Build an Electronic Dummy Load
An electronic dummy load is one of the most useful instruments for testing power supplies, battery chargers, DC-DC converters, batteries and solar systems. Instead of connecting a real device, the dummy load safely draws a controlled amount of current, allowing engineers and technicians to evaluate the performance of a power source under different operating conditions. Unlike power resistors, an electronic load can maintain a constant current even when the supply voltage changes.
Project Overview
The electronic dummy load uses one or more power MOSFETs operating in their linear region as variable resistors.
An operational amplifier continuously monitors the voltage across a precision current-sense resistor and adjusts the MOSFET gate voltage to maintain the selected current.
This constant-current operation allows accurate testing of power supplies over a wide voltage range.
Project Difficulty
| Item | Value |
|---|---|
| Difficulty | ⭐⭐⭐⭐ Advanced |
| Build Time | 6–12 Hours |
| Supply Voltage Under Test | 1–60 V DC (Design Dependent) |
| Load Current | 0–20 A (Expandable) |
| Maximum Power | 100–300 W (With Cooling) |
Operating Modes
| Mode | Description |
|---|---|
| Constant Current (CC) | Draws a fixed current regardless of voltage. |
| Constant Resistance (CR) | Behaves like an adjustable resistor. |
| Constant Power (CP) | Automatically adjusts current to maintain constant power. |
| Battery Discharge | Discharges batteries while monitoring capacity. |
A simple DIY version generally operates in Constant Current mode, while microcontroller-based versions may support multiple operating modes.
Main Sections
| Section | Function |
|---|---|
| Input Terminals | Connect the power source under test. |
| Power MOSFET | Dissipates electrical power as heat. |
| Current Sense Resistor | Measures load current accurately. |
| Operational Amplifier | Controls the MOSFET gate voltage. |
| Cooling System | Removes heat from the MOSFETs. |
| Display (Optional) | Shows voltage, current and power. |
Typical Components
| Quantity | Component |
|---|---|
| 2–6 | Power MOSFETs (IRFP250, IRFP260, IRFP150, etc.) |
| 1 | Operational Amplifier (LM358, TL081 or Similar) |
| 1 | 0.1 Ω or 0.05 Ω Precision Shunt Resistor |
| 1 | Current Adjustment Potentiometer |
| 1 | Large Aluminium Heat Sink |
| 1 | 12 V Cooling Fan |
| 1 | Voltmeter/Ammeter Module (Optional) |
| 1 | Project Enclosure |
How the Circuit Works
The current flowing through the load develops a small voltage across the current-sense resistor.
The operational amplifier compares this voltage with the reference voltage set by the current adjustment control.
If the measured current is too low, the amplifier increases the MOSFET gate voltage. If the current is too high, it reduces the gate voltage.
This closed-loop feedback system keeps the load current constant even when the input voltage changes.
Applications
- Bench power supply testing.
- Battery capacity measurement.
- Solar panel evaluation.
- DC-DC converter testing.
- USB charger testing.
- LED driver evaluation.
- Laboratory experiments.
- Power electronics development.
Testing
- Inspect all wiring and MOSFET insulation.
- Verify the cooling fan operates correctly.
- Set the current adjustment to minimum.
- Connect a low-voltage power supply.
- Increase the load current gradually.
- Compare the measured current with a calibrated ammeter.
- Monitor MOSFET temperature during operation.
- Test at several voltages and current levels.
Troubleshooting
| Problem | Possible Cause |
|---|---|
| No load current | Faulty MOSFET, op-amp or wiring. |
| Current unstable | Incorrect compensation or poor grounding. |
| MOSFET overheating | Insufficient heat sink or fan failure. |
| Incorrect current reading | Shunt resistor tolerance or calibration error. |
| Fan not operating | Power supply or wiring fault. |
Project Improvements
- Add an OLED or TFT display.
- Control the load using an ESP32.
- Implement Constant Power and Constant Resistance modes.
- Add battery capacity (Ah and Wh) measurement.
- Automatically control the cooling fan speed.
- Provide USB communication with a PC.
- Store test results on a MicroSD card.
- Add over-temperature and reverse-polarity protection.
Skills Learned
- Power MOSFET operation in the linear region.
- Current sensing techniques.
- Operational amplifier feedback control.
- Thermal management.
- Power dissipation calculations.
- Electronic test equipment design.
Safety Notes
- Power MOSFETs can become extremely hot during operation.
- Always use an adequately sized heat sink and cooling fan.
- Observe the maximum voltage and current ratings of all components.
- Ensure good electrical insulation between MOSFETs and the heat sink if required.
- Never exceed the safe operating area (SOA) of the MOSFETs.
Key Points
- An electronic dummy load provides a controlled load for testing power sources.
- Constant-current mode is the most commonly used operating mode.
- Power MOSFETs dissipate the absorbed energy as heat.
- Proper cooling is essential for safe operation.
- An electronic load is an indispensable instrument for power electronics design, testing and repair.