Resistors 101
meh 10/2002
A resistor is an electronic component whose purpose is to "resist" the
flow of electrical current. All resistances are measured in ohms (symbol:
Ω), which is the units for volts divided by amps. Ohm's law relates
these three quantities:
- Volts = Amps × Ohms or
E = I × R
Large values of resistors use the scalars "Kilo" (k) and "Mega" (M) to represent "times 1,000" and "times 1,000,000" respectively.
Resistors dissipate power by doing their "resisting" job, and therefore all resistors also have a power rating - this rating can only be judged by size on smaller resistors, and common ratings include 1/8, 1/4, 1/2, 1, and 2 watts. A resistor which dissipates too much power will burn up, producing that hopefully not-too-familiar "burnt electronics" smell.
In tube audio amplifiers, resistors have several common uses:
- Plate resistor (usually a load)
- Cathode resistor (usually for biasing, sometimes a load)
- Grid resistor (to limit ultra HF response)
- Screen resistor (to limit screen current)
- Bias resistor (to keep the grid at a constant DC level, often zero or ground)
- Voltage divider (to cut down the signal level, particularly in high gain amps)
- Part of an RC low-pass filter (to both lower the voltage and reduce the ripple in the "downstream" parts of a power supply)
This page contains an introduction to the standard resistor color codes.
These color codes tell you how many ohms the resistor you're looking at is
rated. There are slightly different codes for resistors, but they
all are just a variation on a theme. First, the digits 0 to 9 are given
colors:
| Black | 0 | Green | 5 | |
| Brown | 1 | Blue | 6 | |
| Red | 2 | Violet | 7 | |
| Orange | 3 | Gray | 8 | |
| Yellow | 4 | White | 9 |
One old menomnic to help you remember this sequence is an old (and slightly PC-ified) one: "Bad Boys Rate Our Young Girls But Violet Grins Willingly."
These digit codes may represent significant digits, as in the 2 and 0 in "2.0", or a power of ten multiplier. For example, the simplest and most common code has three bands. "brown red orange" translates to 1 2 3, which can be interpreted as "12 × 103 or 12000 (12K) ohms.
An easy way to remember how to figure the multiplier is "it says how many zeroes to add on to the end."
Ah, if were always so easy! The colors gold and silver may also be used to
indicate a power of 10 or a tolerance (how far away from the intended
resistance the resistor is allowed to be.)
| Color | as Tolerance | as power of 10 |
|---|---|---|
 Gold |
5% | × 10-1 (.1) |
 Silver |
10% | × 10-2 (.01) |
| (None) | +/- 20% | -- |
For example, "brown black silver gold" would mean 10 x 10-2 with a tolerance of 5%, or .1 ohms (+/- 5%, which means it could actually be .095 or .105 ohms).
In general, 20% tolerance resistors have three color bands, 10% and 5%
tolerance resistors have four color bands (the last being silver or gold),
and 2% and 1% resistors also have four color bands (three significant
digits and a multiplier) but the last is normally not gold or silver.
This is summarized below:
| Tolerance | Bands | Significant Digits |
|---|---|---|
| 20% | 3 | 2 |
| 10% and 5% | 4 | 2 |
Some Mil-spec metal films actually have the digits printed right on the resistor (tolerance is assumed to be 1%), for example "1002" would be 100 × 102 or 10000 Ω (10KΩ).
Note that I have (rarely) seen 5% resistors that have five bands, the fifth being some sort of failure rate spec or temperature tolerance. The tipoff is the fifth band being something wierd, like yellow. These are extremely unusual, and hopefully you'll never see one.
You'll sometimes see some questionable colors (um, is that brown or orange?) and sometimes it will be difficult to tell which end to start from. Even when you know these codes, your best friend is always an ohmmeter to confirm the resistance you're looking at!
The next page has some examples in picture form.