Enter LED Circuit Details
Example Data Table
| Supply |
LED Type |
Forward Voltage |
Series LEDs |
Current |
LED Drop |
Approx Resistor |
| 5 V |
Red |
2.0 V |
1 |
20 mA |
2.0 V |
150 Ω |
| 9 V |
Blue |
3.2 V |
2 |
20 mA |
6.4 V |
130 Ω |
| 12 V |
White |
3.1 V |
3 |
20 mA |
9.3 V |
135 Ω |
| 24 V |
Green |
2.2 V |
8 |
15 mA |
17.6 V |
427 Ω |
Formula Used
Temperature adjusted LED voltage: Vf adjusted = Vf + (Operating temperature - Reference temperature) × Temperature coefficient.
Total LED voltage drop: LED drop = Adjusted Vf × LEDs in series.
Available resistor voltage: Vr = Supply voltage - LED drop - Wire drop.
Recommended resistor: R = Vr / LED current.
Actual current: I = Vr / Used resistor.
Resistor power: P resistor = Vr × I.
Total LED power: P LED = LED drop × Current × Parallel strings.
Efficiency: Efficiency = LED power / Supply power × 100.
How to Use This Calculator
Enter the supply voltage first. Add the LED forward voltage from the datasheet. Enter the number of LEDs placed in series. Add the number of parallel strings if the circuit has more than one branch.
Enter the target current in milliamps. Leave the known resistor field empty if you want a suggested resistor. Add wire resistance, temperature, tolerance, and duty cycle for deeper checks. Press the calculate button to view results above the form.
Use the CSV button for spreadsheet records. Use the PDF button for a simple printable report.
Understanding LED Voltage Drop
An LED does not behave like a simple lamp. It has a forward voltage. This is the voltage needed before useful light appears. Red LEDs often need less voltage. Blue and white LEDs often need more. The value also changes with current, heat, and manufacturing tolerance.
Why Voltage Drop Matters
Voltage drop decides how much voltage remains for the current limiting resistor or driver. If the supply is too low, the LED string may look dim or fail to turn on. If the remaining voltage is too high, the resistor wastes more power and creates heat. Good planning keeps current stable.
Series and Parallel Strings
LEDs in series share the same current. Their voltage drops add together. Three LEDs rated at 2 volts each need about 6 volts before resistor losses. Parallel strings increase total current. Each parallel string should normally have its own resistor. This improves balance and reduces stress.
Temperature Effects
LED forward voltage usually falls as temperature rises. This calculator includes a temperature coefficient. A negative coefficient means the LED needs slightly less voltage when hot. That can raise current if the resistor or driver is not selected carefully. Heat should always be considered.
Resistor and Power Checks
The resistor absorbs the unused voltage. Its value is found by dividing resistor voltage by target current. Its power is current multiplied by resistor voltage. A safety margin is helpful. Many designers choose a resistor wattage rating above the calculated dissipation.
Using the Results
Use the calculated LED drop to check supply headroom. Use resistor voltage to judge current control. Use power values to select components safely. Review efficiency when battery life matters. The calculator is a guide, not a substitute for datasheets, testing, and proper thermal design.
Practical Design Tips
Always check the LED datasheet at the intended current. Use typical values for estimates. Use maximum values for worst case checks. Leave enough voltage for the resistor or driver. Avoid placing unmatched LEDs directly in parallel. Measure the real circuit after assembly. Small changes in parts can cause visible brightness differences.
For critical products, test several samples across temperature, supply range, and expected aging before approving the final design for production use and service.
FAQs
What is LED voltage drop?
LED voltage drop is the forward voltage across the LED when it conducts. It depends on LED color, current, temperature, and part tolerance.
Why does an LED need a resistor?
A resistor limits current. Without current control, a small voltage change can cause high current, overheating, and LED failure.
Can I place LEDs directly in parallel?
It is not recommended. Small voltage differences can make one LED take more current. Use separate resistors for better current sharing.
What happens if supply voltage is too low?
The LED string may not reach its forward voltage. It may stay off, appear dim, or behave inconsistently as conditions change.
Why does temperature affect LED voltage?
LED forward voltage usually decreases as temperature rises. This can increase current if the circuit uses only a fixed resistor.
How do I choose resistor wattage?
Calculate resistor power, then choose a rating above that value. A safety margin helps reduce heat and improve reliability.
What is duty cycle used for?
Duty cycle estimates average power in pulsed circuits. A lower duty cycle reduces average power, but peak current still matters.
Should I use datasheet values?
Yes. Datasheets provide typical and maximum forward voltage values. Use them for safer estimates and better worst case checking.