Basics
A resistor controls the relationship between voltage and current. In many ordinary circuits it follows Ohm's law closely:
I = V / R
R = V / I
I = 5 / 1000 = 0.005 A = 5 mA
Resistance units and prefixes
| Written value | Meaning |
|---|---|
| 1 Ω | One ohm |
| 1 kΩ | 1,000 ohms |
| 1 MΩ | 1,000,000 ohms |
| 4.7 kΩ | 4,700 ohms |
| 2.2 MΩ | 2,200,000 ohms |
On schematics and parts lists, the multiplier letter may replace the decimal point: 4K7 means 4.7 kΩ, and 2M2 means 2.2 MΩ. The letter R may be used for values below 1 kΩ, so 4R7 means 4.7 Ω.
Four-band color code
On a common four-band resistor, the first two bands are digits, the third is the multiplier, and the fourth is tolerance. Read from the end where the digit bands are grouped more closely; the tolerance band is often gold or silver and is usually separated slightly from the others.
| Color | Digit | Multiplier | Common tolerance use |
|---|---|---|---|
| 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 | ×10,000,000 | ±0.1% |
| Gray | 8 | ×100,000,000 | ±0.05% |
| White | 9 | ×1,000,000,000 | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
4, 7, ×100, ±5%
47 × 100 = 4,700 Ω = 4.7 kΩ, ±5%
Five-band precision resistors normally use three digit bands, followed by a multiplier and a tolerance band. Six-band codes may add a temperature coefficient. Color can be hard to judge under poor lighting, and body colors can be confusing, so verify uncertain parts with a meter.
Tolerance
Tolerance states how far the actual resistance may be from the marked value. A 1 kΩ resistor with ±5% tolerance may be between 950 Ω and 1,050 Ω when new under its specified reference conditions.
Power and heat
P = I2R
P = V2 / R
P = 102 / 1000 = 0.1 W
A 1/4 W resistor has enough nominal rating for this simple example, but good design still allows margin.
Series resistors
Series resistances add because the same current passes through each one:
Parallel resistors
Parallel resistors share the same voltage and provide multiple current paths:
For two resistors:
Voltage dividers
Two series resistors can produce a fraction of an input voltage:
A connected load changes the divider because it appears in parallel with the bottom resistor. Voltage dividers are therefore best for signal and reference levels, not for powering loads that draw significant or changing current.
Current-limiting example
A resistor is often placed in series with an LED. Subtract the LED's expected forward voltage from the supply voltage, then divide by the desired current:
R = (5 − 2) / 0.010 = 300 Ω
A nearby standard value such as 330 Ω would give a little less current. The LED datasheet and supply range should be checked before final selection.
Measuring resistance
- Remove circuit power.
- Discharge capacitors safely.
- Place the meter on an appropriate resistance range.
- Measure across the resistor.
- For an accurate value, lift one lead if other circuit paths affect the reading.