Electrical Dumbass-Check

[willskyfall]

I have a safety question for the electronically-minded among you; I dearly want to go through with a particular test (as I'm hungry for knowledge of the world), but I have a strong suspicion that I might set a multimeter on fire in the process.

Comments

[ziqueenmab]

A common-sense observation from someone with experience only in non-XTREEM multimeter use:

A multimeter set to measure voltage will have a very high internal resistance (it's connected in parallel across the thing you're measuring so any current that went through it instead of the thing would affect the very voltage you were trying to measure). A multimeter set to measure current will have a very low internal resistance (connected in series with things, so any internal resistance will mean there's some voltage drop across it which in turn can affect the circuit you're measuring). So it stands to reason that the multimeter-innards you're working with (and possibly pushing the limits of) depend on what you're set to measure.

So the 10A cap on measuring current probably doesn't apply so strongly to measuring voltage. I wouldn't take this as a "totally go ahead and do it", though, since 20A is still a lot of current in my mind.

[ywalme]

True. To get quantitative, I just looked up the internal resistances of multimeters these days and they seem to be on the order of megaohms. So, unless the resistive element you're measuring across has comparable resistance, you shouldn't be drawing too much current if you're in voltmeter mode. And if you're measuring across the internal resistance of the supply itself, power supplies tend to have very low internal resistance -- I pegged a lab supply at a measly 37 ohms last semester -- so unless something is very wrong, your power supply should not have a resistance comparable to your multimeter's. Still, you should check on this. (Especially since I've never done this on a computer power supply, and I don't know exactly how their innards are set up

[willskyfall]

If you really feel you must do it, at least keep one hand in your pocket.

I can do that. It might make placing the multimeter probes a little more awkward but I definitely understand the sentiment.

I know that "it's the current that kills" and all that, and I understand that 20 A is a pretty ridiculous value compared to other systems, but wouldn't the overly-low voltages that I'm testing (5V, 12V) make things a bit safer?

This may be an abuse of Ohm's Law, but if the power supply is pushing a high current through a low voltage drop, it's because the internal resistance of the power supply is miniscule. Would the resistance of human skin be enough to significantly cut the current if something went wrong?

. . . maybe I should wear rubber gloves too. :)

No real help here, just a data point

[ziqueenmab]

I've seen a person's resistance (measured with one multimeter probe held in each hand) vary between the low tens of thousands and ~1 million ohms, probably mostly dependent on the clamminess of the person's skin.

[ywalme]

Well, at this point we're at the level of me googling the answers to your questions, which you can do as well as I can :) but the short answer is that the resistance your body provides can vary wildly (for instance, simply sweating can cut it down an order of magnitude or two, as Julie mentioned, and I don't recall you being in the habit of wearing rings made out of conducting metals, but if they happen to be the contact point, that can also be quite hazardous