We often field requests for resistors based on description, such as RES 0805 1K 5% 1/4W, RES TH 4K7 2W 10%, or RES 0402 1% 2.2K .1W. What do all these obscure-looking numbers mean, and is it possible to find these components without a degree in electronics?
Absolutely you can.
Let’s break down what each of these indicators is, and how you can use it to locate orderable parts.
RES is simply common shorthand for “Resistor” and indicates the type of part you’re looking for. You may also see “CAP” for capacitor, “IND” for Inductor, and so forth, but for now we’ll focus on resistors.
0805 is an example of a chip or SMT resistor size code. These size codes determine the physical size of the part. Size codes must match - you cannot replace an 0805 part with an 0603 part without redesigning your board. Common size codes include 0201, 0402, 0603, 0805, 1206, 1210, 2012, 2512, and so forth. These size codes will become second nature as you work with these components, but in general any four-digit number without any other sign or unit is a size code.
TH means the part is a through-hole resistor. These are also often called “axial” resistors, and they’re what many hobbyists, makers, and older techs or engineers think of as “standard” resistors. They’re inexpensive and easy to work with, but tend to be much larger than chip resistors and are not as useful for automated assembly.
1%, 5%, and 10%, and any other number with a “%” sign next to it, are all different values for tolerance. Tolerance is the range from which a resistor can vary from its listed resistance value (called its nominal resistance) and still be considered by the manufacturer to be within its specifications. For example, a 1k-ohm resistor (one kilo-ohm, or one thousand ohms) with a ±10% tolerance can vary as far as 10% away from 1000 ohms. This means a physical example of the part can be anywhere from 900 ohms to 1100 ohms and still be considered within spec. Note that tolerance values given without a sign, such as “10%”, are considered to be plus or minus their value. The more precise way to write this is ±10%. Some parts, especially very old ones, will have different values for the plus and minus sides of tolerance, such as +80/-20%. This means the part can vary twenty percent below its nominal value, but as high as eighty percent above. Smaller tolerance values are better - 1% is much tighter and thus a higher quality part than 5% or 10%. All else being equal, you can always use a lower tolerance part in place of an equivalent part with higher tolerance. Consult an engineer familiar with your project if you have to use a higher tolerance part.
0.1W, 1/4W, 2W, and any other number with a ‘W’ next to it is the resistor’s power or wattage rating. This is how much energy the resistor can safely dissipate in the form of heat, measured in watts. All resistors will always create and dissipate heat while in operation, this is an unfortunately unavoidable result of physics. Higher wattage is better - a 1W part can dissipate twice as much heat as a 0.5W part. All else being equal, you can always use a higher wattage value part in place of a part with a lower wattage value. Do not go lower unless you have confirmed with an engineer that your project can withstand a lower value, this can cause damage to your device.
1K, 4K7, and 2.2K are all examples of resistance value. This is the most important specification for a resistor and is the reason it gets put on a board. You’ll almost always see these values with one of the following letters in them: R, K, M, or G. These are essentially multipliers, with ‘K’ being a kilo-ohm part, ‘M’ being a mega-ohm part, and ‘G’ being a giga-ohm part. ‘R’ is a little different in that it indicates no multiplier - a 1R part is simply one ohm. When you see the letter in the middle of a number, such as 4K7, this means the letter is acting as a decimal point. 4K7, therefore, would be 4.7 kilo-ohms, while 4R7 would be 4.7 regular ohms. This should match. Some projects might be able to get away with a close-enough value, but if you don’t know if yours is one of them, match the value to the best of your ability.
Worth noting is that due to the manufacturing history of these products and how manufacturers of old dealt with the issue of manufacturing tolerances, resistors don’t break down into the same values one might expect. ‘Even’ values such as 1.5k, 2k, 5k, and the like are often quite rare, and tend to be very expensive military-spec parts. One common example is that 5k resistors are rare, expensive, and special-purpose, while 4.99k is considered a standard value available for general use. If you don’t see your target value in our system, check for close values near it. This can be especially true with tolerances involved - a 5% 4.99k part will be cheaper and more readily available than a 10% 5k part, and the 5% 4.99k is not able to exceed the tolerances already allowed for within the 10% 5k. It may take a bit of math, but it could lead you to a much easier and more affordable source of parts.
And that’s it! Armed with this knowledge, you should be able to hunt down basic resistors as needed within DigiKey’s website. As always, if you’re having trouble, feel free to contact DigiKey Applications Engineering for assistance.