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Author Topic: Measuring AC-DC  (Read 1528 times)

Mike Sokol

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Measuring AC-DC
« on: April 08, 2016, 03:28:33 pm »

Last night I had a call from an RV owner who said he was feeling a shock from his RV chassis while laying underneath it to change an oil filter. He had stuck a screwdriver in the dirt and measured the voltage between it and the RV's bumper, chassis, lugnuts, etc... He didn't realize that virtually everything metal on an RV is bonded to the chassis, so he didn't have to measure everything individually. He measured this voltage with the generator on, the generator off, the inverter on, the inverter off, and a few other iterations I can't remember. But what was interesting was that his measurements were showing only few hundred millivolts at most difference between the frame of the RV and the screwdriver in the dirt, yet he still said he could feel a shock when he stood on damp concrete and touched the frame of the RV. Of course, it would be impossible to feel 3/10 of a volt shock, but when I asked him what kind of meter he was using I discovered that he had it set to the DC voltage scale, not the AC scale as would be needed to measure AC hot-skin voltage. He said he selected the DC scale because he thought the shock was coming from the 12-volt battery in the engine compartment of the RV. Of course, if he has a failed EGC safety ground on his shore power connection, it's going to be AC voltage as a hot-skin. And if you set a meter to DC and try to read AC voltage it will show up as a few millivolts when reading 120-volts AC. I wonder just how many sound techs do the same thing? My beginning college students will do this in at the start of my audio electronics class, until I beat the difference between AC and DC voltage into them.  And don't get me started on students who set the meter to the ohms scale, then plug the probes into 120-volts AC. I have at least one student a semester do this no matter how much I tell them not to. Ugh...
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Mike Sokol
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Chris Hindle

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Re: Measuring AC-DC
« Reply #1 on: April 08, 2016, 03:45:32 pm »

I wonder just how many sound techs do the same thing? My beginning college students will do this in at the start of my audio electronics class, until I beat the difference between AC and DC voltage into them.  And don't get me started on students who set the meter to the ohms scale, then plug the probes into 120-volts AC. I have at least one student a semester do this no matter how much I tell them not to. Ugh...
My goodness. I think I was 5 when my Dad showed me how to use his Triplett. He wouldn't let me use the Simpson or VTVM untill i was 12, when he also showed me the 'scope, an audio oscillator, and a bench power supply. Interesting childhood. Between the electrical and a chemistry set, I did a LOT of "experimenting"
At least i got to introduce him to computers.
Chris.
« Last Edit: April 08, 2016, 03:47:33 pm by Chris Hindle »
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Ya, Whatever. Just throw a '57 on it, and get off my stage.

John Roberts {JR}

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Re: Measuring AC-DC
« Reply #2 on: April 08, 2016, 03:51:18 pm »

Sounds like a good job for the cheap neon lamp probes.

they will light with DC or AC.

JR
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David Buckley

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Re: Measuring AC-DC
« Reply #3 on: April 09, 2016, 06:45:26 pm »

I've got meters that measure both simultaneously.  Useless, 99.9% of the time, but...
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Mike Sokol

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Re: Measuring AC-DC
« Reply #4 on: April 09, 2016, 07:40:36 pm »

I've got meters that measure both simultaneously.  Useless, 99.9% of the time, but...

These same users seem to interchange the words VOLTS and AMPS a lot, so it's no wonder they don't understand the difference between the AC and DC voltage settings on a meter. Perhaps 25 years ago I was hiring a tech for my computer repair shop, and would give every applicant a simple test. I drew out a very basic circuit showing a battery, a switch, a fuse, and a light bulb. Then I would draw on test points and ask them to troubleshoot what was wrong with a few hypothetical voltage readings such as measuring 9 volts across the fuse. I had guys who had recently graduated from a local computer tech school who couldn't tell me that reading 9 volts across the fuse showed the fuse was open. Yikes...
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Mike Sokol
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