A DC electronic load is a bench instrument that draws a precisely controlled current from a DC power source under test — batteries, power supplies, chargers, and solar panels — in place of the real load the device will eventually see. Instead of building a bank of resistors to test a power supply, an electronic load lets you dial in exactly the current, voltage, resistance, or power profile you want, and change it in software or at the push of a button. This guide covers how electronic loads work and what to look for when choosing one.
How an Electronic Load Works
Inside an electronic load, a bank of power MOSFETs is driven in its linear region to dissipate the electrical energy delivered by the source under test — essentially acting as a controllable resistor, but one that can hold a fixed current or fixed power regardless of the source voltage changing. A feedback control loop continuously adjusts the MOSFETs to maintain whichever operating mode is selected, while an internal heatsink and fan dissipate the resulting heat.
The Four Operating Modes
| Mode | Behaviour | Typical use |
|---|---|---|
| CC — Constant Current | Draws a fixed current regardless of source voltage | Battery discharge testing, power supply current-limit checks |
| CV — Constant Voltage | Sinks whatever current is needed to hold the source at a set voltage | Testing a source's regulation and current capability at a target voltage |
| CR — Constant Resistance | Behaves as a fixed resistive load — current tracks source voltage linearly | Simulating a resistive load like a heater or incandescent lamp bank |
| CP — Constant Power | Draws whatever current is needed to hold a fixed power dissipation | Simulating loads like motors or switch-mode converters that draw constant wattage |
Battery Capacity and Discharge Testing
The most common application for an electronic load on the bench is discharging a battery at a fixed current (CC mode) while logging voltage over time. Multiplying the discharge current by the elapsed time to the cut-off voltage gives the battery's actual capacity in amp-hours — the only reliable way to verify a battery's real capacity against its rated figure, and the standard method for checking cell health in a battery bank before it fails in service.
Why CC mode is used for capacity testing, not CR
Power Supply and Charger Testing
Electronic loads are used to verify a power supply's load regulation (does output voltage stay stable as load current changes?), transient response (how quickly does it recover after a sudden load step?), and current-limit or overload protection behaviour — all difficult or impossible to test accurately with fixed resistors, since a resistor bank can't step instantly between load levels or hold a fixed power draw as voltage changes.
Key Specifications When Choosing an Electronic Load
- Maximum voltage, current, and power: All three ratings matter independently — a load rated 150 V / 30 A / 150 W cannot actually do 30 A at 150 V simultaneously (that would be 4500 W). Check the full operating envelope, not just the headline numbers.
- Minimum operating voltage: Important for low-voltage battery testing (single-cell Li-ion, NiMH) — some loads can't sink meaningful current below 1–2 V.
- Dynamic/transient mode: The ability to switch rapidly between two current levels, used to test a power supply's transient response.
- Remote sense: Compensates for voltage drop in the test leads so the load regulates based on the actual voltage at the source's terminals, not at the load's own input.
CIE supplies Vartech DC electronic loads for battery, power supply, and charger testing. Contact us to match a load's voltage/current envelope to your test requirements.