Free · Physics models
Ideal LED Current-Limiting Resistance
Estimate a shared current-limiting resistor and its dissipation for an ideal LED arrangement.
- Formula & worked example
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Calculator inputs
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How to use this calculator
- Enter the known values in the units shown. Results update as you type.
- Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
- Use the worked example to check the method. Reset restores the starting fields.
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Formula and method
resistance = 400.00; powerResistor = 0.16000. This worked example uses the displayed units and declared assumptions.
resistance = (supplyVoltage - ledVoltage*(numberofLeds*ledsConnection + 1 - ledsConnection)) / (ledCurrent* (ledsConnection+ numberofLeds*(1-ledsConnection))) *1000; ledVoltage = color; powerResistor = ledCurrent ^2 *(ledsConnection+ numberofLeds*(1-ledsConnection))^2* resistance /1000000; powerSingleLed = ledVoltage * ledCurrent /1000; powerAllLeds = ledVoltage * ledCurrent * numberofLeds /1000
Worked example
Enter these known values and leave the other values blank.
- LEDs connected in...
- 1
- Number of connected LEDs
- 2
- Supply voltage
- 12 V
- Current through the LED
- 20 mA
- Voltage drop across LED
- 2 V
- Resistance
- 400 Ω
- In a single LED
- 0.04 W
- In all LEDs
- 0.08 W
- In the resistor
- 0.16 W
Assumptions and limitations
- Forward-voltage presets are nominal color estimates, not device specifications. Current is per LED in milliamperes. The parallel calculation assumes identical current sharing, which real LEDs may not achieve with one resistor. Use measured forward voltage and device data for a real circuit.
- Clear an unknown to solve a supported inverse; keep known values fixed with locks. Singular or underdetermined inverses may remain incomplete.
Common questions
What assumptions apply?
Forward-voltage presets are nominal color estimates, not device specifications. Current is per LED in milliamperes. The parallel calculation assumes identical current sharing, which real LEDs may not achieve with one resistor. Use measured forward voltage and device data for a real circuit.
Which unknowns can I solve?
Editable controls preserve the supported captured inverse branches. Clear the desired unknown and provide enough independent known quantities; singular or underdetermined inverses may remain incomplete.
References
- Independent physical model reference
- OpenStax physical model reference
- Calculation definition and unit reference
Resistor dissipation is P=I²R, so the physical current inverse must use the positive root. Captured current inverse had the negative denominator and violated its own positive-current domain. Remove singular/nonphysical arrangement and count branches from this dissipation equation: series dissipation contains no count information; count still has inverses through resistor voltage and total LED power. Arrangement remains an explicit selector. LED count must be a whole number at most 100000.