Constant-Current Electronic Load

Every power supply and battery has a story it only tells under load: a regulator that looks perfect at no load may sag, ripple or overheat at 2 A. Students usually test with resistors, which are fixed, hot, and imprecise. An electronic load solves this by drawing a precisely controlled constant current, adjustable at a knob. This project builds one around an IRF540 MOSFET bolted to a finned heatsink with a 12 V cooling fan: an op-amp compares the voltage across a current-sense resistor with the setpoint and drives the MOSFET gate, holding the current steady regardless of the source voltage. An Arduino Nano reads the same sense voltage and shows set and actual current on an OLED. Thermal management — heatsink sizing, the fan, and a temperature cutout — is part of the design, not an afterthought.

1. The supply or battery under test connects to the input banana jacks; the IRF540 MOSFET sits in series as the controlled load element.
2. The adjustment knob sets a reference voltage that represents the desired current.
3. The op-amp compares this reference against the voltage across the current-sense resistor and drives the MOSFET gate to close the loop.
4. As the source voltage varies, the loop adjusts the gate so the drawn current stays at the set value.
5. The Arduino Nano samples the sense voltage and displays set vs actual current on the OLED.
6. If the heatsink temperature crosses the cutout threshold, the firmware pulls the gate low until it cools — the report documents the thermal design behind this.

Tech stack:

• IRF540 power MOSFET
• Op-amp current-regulation loop
• Arduino Nano (ATmega328P)
• OLED display
• Finned aluminium heatsink + 12 V fan
• Current-sense resistor
• Arduino IDE (C/C++ firmware)

• [Op-amp current regulation loop] An op-amp compares the sense-resistor voltage with the knob setpoint and drives the MOSFET gate, holding the drawn current steady as the source voltage changes — the classic constant-current topology.
• [IRF540 power MOSFET stage] The load element is a genuine power device in its linear region, bolted to a finned aluminium heatsink with thermal paste for reliable heat transfer.
• [OLED current readout] The Arduino Nano measures the sense voltage and displays both the set current and the actual drawn current, so regulation error is visible at a glance.
• [Adjustment knob] A front-panel potentiometer sets the target current smoothly from near zero up to the design target — no firmware change needed to test at a new load.
• [Cooling fan] A 12 V fan blows across the heatsink fins, extending the continuous power the unit can absorb — sized from the MOSFET's thermal ratings in the report.
• [Thermal cutout] A temperature sensor on the heatsink shuts the gate drive down if the MOSFET runs too hot, protecting the device from thermal runaway in the linear region.
• [Banana-jack input terminals] Standard 4 mm jacks accept the supply or battery under test with thick, low-resistance leads.

• Working constant-current electronic load prototype
• Arduino Nano firmware with OLED readout and thermal cutout
• Schematic and wiring documentation
• Thermal design notes (heatsink sizing, fan selection)
• Load-test procedure manual (how to test supplies and batteries)
• Project report PDF (MOSFET linear operation, feedback control, thermals)
• PPT presentation for final review
• Viva Q&A preparation document

• The 3 A / 30 W figures are design targets set by thermal limits — sustained dissipation depends on ambient temperature and airflow; no measured thermal curve is claimed.
• MOSFETs in the linear region are thermally fragile; the cutout protects the device but abusive overloads can still damage it.
• Current accuracy is bounded by the sense-resistor tolerance and op-amp offset — it is a lab instrument, not a calibrated source-measure unit.
• Constant-current mode only; constant-voltage and constant-power modes are not implemented.
• DC only — it cannot load AC sources.