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So I have an Arduino Nano that I think might have gone bad. The Vin and ground pins are shorting (I'm checking continuity with a multimeter). But it only shorts when:

  • The Arduino has power
  • The red lead of the multimeter is on ground and the black is on Vin (not the other way around)

I've tried checking with the leads both ways (red to ground as well as red to vin) while both on and off, and with two different multimeters, one of which is brand new. Completely at a loss for words because this isn't really how physics is supposed to work (continuity shouldn't depend on lead orientation from the multimeter or whether the circuit is powered, right?). Any insight would be appreciated. Do I have a bad board?

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    Sounds normal to me. You shouldn't measure continuity when power is applied. As to point two. For the multimeter to measure resistance, it has to apply a small voltage. The Arduino however is using this voltage and tries to run on it. This in turn will drop the voltage making the multimeter confused. Measuring components that are in circuit can be hard or even impossible in certain situations. – Gerben Feb 18 '17 at 12:40
  • if a circuit has DC continuity in one polarity and not the other, there is a diode across the circuit. you describe a "reverse input polarity protection" diode doing it's job. – dandavis Feb 19 '17 at 8:58
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But it only shorts when: The Arduino has power

You must not use a continuity checker or ohm meter on a powered circuit.

Not only will it not work, you risk damaging the meter.

It's also not necessarily safe to use such a meter on an unpowered active electronic circuit like an IC or eval board, as the meter must apply a small amount of power to the circuit to make a measurement - how much power varies between meters and settings, and how much a circuit can tolerate in an unintended way depends on things like polarity and the state of other parts of the circuit. However, it is common practice to do so - especially for example when there's a fear that the device might already be damaged.

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It is not clear from your question what you are measuring with the multimeter (volts, ohms, amps etc.) but if you are using the ohms scale when your Nano device is not connected to the power, your multimeter charging capacitors on the board and reversing the leads will then cause a discharge through the multimeter, maybe giving a reading which leads you to believe there is a short circuit.

  • I'm checking continuity (whether or not there is a closed circuit). – theasianpianist Feb 18 '17 at 10:06
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Also if you have this setup and set it to measure Amp, then it will effectively shortcut the leads with as small resistace as mutltimeter can.

Also check, if those connectors (usually crocodiles) cannot touch something they should not (each other mainly)

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