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As in this Armbian forum I am regularly diagnosing power issues. I've answered with this info on a few different posts over the years but I wanted this in one place so my students and the Armbian forum could discuss it.

You will read over and over the importance of good power, both in the supply, the cables, and the connection. Anything that causes the voltage to drop below each boards threshold will cause problems that seem to have no explanation. The board may run and appear to be 'working' but operations that draw more power may fail and let the processor continue running. You might think a fully functional board has software failures. It's hair pulling, especially for those uneducated in power issues.

Without attempting to teach a power class I've come up with some tools that can 'prove' weather the power supply and cables can deliver the power you expect. There's deep math and science to all of it, but what we really need is to be able to 'trust' our power. These tools can help us 'prove' our power is at least staying above the levels we expect.

When there are so many variables in a situation we have to eliminate things to narrow down the problem. Since power is frequently the problem and fairly easy to prove, it's smart to start there. But how? We use our tools to test what we need to eliminate.

We used to use banks of resistors and separate power and current meters to measure power draw and voltage drop, but these days there are inexpensive tools called DC Electronic Loads that do this for us. I've purchased numerous versions and models, and they've all been good at what they are designed for yet lacking in some other things. A trade off of quality for expense, but good enough really is good enough.

Basically they have a transistor and a very low value resistor across the output and your power supply is pulsed with a PWM across that transistor. The width of the pulse determines the current draw on your power supply, effectively 'simulating' power draw from your device while displaying the voltage at the point of the load and the draw current. You simply attach your power supply and increase the load until the voltage drops below the supply's rating. If the voltage stays above its voltage rating at its rated current, it passes. If not, it fails. Almost as simple as that.

I usually let the thing run for a while and check for heating, voltage drop over time and shutdown. Sometimes failure comes with heat.

If you have a 5v power supply that's rated at 4 amps, and your load reports 4.9v at 4A draw, your supply has failed.

The image below shows a successful test of a popular power supply, the Meanwell RD65A., a dual voltage supply rated for 3A at 12v, and 6A at 5v.  As you can see in the photos the voltage stays above its rating at the rated current. This particular photo was taken after approximately 40 minutes running at full load, which is highly inadvisable for this kind of power supply. Usually one would never run over 80% load continuously, but hey, this was a burn test.  The only thing that failed here was the load, as the fan sensors on these particular models are somewhat flawed and overheat at less than their rating and shut down. Both the load and supply were run to their limits and we 'proved' this is a viable supply for our purposes.

Here is an ebay search in the US that has similar inexpensive loads -

and Amazon -

They can be inexpensive and the fancy high power ones can be quite expensive ... find one that suits your needs.

With another voltmeter you can check the drop in your cables. Set up the load to draw what you think is appropriate, note the voltage at the load. Then using the voltmeter measure the voltage at the power supply. Note the difference between the voltage at the supply and the voltage at the load. Viola, there's your drop. Is it good enough to run your board?

Most 5v boards complain at 5v. They want above 5v, like 5.1.

This is only a basic test, you can go a lot further but this will definitely prove your supply or fail it. If you really get into testing get some inline current measuring tools so you can see what your board is actually drawing. Most voltmeters have inline current meters and many are quite accurate and detailed. Watching the current draw on your board as you perform various operations can tell you if you have a hardware problem; if it draws more than the manufacturer specifies then there may well be a hardware issue.

I hope this motivates someone to go farther with testing and proving their power.

Board: Not on the list

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