Calculator Inputs
Example Data Table
| Example | Inputs | Ideal Resistor | Power | Recommended Choice |
|---|---|---|---|---|
| 12 V source to 5 V load at 20 mA | Vs = 12 V, Vl = 5 V, I = 20 mA | 350 Ω | 0.14 W | 360 Ω, 0.5 W |
| 24 V resistor drop at 50 mA | Vr = 24 V, I = 50 mA | 480 Ω | 1.2 W | 470 Ω, 3 W |
| 3.3 V drop with 0.5 W target | Vr = 3.3 V, P = 0.5 W | 21.78 Ω | 0.5 W | 22 Ω, 1 W |
Formula Used
Ohm's Law:
R = V ÷ I
Power from voltage and current:
P = V × I
Power from voltage and resistance:
P = V² ÷ R
Resistance from voltage and power:
R = V² ÷ P
Resistance from current and power:
R = P ÷ I²
Minimum resistor wattage:
Rated Wattage ≥ Dissipated Power × Safety Factor
For the supply-to-load mode, the resistor voltage is calculated first: Vresistor = Vsupply − Vload. The calculator then sizes the resistor and checks heating and practical standard values.
How to Use This Calculator
- Choose the calculation mode that matches the values you already know.
- Enter voltage, current, or power values in the appropriate fields.
- Set tolerance, safety factor, duty cycle, and preferred resistor series.
- Press Calculate Load Resistor to show the results above the form.
- Review the recommended standard resistor and wattage before building the circuit.
- Use the chart and exports for documentation, comparison, or sharing with others.
FAQs
1. What does a load resistor do?
A load resistor limits current, drops voltage, or provides a controlled electrical load. Designers use it to protect components, simulate demand, or dissipate power safely in test and operating circuits.
2. Why is resistor wattage important?
Wattage tells you how much heat the resistor can safely handle. If the actual dissipation exceeds the rating, the resistor can overheat, drift in value, fail early, or damage nearby parts.
3. Why does the calculator suggest a standard resistor value?
Real resistors come in preferred values such as E12, E24, and E96. The nearest standard value helps you select a part you can actually buy instead of an ideal theoretical value.
4. Should I always use a safety factor?
Yes. A safety factor gives thermal margin for real environments, airflow limits, and tolerance drift. Many practical designs use 1.5× to 3× the calculated dissipation for more reliable operation.
5. What is the difference between ideal and actual current?
Ideal current comes from the exact calculated resistance. Actual current reflects the nearest standard resistor value. Because standard values are rounded, the delivered current usually shifts a little.
6. When should I use the supply and load voltage mode?
Use that mode when a resistor sits in series between a source and a load. The calculator finds the voltage dropped by the resistor, then computes the needed resistance and heat.
7. Can I use this calculator for LEDs or test loads?
Yes. It works well for LED series resistors, simple current-limiting tasks, and dummy-load estimates, as long as the underlying assumptions match your circuit and the load current is known.
8. Does tolerance change circuit behavior?
Yes. Tolerance means the real resistor may be above or below its nominal value. That changes current, voltage drop, and power slightly, so tighter tolerance can improve predictability.