What exactly is a resistor, and how do identify theses things?   A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. The resistor is a passive device and doesn't do anything actively to your circuit. Its actually a pretty boring device. If you add some voltage to it, nothing really happens. Well, maybe it gets warm, but thats it. BUT, by using resistors, you can design your circuit to have the currents and voltages that you want to use to control or limit active devices. So, by itself it may be boring, but in a circuit the active components performance is dictated by the resistor value. This why the resistor gives the designer control over his circuits!   Most through hole resistors (AKA axial or leaded) in the 2 watt or below have stripes all around denoting the value. There are no stripes warning you of the power level. In time you can estimate that by looking at the size of the resistor. If you look at antique resistors, they may be the size of a modern 10 watt resistor, so be cautious. However, the size for performance does not matter. What I mean is if you use a 100 watt resistor in place of a 1/2 watt it will do the same thing, and have no effect on your circuit. If you replace a 100 watt with a lower watt resistor of the same value, then you will burn out the resistor. So you can always go higher in watts. I recommend the 2-3 watt resistors for unknown power levels in antique devices. This goes for most of your circuits, but not always for power circuits, like power supply or amplifier circuits. These are examples of modern, vintage, and some antique 1/2 watt resistors: These are a few examples of 10 watt resistors: This is an example of an antique resistors (AKA Dog Bone resistors): Reading Resitor Codes:   Let's start with modern resistors that are marked with stripes. The color code is universal and applies to most electronic and electrical devices through the ages. As well as wire and capacitors. Take a look at the table below. This is a visual example, but not for all of them. Some resistors print the numbers on the body. the resistor shown near the bottom is a precision type, used mostly in the military and medical industries. Or used where 1% or better tolerance is required. These resistors add a third value color and is broken down to be used for more accurate resistance. If the third band was red for instance, then the value below would be 4.72K.   One more note about tolerance markings. Antique resistors did not mark for tolerance in most cases. I am sure there are exceptions, but I haven't noticed. Older or vintage resistors would leave off the forth band indicating a 20% tolarance. I do see these quite often when working on vintage electronics. I'm not sure you can purchase new resistors with a 20% tolerance. I have observed tighter tolerance resistors have become more main stay in the market and cheaper as time goes on.   We spoke earlier about trying to calculate the wattage of a resistor by size. The picture below was pulled from a google search, and does a great job lining these up. This isn't always correct because the older resistors were bigger than today's even though the power (wattage) is equal. The chart below shows a size comparison for reference. This is not to scale but is informative. There are too many variations to show them all.   Decoding Resistors: The 4 band resistors are the most common of all. You get the two value numbers, then a multiplier, and the tolerance band.   The 5 band are precision type, and usually offer tight tolerance with a more precise resistance value. Such as 4.99K rather than 5.0k. You get three value bands, then a multiplier, and a tolerance band. The six band type are exactly like the 5 band type except they add a temperature coefficient value band. This is used when temperature changes must be taken into account with the circuit. The temp changing will affect the actual value. If you need to be within 1% between 30 F and 90F then your going to need to order resistors that can compensate or stabilize during the change. These are rated as parts per million tested per military specs. They just can not test everyone for temperature coefficient value when producing Millions of them at a time. Resistor With Printed Information: By this time, you should realize the color code is pretty universal. Decoding may change from device to device, but colors always represents the same numbers.   In most cases power resistors have numbers printed on the body of the device. This is due to their size? I believe. In any case these may be coded as well.  There is also a letter or two involved before or after the numbers. This is a tolerance code, or in some cases a multiplier designation. Most are to (K) 10% or (J) 5% but certainly not limited to just these two. In some cases, they will just tell you. There are also cases where they don't tell you. The next three sections will point out one by one the difference. The last image shows us all the known tolerance letter designations.   This example is pretty straight forward, and is common. The first line shows a part number then a coded way to tell you this is a 5 watt resistor. The second line tells us this is a 200 ohm 10% tolerance resistor.   In this example there is no part numbers, and a K as well as a J in the line of charactors. These are read in a straight line, meaning one section at a time. Do not confuse the K with the J. If you break this down you would start with the 5W, then the 8K2, then the J. There is a set format, and it takes time to get used to it. Let's break this down: 5W: 5 watt resistor 8K2: is another way to tell you this is an 8.2K resistor. If there was an R, then it would be an 8.2 ohm. If there was an M, then you guessed it! Its an 8.2M resistor. J: This is the tolerance designation, and tells us its a 5% part. This is very much like the one above, however the information is varied slightly. This reads as. 10 watt, 10 ohm, 5% resistor. Like before when you see an R after the value, or in between then its value is in ohms. If you see a K then its in K ohms. If this was 10R5, then it would be a 10.5 ohm resistor. The tolerance is usually the last designation.   As you can see there is more than just two letter designations for tolerance. The most common are J and K, but any of these could be used. I have never seen P or Z on a resistor. However, there are many old capacitors with this kind of tolerance. The D is a special designation for capacitors. This makes sense since this is a universal color code that applies to other types of components, like capacitors. Resistor TYPES: There are many types of resistors from fixed to variable, adjustable, tapped, fixed tapped, rheostats. Then we have many materials they can be made of. Like metal film, carbon, wire wound, and others. We will cover some of the types here to keep it as simple as we can. For antique electronics wire wound were the most common. For vintage electronics carbon seem to be the most common. Most wire wound resistors tend to be high power types above 2 watts. I am sure prices, or cost was the determining factor. However, in some cases you want to use a certain material in your design. I think for this exercise we are not concerned with new technologies when it comes to resistors. We will stick with carbon and wire wound as far as materials go. AXIAL/RADIAL: I think we covered the general fixed axial resistors. This is simply a single device with one unchangeable (fixed) resistance, and two leads coming from each end. At this point we have not shown any radial type, but they were seldom used in antique electronics. Used in some high power circuits in vintage electronics, like amplifiers. All aspects concerning power, value, and tolerance is exactly the same. Just the orientation of the leads are different. The leads or connection points can come from anywhere but the sides. Meaning the top, ends, middle, bottom. A few examples shown below will help you determine this. Tapped Variable: I think we covered the general fixed axial resistors. This is simply a single device with one unchangeable (fixed) resistance, and two leads coming from each end. At this point we have not shown any radial type, but they were seldom used in antique electronics. Used in some high power circuits in vintage electronics, like amplifiers. All aspects concerning power, value, and tolerance is exactly the same. Just the orientation of the leads. The leads or connection points can come from anywhere but the sides. Meaning the top, ends, middle, bottom. A few examples shown below will help you determine this.   This resistor is usually a radial type to make it ergonomically correct. Meaning this makes adjusting them easier once in a circuit or mounted to a chassis. Mostly found in antique electronics for many reasons. You would use these to make adjustments after the device was built. In most cases its used to compensate for circuit tolerances, or drift as the device warms up. Also, can be used as a voltage divider to supply certain circuits with certain voltages. Keep in mind antique devices had very high tolerances with components than later devices. Plus, more drift due to extreme heat and electrical changes of the tubes. As electronic components and design progressed, less and less of these resistors were used.  The center tap, or adjustment lead is usually loosened with a screw, and has a bump out that touches exposed wires alone the resistor. Once loose the tap can be moved back and forth along the internal wires changing the resistance at the tap ONLY. The actual or fixed resistance does not change. These can come in any fixed resistance, and in many cases the adjustment is limited to a certain part of the resistor. Let's take a 10K version. From the two outside leads it will read 10K ideally. One end to the center tap may only have a range of 3k to 8K.   What does Ideal mean: Simply put, IN THE PERFECT WORLD It was designed and build to be a specific value.   Resistors like most of us drift over time. This is where wire wound performs the best. They tend not to drift unless heated beyond specs. This is where they are not at their best. Used properly they will be pretty stable after many years. Once use incorrectly the resistance can change dramatically. For manufacturing, a tolerance is put in place so they can be manufactured at less expense. Like everything else, cost is always a factor. So a device with 20% tolerance is generally cheaper to purchase than one with 10% or better. Today they make millions at a time, then test them before they are marked. The resistors that are within 1% are marked 1% and so-on. Yet the 1% will cost more even though they came from the same run. The point is, they don't make a 1% run. They make a run and sort it out afterwards. Below are a few examples for review. Multi-tapped Resistors: Are also fixed, but not adjustable like the fixed adjustable type. These can have multiple taps, not just one. The multiple tapped usually have more than one tap, although there are many with just one tap.   There are used to simplify the manufacturing build, or space, or cost. Again, cost is always the first concern when building a product. You can get one 20 watt resistor that needs to be tied to one place and do many things at the same time. They can also be used to make more space in a smaller chassis. The most common use in antique devices is power distribution. They tie one end to B+ then tap off many voltages from that source. Again, like the fixed tapped resistor, these are primarily found in power resistor category over 3 watts. I have found that when these go bad its usually one section. Just a tip: If this happens then just replace that one section with a standard resistor. As far as values, power, and marking goes? Don't expect much, because they are not always marked. In many cases I see a part number rather than specifications. If your repairing something then your probably using the schematic, or parts list. The next photo's shows us some examples.   Variable/potentiometer/rheostat Resistors:   This is very commonly used for volume and tone controls. They are called variable resistors, pots, even rheostats depending on how they are wired. These are actually called Potentiometers. They are used for many purposes in a circuit as well. Calibration, compensation, limiting, current limiter (rheostats), voltage dividers, and more!   This is a fixed resistor with a tap (wiper) that is moved across the entire resistor. So it has three leads, one connected to the wiper, and the other two connected to the resistor. These also have power ratings, and are marked very similar to the power resistors.   Let's discuss a rheostat first, just to get that out of the way. The real physical different between a rheostat and potentiometer is the way they are wired. A true rheostat would have two leads, or the wiper tied permanently to one of the resistor leads. Any potentiometer can be a rheostat just by connecting the wiper lead to one of the resistor leads. Below are the schematic diagrams of each to help clarify. Potentiometer Rheostat Tapped Potentiometers also have many physical types, such as slide, trimmer, dual to quad, and tapped. Although the general idea and functionality are the same.   A tapped potentiometer would have a forth lead from somewhere in the body. This has two fixed resistors in one, but the tap is referenced to the internal resistance, and not connected to the wiper. The tap can be anywhere within the resistor. It's usually near the center of the resistance value. So on a 10K ohm potentiometer, the tap is most likely in the 3 - 8K area.   A slider potentiometer is like any other in the fact that it does vary resistance. The only difference is the physical means. A slider moves back and forth, where others move in a circular fashion.   What we call Trim Pots can vary within themselves. However, they are usually small and have an opening for a small screwdriver to set the resistance. They are not made for continuous use. They are used in circuits to make a calibration or set point adjustment. These come with multiple turns as well. Most potentiometers are a single turn, but many trimmers can be 10 turns. This why they are great for set points and calibrations. 10 turns will give you a much finer and slower adjustment for accuracy.   Dual or Quad potentiometers are exactly like any other EXCEPT they have more than one variable resistor internally. These can come as slider or single turn. I have never seen a dual or quad multi-turn potentiometers, however it wouldn't surprise me. These can also be tapped. TAPPED SLIDER TRIMMER Single turn TRIMMER 10 turn DUAL QUAD How do we USE Resistors?   Let's start with ohms law, and let's start with a chart! I'm not going to cover ohms law for this article, but at least show you what it takes to figure out resistance or how to figure out the resistance you need in a circuit. There are a couple versions of the chart I just mentioned. The first is the simple version and the one we will focus on. The second is a reference for you. As you can see they are quite different, and similar. The Pie represents a multiple (horizontal lines) or a division (vertical line). So, P would be divided by either   This chart is what we will talk about and adhere to. Basically, this represents three formulas just as it is depicted. Voltage can be represented as "V" or "E". I have no idea why, but that's what it is. Then "I" represents current and AMPS, again I don't know why since we measure amps as "A". The "R" for resistance is the only one that makes sense. to simplify we will break it down the basic three formulas. E = I x R R = E / I or E over I I = E / r or E over R When one letter is over another you divide, when one is side by side you multiply.   This chart is very much like the first chart, but offers many ways to figure out the same parameters. This chart also adds "P" wich stands for power (watts). You will always need at least two electrical parameters, but you wont always have the same two. lets look at R (resistance). You can figure this out with (power and current) or (power and voltage) or (current and voltage). Use up the resistor we have:   Resistors can be added or divided, and certain values can be made by combining multiple resistors. You can also use resistors to compensate for power. If you have a 2 watt load you can use two 1 watt resistors to carry that load, but the value of each resistor would have to change. These circuits are called series or parallel resistance. You can also use them in a series-parallel design. Below are examples starting with series resistance, then parallel, then series-parallel. We will make the "RT" Resistance total, the same in all three using different resistors. Let pretend that's the only resistors you don't have are 10K, and your in a hurry to get this done. Series Circuit: They add up, RT = R1 + R2   The power rating for this configuration would only be as high as the lowest power value. So if R1 is a 1 watt and R2 is a 2 watt, then the total wattage only 1 watt. Parallel Circuit: They divide when RT = R1/# resistors.   RT = R1/2. This only works when all the resistors are  the same value. When they are not, we need to use the equation shown below. Power in this configuration can be shared. SO, if both resistors were 1 watt, then the total power rateing will be 2 watts. Series-Parallel Circuit: All the same rules and calculations apply. The trick is to separate the two types. Calculate the parallel resistance (R1 and R2) first, then add that resistance to the series resistor R3.   Power is unique in this configuration though. In this case the series resistor OR the parallel is going to dictate the max power value. However, you can mix up power values. R1 and R2 can be 1 watt, and if R3 is a 2 watt then you have a 2 watt circuit. BUT if R3 is a 1 watt, then you have a one watt circuit. This works in reverse too. Let's say R1 and R2 are 1/2 watt, and R3 is a 2 watt resistor. This means you still limited to a 1 watt circuit. Since we separate to calculate, we have to calculate all parameters. For power we always use the smallest value as our MAX power.   Sooooo, let's re-cap! This article was not intended to go over Ohms Law, but rather a perspective for information about resistors and their use. Safety is another concern so power and its role with resistors was added. Always be safe! You can always Google Ohms Law Calculators. They are easy and useful, and I use them all the time. I hope you found this informative and let's hope it has some influence with your testing, troubleshooting, and repairs.   Disclaimer: The information contained within this website www.wjoe.com, or any affiliate, is derived from theoretical information based on experience and knowledge obtained in experience. The reader "you", are ultimately responsible for any and all information used from this site. Any damage, or any consequences you experience from this information is solely your responsibility. This information is a free service, so please enjoy it! Any other use, retention, dissemination, editing, selling, or copying anything from this site, for any intent is strictly prohibited. Unless you obtain written permission by me at joe@wjoe.com. Any information on this web site is owned by myself and is intended to aid you in your research about Antique or Vintage capacitors. Please enjoy!!!!!! Written by WJOE Radio 08/10/96, LLC Edited 11/07/19