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Project Notes

#513 Simple USB Dummy Load

Examining and testing a simple USB resistive dummy load.



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!


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!


Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.