Electronic Loads and Load Resistors
A power supply that reads a perfect 12.0 V with nothing connected can still collapse to 9 V the moment you ask it for 3 A. A battery that shows a healthy voltage can still be nearly worn out. The only way to find out what a supply or battery really delivers is to make it work — to connect a known load and measure what happens. A load resistor or an electronic load does exactly that, safely and repeatably, on the bench.
Why Test Under Load?
Every real power source has some internal resistance. With no current flowing, that resistance drops no voltage and the source shows its full open-circuit voltage. As soon as current flows, the internal resistance drops a voltage (V = I × R) and the terminal voltage falls. A good regulated supply hides this by adjusting its output; a weak battery, a failing capacitor or a cheap adapter cannot.
Testing under load answers the questions that matter:
- Does the supply hold its voltage at its rated current, or does it sag?
- Does the current limit or overload protection actually work?
- How much ripple and noise appear on the output when it is working hard?
- How many amp-hours can this battery really deliver before it reaches its cut-off voltage?
Load Resistors
The simplest load is a power resistor. Ohm's Law gives the value and the power formula gives the rating:
R = 5 ÷ 2 = 2.5 Ω P = 5 × 2 = 10 W
Use a 2.5 Ω resistor (or two 5 Ω resistors in parallel) rated for at least 20 W, so it runs well within its rating.
Suitable resistors include aluminium-housed wirewound resistors (designed to bolt onto a heatsink), large ceramic-cased wirewound resistors, and banks of smaller resistors in series and parallel. Two things to remember:
- The current depends on the voltage. A resistor draws less current if the supply sags, which can hide exactly the weakness you are looking for. That is fine for a quick check, but an electronic load gives a truer test.
- Resistance changes with temperature. A hot resistor may drift a few percent. Measure the actual current with a meter rather than relying on the nominal value.
Car headlamp bulbs make a handy rough load for 12 V supplies: a 55 W bulb draws about 4.5 A. Their resistance is much lower when cold, so they draw a large surge at switch-on — a useful test of a supply's start-up behaviour.
Electronic Loads
An electronic load replaces the fixed resistor with a power transistor (usually a MOSFET) controlled by a feedback circuit. The circuit measures the current through a small shunt resistor and adjusts the MOSFET so that the current stays at whatever value you set, regardless of the supply voltage.
Most electronic loads offer several modes:
| Mode | What it holds constant | Typical use |
|---|---|---|
| Constant current (CC) | Current, e.g. exactly 2.00 A | Supply regulation tests, battery capacity tests |
| Constant resistance (CR) | Load resistance | Imitating a resistive load such as a heater or lamp |
| Constant power (CP) | Power drawn | Imitating a switching converter or a device that draws fixed power |
| Constant voltage (CV) | Terminal voltage | Testing chargers and solar panels |
Inexpensive USB electronic loads (typically up to 3–4 A and 25–35 W) are ideal for testing chargers and power banks. Bench electronic loads handle 150 W or more with fan cooling and can log voltage, current and capacity to a computer. A simple constant-current load can also be built from an op-amp, a logic-level MOSFET, a 0.1 Ω shunt and a large heatsink — a classic workbench project.
Testing a Power Supply
A basic supply test takes only a few minutes:
- No-load voltage: Measure the output with nothing connected.
- Load regulation: Apply 25%, 50%, 75% and 100% of the rated current, measuring the output voltage at the supply terminals each time. A good regulated supply changes by well under 1%; an unregulated adapter may drop 20% or more.
- Ripple: At full load, look at the output on an oscilloscope (AC-coupled) or measure it with the meter's AC voltage range. Ripple that grows sharply with load often points to a tired filter capacitor.
- Current limit: Increase the load past the rating. The supply should limit the current or shut down cleanly, not overheat or let the voltage oscillate.
- Soak test: Run at full rated load for an hour and check that the supply's case and the output voltage stay steady.
Load regulation = (12.3 − 11.6) ÷ 12.3 × 100% ≈ 5.7%.
Acceptable for LED strips, but too loose for a circuit that needs a stable 12 V — add a regulator or choose a better supply.
Measuring Battery Capacity
A battery's rated capacity is what it delivers when new, at a stated discharge rate and temperature. To find what a battery delivers now, discharge it at a constant current down to its cut-off voltage and time how long it takes:
Capacity = 0.35 × 14 = 4.9 Ah — 70% of rating. The battery is ageing and will need replacing soon.
Electronic loads with a capacity mode do this automatically: set the current and the cut-off voltage, and the load stops at the cut-off and reports the amp-hours and watt-hours delivered. Always use the correct cut-off voltage for the chemistry (about 3.0 V per cell for Li-ion, 2.5 V for LiFePO4, 1.75 V per cell for lead-acid) — discharging below it damages the battery.
Managing Heat
Every watt a load absorbs becomes heat. A 60 W load is as hot as a 60 W incandescent bulb, concentrated in a much smaller component.
- Mount power resistors and MOSFETs on a heatsink with thermal paste, and add a fan for anything above a few tens of watts.
- Derate: run resistors at no more than half their rated power unless they are on a heatsink sized for the full rating.
- Keep the load away from flammable material and from the supply under test, and never leave a long test unattended without over-temperature protection.
- Use leads and connectors rated for the current. Thin leads drop voltage and get hot, and measure the voltage at the supply terminals rather than at the load end of the leads.
Frequently Asked Questions
Can I use a car headlamp or heater as a test load?
For a rough check, yes. Bulbs and heater elements are cheap, high-power resistive loads. Their resistance changes a lot with temperature, so the current is not well defined, and a filament bulb draws a large surge when cold. For measurements you want to repeat or compare, use a proper load resistor or an electronic load and measure the actual current.
Why does my USB charger's voltage drop under load?
Some drop is normal: the cable and connector resistance alone can lose 0.2–0.5 V at 2 A. A large drop, or a voltage that falls below about 4.75 V, means the charger cannot deliver its rated current or the cable is too thin. Measure at the charger's own output with a short, heavy cable to separate the two.
Is an electronic load safe to use with lithium batteries?
Yes, provided you set the correct cut-off voltage and a sensible current (no more than the battery's rated continuous discharge current). Most electronic loads stop automatically at the cut-off. Never discharge a lithium cell below its minimum voltage, and do not leave a high-current test running unattended.
What is the difference between load regulation and line regulation?
Load regulation is how much the output voltage changes as the load current changes. Line regulation is how much it changes as the input (mains or battery) voltage changes. A good regulated supply keeps both small; an electronic load is the tool for measuring load regulation.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.