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My Arduino came with a 9V 1A power supply to use with it. I decided to test it out with my multimeter. I put the red lead in the connector and the black on the outside. Sure enough I read 9V. However when I tested the amps this did not work. The meter read a low value and the power supply started making a ticking noise and a light started flashing on it.

I did have the multimeter on the correct settings. Looking online I now realize that you cannot test the amps this way and as I was essentially testing a circuit with no load on it.

What I would like to know is what the power supply was doing when it was ticking and flashing. It is alive and well so I suppose it was some sort of internal protection.

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It was "hiccuping" in order to prevent damage from the short circuit condition. Once the short circuit condition is removed, the PSU restores normal operation.

From the reference: "The operation of hiccup is as follows. When the current-sense circuit sees an over-current event, the controller shuts off the power supply for a given time and then tries to start up the power supply again. If the over-load condition has been removed, the power supply will start up and operate normally; otherwise, the controller will see another over-current event and shut off the power supply again, repeating the previous cycle."

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I was essentially testing a circuit with no load on it.

No, you were imposing a very high load. An ammeter (amp meter) should be in series with the circuit under test. For example:

----(-)-- Power supply (+)-----(+)- Amp meter (-)---|
|                                                   |
|                                                   |
---(-)---- Circuit under test ----(+)----------------

When in amps mode the meter has a very low resistance, and measures the voltage drop over that low resistance (probably 0.1 ohms or less). Thus putting the meter across the power supply shorts it out.

Hence the noise it makes while trying to recover. The power supply turns itself off and on until the short goes away.

Also you are lucky you didn't blow the fuse in your meter. They are normally fused in case you accidentally do something like that.


Isn't low resistance considered a low load and not a high load?

A low resistance in series is a low load. A low resistance in parallel is a high load.

Assuming your meter presented a resistance of 0.15 ohms on the 10A range (as mine does) then using Ohms Law we can see that this would give a current of:

I = V / R
I = 9 / 0.15
I = 60 amps

So, you are trying to draw 60 amps from your 1A power supply, which then turns off.


If the multimeter is rated at 10A and the power supply can provide at most 3A, then why is there a risk of blowing the fuse in the meter?

It depends on the range on the meter. Mine has a 10A range and a milliamp range. The 10A range is fused to slightly more than 10A (naturally) and the milliamp range has a maximum of 400 mA. If you had tested with the milliamp range you would have blown the fuse. Yes, it would survive on the 10A range, if your power supply cannot output 10A.

There will be lots of web pages about measuring amps with a meter, here is one.

  • Thanks for the info. Could you clarify a little further to help me understand? Isn't low resistance considered a low load and not a high load? If the multimeter is rated at 10A and the power supply can provide at most 3A, then why is there a risk of blowing the fuse in the meter? – denver Apr 13 '17 at 13:22
  • See amended answer. – Nick Gammon Apr 13 '17 at 21:39

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