Precision Load Box
A programmable electronic load for characterizing batteries and power supplies under constant-current, constant-voltage, or constant-resistance conditions.
Overview
Instead of a bank of fixed resistors, the load uses a power MOSFET running in its linear region as a variable resistance, controlled by a feedback loop. A DAC sets the target (current, voltage, or resistance depending on mode), an op-amp compares that target against the sensed value, and continuously adjusts the MOSFET's gate to hold it there.
Because the MOSFET dissipates the full test power as heat rather than switching it, thermal design carries as much weight as the electronics — the heatsink and airflow are sized around the worst-case continuous power the load is meant to sink, not just the electrical spec.
Block Diagram
Key Components
- Power MOSFET(s) rated for continuous linear-mode operation (not just switching)
- Large heatsink with forced-air cooling
- Precision current-sense resistor
- Op-amp control loop plus DAC for the set point
- ADC for voltage/current readback and logging
- MCU with a mode selector for constant-current, constant-voltage, and constant-resistance
- Display for live readout of voltage, current, and power
Bill of Materials
| Ref | Part / Value | Rating | Package |
|---|---|---|---|
| Q1 | IRFP250N — power MOSFET, linear-mode rated | 200V, 30A | TO-247 |
| R1 | Current-sense shunt, 10mΩ | 5W | TO-220 style |
| U1 | INA240A — precision current-sense amplifier | 20V/V gain, 20V bus | SOIC-8 |
| U2 | MCP4725 — 12-bit DAC, I2C | 2.7–5.5V | SOT23-6 |
| U3 | ADS1115 — 16-bit ADC, I2C | 2.0–5.5V | MSOP-10 |
| U4 | Arduino Nano (ATmega328) — mode control, logging | 5V, 16MHz | Module |
| HS1 | Heatsink + fan | ~150W continuous dissipation | — |
Build Requirements
- Thermal design experience — in linear mode, the MOSFET dissipates essentially the full test power as heat
- Control-loop tuning to keep the feedback stable across the load's full range
- A calibrated reference meter to verify the load's own readings
- Comfort with both analog control loops and MCU firmware