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Electronics => Beginners => Topic started by: SirRage on June 05, 2016, 08:23:21 am

Title: Mixing AC and DC current modes on a Fluke 87v
Post by: SirRage on June 05, 2016, 08:23:21 am
Hey there,

So this sounds like a dumb question but I haven't been able to find an answer to it in the manual or online so far. I was wondering if you would damage the meter if you accidently left the unit in DC current mode and stuck the probs into an AC source while diagnosing another issue. I imagine there is some kind of protection circuitry in there to prevent damage.
Title: Re: Mixing AC and DC current modes on a Fluke 87v
Post by: Simon on June 05, 2016, 08:25:24 am
I don't believe it would usually do any damage but your manual should specify maximum ratings for the input. It would however measure 0 V as the average of a perfectly symmetrical AC signal is 0. I have before measured the output of a 9 V AC transformer with the meter set to DC with no problem and then wondered why a capacitor on the output blew up  :-DD
Title: Re: Mixing AC and DC current modes on a Fluke 87v
Post by: retrolefty on June 05, 2016, 10:31:55 am
Hey there,

So this sounds like a dumb question but I haven't been able to find an answer to it in the manual or online so far. I was wondering if you would damage the meter if you accidently left the unit in DC current mode and stuck the probs into an AC source while diagnosing another issue. I imagine there is some kind of protection circuitry in there to prevent damage.

 The most common damage mode is to have the meter in current mode (AC or DC) with the probe in the proper current jack and then in error go and attempt to measure a voltage source (AC or DC). That's called a brain fart but the meter will protect itself via it's internal current protection fuse but the external circuit may be damaged by the momentary high current before the fuse opens.

Title: Re: Mixing AC and DC current modes on a Fluke 87v
Post by: Simon on June 05, 2016, 10:45:37 am
It shouldn't actually be a problem because the DC mode is made to take current and voltage in both directions which is all AC is. It's just that your measurements are going to be rather wrong.
Title: Re: Mixing AC and DC current modes on a Fluke 87v
Post by: jeroen79 on June 05, 2016, 11:08:43 pm
As illustrated by Simon, the biggest risk of accidentally measuring DC on an AC source (or vice versa) is that it may lead you to believe that the source is not live and safe to touch.