Resistor Calculator
Calculate resistor values using color bands, determine resistance from voltage and current, calculate resistor power, and find equivalent resistance for series and parallel resistor combinations.
What Is a Resistor?
A resistor is a passive electronic component that limits or controls the flow of electrical current in a circuit.
Resistance is measured in ohms (ฮฉ).
Voltage โ โผ โโโโโโโโโโโโ โ Resistor โ โโโโโโโโโโโโ โ โผ Current
Resistors are used for current limiting, voltage division, biasing, pull-up and pull-down circuits, signal conditioning, feedback networks and many other applications.
Ohm's Law
The relationship between voltage, current and resistance is described by Ohm's law.
V = I ร R I = V รท R R = V รท I
Where:
- V = Voltage in volts (V)
- I = Current in amperes (A)
- R = Resistance in ohms (ฮฉ)
Calculate Resistance
Calculate Resistor Power
A resistor converts electrical energy into heat. The power dissipated by a resistor can be calculated using:
P = V ร I P = Iยฒ ร R P = Vยฒ รท R
Resistor Color Code
Through-hole resistors commonly use colored bands to indicate their resistance value and tolerance.
A typical four-band resistor uses:
Band 1 โ First digit Band 2 โ Second digit Band 3 โ Multiplier Band 4 โ Tolerance
Resistor Color Values
| Color | Digit | Multiplier |
|---|---|---|
| Black | 0 | ร1 |
| Brown | 1 | ร10 |
| Red | 2 | ร100 |
| Orange | 3 | ร1,000 |
| Yellow | 4 | ร10,000 |
| Green | 5 | ร100,000 |
| Blue | 6 | ร1,000,000 |
| Violet | 7 | ร10,000,000 |
| Grey | 8 | ร100,000,000 |
| White | 9 | ร1,000,000,000 |
Resistor Tolerance
The tolerance indicates how much the actual resistance may differ from the nominal marked value.
| Color | Tolerance |
|---|---|
| Brown | ยฑ1% |
| Red | ยฑ2% |
| Gold | ยฑ5% |
| Silver | ยฑ10% |
Four-Band Resistor Calculator
Example โ 4.7 kฮฉ Resistor
A 4.7 kฮฉ resistor can be represented by the first two digits 4 and 7, followed by a multiplier of 100.
47 ร 100 = 4700 ฮฉ 4700 ฮฉ = 4.7 kฮฉ
For example, a resistor with yellow, violet and red bands represents 4.7 kฮฉ before considering the tolerance band.
Resistors in Series
When resistors are connected in series, their resistances add together.
Rtotal = R1 + R2 + R3 + ... Example: 100 ฮฉ + 220 ฮฉ = 320 ฮฉ
Resistors in Parallel
For two resistors connected in parallel:
Rtotal = (R1 ร R2) รท (R1 + R2)
For multiple resistors:
1 โโโโ = 1/R1 + 1/R2 + 1/R3 + ... Rtotal
Voltage Divider
Two resistors can be used as a voltage divider.
R1
Vin โโโโโโโ/\/\/\โโโโโโโ
โ
โโโ Vout
โ
R2 โ
GND โโโโโโโ/\/\/\โโโโโโโ
The output voltage is:
Vout = Vin ร R2 / (R1 + R2)
LED Current-Limiting Resistor
A resistor is commonly placed in series with an LED to limit its current.
Supply โโ Resistor โโ LED โโ GND
The approximate resistor value can be calculated using:
R = (Vsupply - VLED) รท ILED
Resistor Power Rating
A resistor should have a suitable power rating for the circuit.
The calculated power should not normally be equal to the resistor's maximum rating. A suitable safety margin should be considered when selecting a resistor.
Common resistor power ratings include:
- 1/8 W
- 1/4 W
- 1/2 W
- 1 W
- 2 W
- 5 W
- 10 W
- Higher-power wirewound and other specialized resistors
Resistor Voltage Rating
A resistor also has a maximum working voltage. A resistor can have adequate power capability but still be unsuitable if its voltage rating is exceeded.
This is particularly important in high-voltage circuits.
Resistor Tolerance
Tolerance determines how close the actual resistance is expected to be to its nominal value.
For example, a 1 kฮฉ resistor with ยฑ5% tolerance can have an actual resistance within the specified tolerance range around its nominal value.
Common Resistor Values
Standard resistor values are organized into preferred-value series such as E6, E12, E24, E48, E96 and E192.
The appropriate series depends on the required tolerance and component availability.
Resistor Selection
When selecting a resistor, consider:
- Resistance value
- Tolerance
- Power rating
- Maximum working voltage
- Temperature coefficient
- Physical size
- Construction type
- Operating temperature
- Pulse capability
Resistor Types
- Carbon film resistors
- Metal film resistors
- Thick-film resistors
- Thin-film resistors
- Wirewound resistors
- Power resistors
- Cement resistors
- Shunt resistors
- Variable resistors
- Potentiometers
Resistor Troubleshooting
A resistor can fail in several ways.
- Open circuit
- Resistance value changes
- Physical burning
- Cracking
- Overheating
- Intermittent connection
When a resistor is visibly damaged, replacing it with another resistor of the same resistance alone may not solve the problem. The reason for the original failure should also be investigated.
Testing a Resistor
- Switch off and isolate the circuit where practical.
- Discharge capacitors where necessary.
- Set the multimeter to resistance mode.
- Measure across the resistor.
- Compare the measured value with the marked value.
In-circuit measurements can be affected by other components connected to the resistor. For an accurate measurement, one end of the resistor may need to be disconnected.
Resistor Faults
| Symptom | Possible Cause |
|---|---|
| Open resistor | Excessive power, surge or component failure |
| Resistor burned | Excessive current or power dissipation |
| Resistance too high | Damage, aging or overheating |
| Resistance unstable | Cracked body, poor connection or damage |
| Resistor repeatedly fails | Underlying circuit fault or incorrect power rating |
Key Points
- Resistance is measured in ohms.
- Ohm's law relates voltage, current and resistance.
- Resistors limit current and are used in many circuit functions.
- Power rating is an important resistor specification.
- Voltage rating can also be important, particularly in high-voltage circuits.
- Color bands can identify the resistance and tolerance of many through-hole resistors.
- Series resistances add together.
- Parallel resistances combine to produce a lower equivalent resistance.
- Voltage dividers use resistor ratios to produce a desired voltage.
- LEDs commonly require a current-limiting resistor.
- A resistor that repeatedly fails usually indicates an underlying circuit problem.