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I am building a device that can power other devices (it has a voltage regulator, battery controller, etc...).

What is a good way to measure how much current it will provide, and/or test if it will provide a specific maximum current?

Normally, my device will be asked to provide 100-200 mA, but, I would like it to be able to provide 1000 mA max.

I can try to power another device like an Arduino (50-100 mA) or a 1000 mA motor I have, but I am thinking there should be a better way.

That wouldn't test intermediate levels like 200 mA or 500 mA. Plus the 1000 mA motor produces feedback which could be bad.

RawBean
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Matt
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4 Answers4

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Put a maximum load on the power supply. Power resistors, home-made power resistors, incandescent lamps (for example car headlights for a 12V load). I use a couple electronic loads which can be programmed to draw a certain steady current, a constant power (negative resistance) or to simulate a given positive resistance. Many real loads are not constant - the load may draw 1A average, but in 5A pulses, so that can be simulated too.

Also check minimum load. Sometimes power supplies will misbehave with a light load (for example commercial DC-DC converters that output 25% more than rated voltage).

Check start-up and shut-down for overshoot and undershoot conditions using appropriate equipment—including pathological cases such as power interruption and restoration during start-up if you have a sensitive load. Check sequencing if applicable under all conditions (for example, the 12V supply may be required to always be higher or equal to the 5V supply during all conditions).

Finally, repeat these tests at extremes of temperatures and any other applicable environmental conditions (high altitude reduces heat sink effectiveness). Typical issues are overheating (measure internal temperatures and compare with design expectations) at high temperatures or failure to start after a soak at minimum temperature. I usually test beyond the specified limits by a considerable margin if the test article is going to stay in the lab and never be used.

JYelton
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Spehro Pefhany
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  • If you're not sure what the maximum is, isn't putting a load on at what you think the maximum might be just asking for the power supply to blow up? – naught101 Dec 31 '14 at 00:17
  • The maximum is what you've **designed it for** not where it breaks. If your design has no errors then it should work well at the (your) rated maximum, with sufficient margin. If it isn't, well you're probably about to learn something, and it seems fair that one must pay a bit of tuition for a lesson. – Spehro Pefhany Dec 31 '14 at 14:46
  • Yeah, fair enough. My question is more closely related to http://electronics.stackexchange.com/questions/76577/how-to-measure-max-current-of-a-power-supply , which I only found after this one. – naught101 Jan 02 '15 at 01:04
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If you have a pile of money burning a hole in your pocket, you could use an electronic load. This will allow you to quickly vary the load current, and will probably includes a meter to measure the voltage sag of your source.

Otherwise, it's common to simply combine power resistors with low enough values to draw the current you want to test. Combined with some switches, you can make a variable load pretty easily.

The Photon
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    I use incandescent light bulbs as loads. They can stand some abuse, and they provide a visible indication :) – Wouter van Ooijen Feb 20 '14 at 07:30
  • @WoutervanOoijen That is a valid answer and could be posted as an answer. – Eduard Florinescu Feb 20 '14 at 09:27
  • I see it as an addition/specialization of the power resistors already mention by the Photon. I think of a lamp as a power resistor with sort-of built-in heatsink and power indication. – Wouter van Ooijen Feb 20 '14 at 13:40
  • @WoutervanOoijen LOL, when I worked at a battery company, we had a ton of very expensive programmable loads, and one of the consultants called bullshit and used lightbulbs instead :) – Potatoswatter Feb 20 '14 at 13:44
  • I (or rather, those around me) recognize the attitude :) – Wouter van Ooijen Feb 24 '14 at 22:20
  • The electronic loads are very useful and the offshore ones are not that expensive. It wastes a lot of $$$ time to have a consultant looking for just the right resistor value or bulb combination. It's also nice to be able to flip back and forth between loads so you can log the transient response of a supply (for example, as you ramp the temperature over the whole range in an environmental chamber). Otherwise you're just guessing. Don't laugh, I've seen circuits misbehave over a narrow range of temperature like 0 to 5 degrees C. Rare, but possible, and it indicated latent issues. – Spehro Pefhany Dec 31 '14 at 14:50
  • @SpehroPefhany, It's not obvious to me whether OP is asking from a professional or hobbyist point of view. – The Photon Dec 31 '14 at 15:34
  • @ThePhoton Indeed.. I just mention on case it's useful to those who have budget for such things. – Spehro Pefhany Dec 31 '14 at 16:13
  • @SpehroPefhany , I thought I already mentioned some of the advantages of an electronic load in my answer, even if I was a bit flippant about the cost. Also, you also posted an answer where you discussed their capabilities. If that discussion was incomplete, you could edit your own answer. – The Photon Dec 31 '14 at 16:28
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The best test you can do is by using theory to analyse the expected performance of your circuit. This will tell you what parts are the weaker areas and will likely shed a lot of light on your design.

This test can take hours but will find the majority of problems without even buying one component.

This is fundamental to good design.

Andy aka
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I would recommend separating your sensitive command circuitry from higher power passive circuitry, also motors since they have fly-back currents due to inductance do burn sensitive semiconductor circuitry. As a kid when I worked with germanium transistor I used to burn them all time until I knew how to protect the circuit.

To protect from this you need a flyback diode, the coil here can the winding of the motor or the coil of an relay.

I recommend to use an relay also you could make available from your powersupply two voltages to the same board 5V for command and 12V for motors which more than halfs the current for the same power and which is basically how old computer boards dealt with motors in the first place.

schematic

simulate this circuit – Schematic created using CircuitLab

RedGrittyBrick
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Eduard Florinescu
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  • How does this answer relate to the question about testing max current of the psu? The motor was just mentioned as an idea for a test load. – alexan_e Feb 20 '14 at 07:59
  • @alexan_e I think relates since you don't need to test the current if you use a relay to power the motor, and also it answer to his concern: ___"Plus the 1000ma motor produces feedback which could be bad."___. . – Eduard Florinescu Feb 20 '14 at 08:03
  • There is no motor in his circuit, he only mentions the motor to use it as a test load and you suggest him to design a pcu with separate outputs for the logic level parts and 12v for the (non existing) motor or use a relay for it, how does this help him test the max current of the existing psu? – alexan_e Feb 20 '14 at 08:27
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    @alexan_e Now I see, I misunderstood his question. I thought he needed to power a motor in the first place. – Eduard Florinescu Feb 20 '14 at 09:09