Calculator Inputs
Formula Used
The calculator uses the correct equation for the selected input set.
- Current and resistance: P = I² × R
- Voltage and resistance: P = V² ÷ R
- Current and voltage: P = V × I
- Average power: Pavg = Pworst × duty cycle
- Required wattage: Pavg × safety factor ÷ derating factor
- Estimated body temperature: ambient + power per shunt × thermal resistance
How to Use This Calculator
- Select the input method that matches your known values.
- Enter current, resistance, or voltage drop as required.
- Add duty cycle and pulse width for pulsed loads.
- Enter tolerance, derating, safety, and thermal values.
- Press Calculate to view wattage and temperature checks.
- Use CSV or PDF buttons to save the report.
Example Data Table
| Case | Input | Duty | Base Power | Suggested Rating |
|---|---|---|---|---|
| Battery monitor | 100 A, 0.5 mΩ | 100% | 5 W | 20 W with 2× safety and 0.5 derating |
| Motor pulse | 80 A, 0.75 mΩ | 40% | 4.8 W | 7.68 W average rating target |
| Current sense drop | 200 A, 50 mV | 100% | 10 W | 40 W with conservative margin |
Why Shunt Power Matters
A shunt resistor carries measured current directly. Its value is usually small. Even small resistance can create serious heat. Power rises with the square of current. That makes overloads important. A meter may read correctly, while the part runs too hot. This calculator helps compare electrical load with safe wattage.
Design Planning
Start with the real current range. Add peak current if the load surges. Enter resistance or measured voltage drop. The tool can also use voltage and current together. For pulsed loads, duty cycle reduces average heating. Pulse width also helps estimate energy per pulse. These checks support battery monitors, motor drivers, power supplies, and test benches.
Heat And Derating
Rated power is not always usable power. Many resistors need derating at high ambient temperature. Airflow, copper area, enclosure size, and mounting style also matter. Use a derating factor below one when cooling is poor. Use a safety factor above one when failure is costly. The recommended rating combines these choices.
Accuracy Notes
Shunt accuracy depends on tolerance and temperature coefficient. Lead resistance can add error. Kelvin connections help remove lead drop from the sense path. The calculator estimates tolerance error from the entered percentage. It also shows voltage drop, because excessive drop wastes energy and may disturb the circuit.
Practical Selection
Choose a resistor with enough continuous rating. Check surge energy separately. Review the data sheet for pulse limits. A part may survive average power yet fail from short pulses. Keep terminals tight. Place sense traces away from noisy switching nodes. After assembly, measure case temperature under the worst load. Adjust the design when temperature rise is too high.
Common Applications
Shunts are common in chargers, inverters, welders, and laboratory supplies. They are also used in energy meters and protection boards. Low side sensing is simple, but it can shift ground. High side sensing keeps the load grounded, but it needs a suitable amplifier. In both cases, power loss becomes heat inside the package. Plan copper area early. Leave clearance around hot parts. Compare several standard wattages. Select the next higher rating when results sit near a limit. This keeps measurement stable during long runs and repeated overload tests inside hot sealed cabinets safely.
FAQs
What is shunt resistor power?
It is the heat created when current flows through the shunt. It is normally calculated from current and resistance, voltage and resistance, or current and voltage drop.
Why does current affect power so strongly?
Power equals current squared times resistance. Doubling current makes power four times higher when resistance stays fixed. This is why overload checks matter.
What derating factor should I use?
Use the resistor data sheet first. A lower factor is safer in hot enclosures, weak airflow, or small copper areas. Many designs use 0.5 as a conservative start.
Does duty cycle reduce shunt heating?
Yes, duty cycle reduces average heating for pulsed loads. Short pulse limits must still be checked separately because peak energy can damage the resistor.
Why include resistance tolerance?
Tolerance changes actual resistance. With fixed current, higher resistance raises power. With fixed voltage, lower resistance raises power. The calculator estimates this worst case.
What is a safe voltage drop?
The best drop depends on the circuit. Higher drop improves sense signal, but wastes power. Low drop saves energy, but needs a cleaner amplifier.
Can several shunts share power?
Parallel shunts can share current and heat. They need matched resistance, equal layout, and good connections. Unequal sharing can overheat one part.
Is the temperature estimate exact?
No. It is a planning estimate based on thermal resistance. Real temperature depends on board copper, airflow, enclosure, mounting, and nearby heat sources.