resistor. Carbon film (labelled B and C) and metal film (labelled D) resistors are common types in stompbox builds. Older guitar effects were usually made with carbon composition resistors (labelled A).

Symbols

In circuit diagrams (or schematics), there are two symbols for resistors. Occasionally one sees a keyboard representation using a string of slashes. Resistors are generally labelled with the capital letter R followed by a number (for example, R3). Usually, this label is a rough guide to where the resistor is located on a schematic or layout: you can expect to find R3 somewhere near R4.

On circuit board layouts, a resistor appears as an outline of its case and leads of the necessary length ending at solder pads. When a resistor is stood on end so that the leads go through adjacent holes, one sees something like the first image.

Values

Resistor values are called resistance, which is measured in units called ohms denoted by the symbol Ω, a capital Greek omega. Stompbox circuits use resistances as small as a few ohms and and as large as several million ohms. To abbreviate values, a resistor rated at 2.2 million ohms is often described as a 2.2MΩ or, more commonly, 2.2M resistor. M stands for mega. A 3,300 ohm resistance is usually denoted 3.3K (where K stands for kilo). Occasionally one also sees 100Ω written as 100R, rather than just 100. The decimal place is often replaced with the order of magnitude letter, as in

Fixed resistors are labelled with standard values grouped by decades in multiples of 10: 1–10, 10–100, 100–1000, and so on. Within a decade the values roughly follow a power series, where the number of values within a decade depends on the precision of the values. For example, ±5% resistors come in (approximate) multiples of 10^1/24≅ 1.1, chosen because it is slightly less than 10^5%=10^1/20 so that the ±5% intervals cover the entire decade interval. Across decades, these 24 standard values are multiplied or divided by powers of 10. The decade series 10, 11, 12, 13, 15, …, 91 becomes 100, 110, 120, 130, 150, …, 910 in the next decade, and so on.

Ratings

Resistors are also rated by power in watts. This is a value that should not be exceeded. In stompbox construction, people generally use 1/4 watt resistors, but one can also use resistors with power ratings larger than 1/4 watts. The next highest rating is 1/2 watts. Note that the cases of resistors get larger as the power rating goes up. This means that 1/2 watt resistors will often not fit as neatly on a published circuit layout as 1/4 watt resistors. Other than this, you will probably notice no difference. For an example of calculating power requirements, see the Power section of resistors: limiting current.

Further Information

For further information on decades and standard values, search the internet for standard EIA resistor decade values.

To learn about reading the color coded value markings on resistors, search the internet for resistor color codes.

To learn more about types of resistors, search the internet for “types of resistor”.

For an expert view on carbon comp resistors, see Using the Carbon Comp Resistor for Magic Mojo by R. G. Keen.

Acknowledgements

Thanks to idlechatterbox for suggesting improvements to this page!