Calculator inputs
Example data table
| Supply (V) | LED Vf (V) | Series LEDs | Target Current (mA) | Exact Resistor | Suggested Standard | Recommended Wattage |
|---|---|---|---|---|---|---|
| 5.0 | 2.0 | 1 | 20 | 150 Ω | 150 Ω | 0.125 W |
| 12.0 | 2.1 | 3 | 20 | 285 Ω | 300 Ω | 0.25 W |
| 24.0 | 3.2 | 5 | 30 | 266.7 Ω | 270 Ω | 0.5 W |
Formula used
1. Total LED forward voltage:
V_led_total = V_f × N_series
2. Voltage across the resistor:
V_r = V_supply - V_led_total
3. Exact resistor value:
R_exact = V_r / I_led
4. Actual current with the chosen resistor:
I_actual = V_r / R_selected
5. Resistor power dissipation:
P_resistor = I_actual² × R_selected
6. Total current for parallel strings:
I_total = I_actual × N_parallel
The tolerance range uses the lowest supply with highest LED drop for minimum current, and highest supply with lowest LED drop for maximum current.
How to use this calculator
- Enter your source voltage.
- Enter the forward voltage of one LED.
- Set the desired LED current per string.
- Enter how many LEDs sit in one series string.
- Enter how many identical strings run in parallel.
- Choose a preferred resistor series such as E12 or E24.
- Add tolerance and power margin values for safer design estimates.
- Press Calculate resistor to show the result above the form.
- Review the preferred resistor, current range, wattage, warnings, and graph.
- Export the result as CSV or PDF if needed.
Frequently asked questions
1. Why does the calculator suggest a higher standard resistor?
A slightly higher resistor reduces current and adds safety margin. That helps protect LEDs from supply variation, forward voltage spread, and resistor tolerance.
2. Can I use one resistor for several parallel LEDs?
It is better to use one resistor per LED string. Parallel LEDs can share current unevenly, causing brightness mismatch and possible overstress.
3. What happens if the supply voltage is barely higher than LED voltage?
Current becomes very sensitive to part variation. Small supply or temperature changes can create large current changes, so extra voltage headroom is usually safer.
4. Should I use this for high-power LEDs?
For high-power LEDs, a constant-current driver is usually better. It controls current more accurately and improves efficiency, thermal control, and lifetime.
5. Why is resistor wattage important?
A resistor that runs near its limit can overheat, drift, or fail. Choosing a larger wattage improves temperature performance and reliability.
6. What does forward voltage tolerance mean?
LED forward voltage varies by batch, temperature, and current. This tolerance helps estimate how much actual current may move away from the target.
7. Why does actual current differ from target current?
Standard resistor values rarely match the exact ideal value. Supply tolerance, LED variation, and using a safer higher resistor also change the final current.
8. Can this calculator improve efficiency?
Yes. It shows how much power is lost in the resistor. Lower resistor drop generally improves efficiency, but too little headroom reduces current stability.