Resistors
| Resistors | |
|---|---|
| What it does | Opposes the flow of current, turning electrical energy into heat |
| Unit | Ohm (Ω); kilohms (kΩ) and megohms (MΩ) |
| Schematic letter | R |
| Governing rule | Ohm's law: E = I × R |
| Ratings | |
| Power | Common leaded sizes 1/8, 1/4, 1/2, 1 and 2 W; larger for power types |
| Tolerance | Typically 5% or 1% |
| Marking | Color bands, or a printed number code on surface-mount parts |
| Series and parallel | Series values add; parallel: 1 / R = 1 / R1 + 1 / R2 + ... |
Resistors are components that oppose the flow of electric current. Unlike capacitors and inductors, which store energy and give it back, a resistor turns energy into heat, and an ideal one behaves the same way at every frequency. Resistors set the current through LEDs and transistors, divide voltages, drain the charge from power supply capacitors and, in a dummy load, absorb a transmitter's entire output. The relationship between voltage, current and resistance is Ohm's law.
The ohm and resistor types
Resistance is measured in ohms (symbol Ω), named after the German physicist Georg Simon Ohm. One ohm passes one ampere when one volt is across it. Larger values are written in kilohms (thousands) and megohms (millions); schematics often put the letter where the decimal point goes, so 4K7 means 4.7 kΩ and R47 means 0.47 ohms. Resistors are sold in standard series of values rather than every number. The widely used E12 series runs 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, repeating in every decade.
Common types include:
- Carbon composition, the traditional type, which handles brief surges well but drifts with age.
- Carbon film and metal film, the everyday resistors of modern kits; metal film is more precise and quieter.
- Wirewound, for high power, but with significant inductance.
- Thick-film power resistors, flat parts that bolt to a heat sink.
- Potentiometers and trimmers, variable resistors used for volume and RF gain controls and for adjustments inside equipment.
Color code
Leaded resistors are marked with colored bands. On the usual four-band resistor, the first two bands are digits, the third is the multiplier (the number of zeros) and the fourth is the tolerance. Five-band resistors use three digit bands before the multiplier.
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | × 1 | |
| Brown | 1 | × 10 | 1% |
| Red | 2 | × 100 | 2% |
| Orange | 3 | × 1,000 | |
| Yellow | 4 | × 10,000 | |
| Green | 5 | × 100,000 | 0.5% |
| Blue | 6 | × 1,000,000 | 0.25% |
| Violet | 7 | 0.1% | |
| Gray | 8 | ||
| White | 9 | ||
| Gold | × 0.1 | 5% | |
| Silver | × 0.01 | 10% |
Yellow, violet, red, gold is 4, 7 and two zeros: 4,700 ohms, plus or minus 5 percent. Brown, black, orange is 10,000 ohms. A traditional memory aid for the digit order is "Better Be Right Or Your Great Big Venture Goes West."
Tolerance is how far the actual value may stray from the marked one. A 1,000-ohm resistor with 5 percent tolerance may measure anywhere from 950 to 1,050 ohms.
Power rating
A resistor's power rating is the heat it can safely get rid of, found with P = I² × R or P = E² / R. A common practice is to choose a rating at least twice the expected dissipation, and more inside a warm enclosure.
For example, a red LED on a 13.8-volt supply drops about 2 volts. To run it at 10 milliamps, the series resistor needs (13.8 − 2) / 0.010 = 1,180 ohms, so the nearest standard value, 1.2 kΩ, is used. It dissipates about 11.8 × 0.010 = 0.12 watts, well within a 1/4-watt rating. A resistor run past its rating discolors, changes value and can burn.
Series and parallel
- In series, resistances add: 100 ohms plus 220 ohms is 320 ohms.
- In parallel, 1 / R = 1 / R1 + 1 / R2 + ... For two resistors this works out to (R1 × R2) / (R1 + R2), so 100 ohms and 150 ohms in parallel make 60 ohms. Equal resistors in parallel divide the value by their number: two 100-ohm resistors make 50 ohms.
Resistors in parallel also share the power, which is how a high-power resistor can be built from many small ones. Two resistors in series form a voltage divider: the output across the lower one is the input voltage times R2 / (R1 + R2).
Non-inductive resistors for RF
At radio frequencies, how a resistor is built matters. A wirewound resistor is a coil, and its inductance makes it look partly like an inductor, so it is unsuitable for dummy loads, RF attenuators or antenna terminations. Film resistors often have a spiral cut into the film to trim their value, which adds a little inductance; this rarely matters at low power on HF but becomes noticeable at VHF and UHF.
Dummy loads and RF terminations use non-inductive resistors: carbon composition in older designs and thick-film power resistors today. Wiring several identical resistors in parallel also helps, since their inductances are in parallel too. The terminating resistor at the far end of a Beverage antenna is another ham use where a non-inductive part matters.