#513 Simple USB Dummy Load
Examining and testing a simple USB resistive dummy load.
Notes
I’ve seen a few people on YouTube using a simple USB dummy load for battery testing - like this one from an aliexpress seller. I picked one up to take a closer look.
These really are at the dumb end of the dummy load spectrum. For example:
- no over temp control
- no over-discharge protection
- no voltage/current/power measurement
Probably a very bad idea to use these for battery discharge tests unless closely monitored. But fine when a simple resistive load is needed for crude bench tests. But if nothing else, a pretty cheap way of sourcing some nice retro 10W resistors!
Construction
The circuit is pretty straight-forward - two 10W resistors that can be switched in separately or on parallel, for three load settings, plus ‘off’ (both resistors switched out).
With a 5V USB supply, expected current and power loads are as follows:
| Resistance | Current | Power |
|---|---|---|
| R1 (4.7Ω) | 1.06A | 5.32W |
| R2 (2.2Ω) | 2.27A | 11.4W |
| R1‖R2 (4.7Ω‖2.2Ω) | 3.34A | 16.7W |
Three LEDs indicate the current operating state:
| LED | Description |
|---|---|
| LED1 | ON when 4.7Ω resistor load enabled |
| LED2 | ON when 2.2Ω resistor load enabled |
| LED3 | ON when USB power present |
My quick re-drawing of the circuit:
Here’s an example under test, with R1 (4.7Ω) switched inline. The current draw and power are close to expected i.e. the resistance value is in spec!