dumb multimeter/electrical question

Paul T · 07-06-2006, 12:02 PM

ok, I am no electrician at all, but I broke out the trusty digital multimeter the other day to check an aging car battery and ran into a dilemma.

A quick check of the voltage and...wtf, 16-17 volts?! That doesn't seem right! So, I switch to AC and check the house outlets for comparison and I get...170! What the hell. Now, knowing nothing, I just figured my multimeter was fried (it's old and cheap), but a quick google search has me believing I might be measuring "peak" as opposed to "rms" voltage? Does this make sense to anyone? I have no idea how I may have changed that setting if so and can't find any reference to it anywhere on the meter. Also, it seems the peak/rms differences only apply to ac voltage, so why the odd reading on the car battery (checked a couple and same thing)?

So, anyway, I go out and buy a nice new digital meter thinking that is the problem. Get home, check the car batt and get 12.5v - ok all is good. But like an idiot I try and check the ac voltage again (and yes I changed to ac voltage) and get a reading of 170 again! Then I go back to the battery and now it's reading 17! Am I somehow frying my meters here? Am I just an idiot? Electrical issues have always been my nemesis....

tobster1911 · 07-06-2006, 01:33 PM

RMS is an average of sorts. To find RMS take the Peak and divide by square-root of 2. SO the AC signal of 170V, if this was peak not RMS, would convert to an RMS of 120.2V which sounds reasonable.

I am not sure I buy that though because of the issue of the DC battery reading. Also multimeter's show RMS when measuring AC voltage. Something weird is going on with your measurement. Here is what I would check.

1. Check that your probes are on the correct terminals. You have to switch where they plug in to do current measurements. Make sure they are set on voltage.

2. Make sure you are on the correct range. Depending on the multimeter, you may have to pick a starting range. Some have auto-ranging.

3. Try to measure again.

Don Wohlfarth · 07-06-2006, 01:34 PM

Don't know about your old meter but it sounds like you are reading peak voltage with the new digital one. I think most digital meters have a little button on the handle that you move for different readings. If you've got a little button try pushing it one time and take a reading (using the same scale) push it again and take a reading.
If all else fails read the directions.

rick-l · 07-06-2006, 10:57 PM

Let me get this straight, you measured battery voltage with the engine off and got 17 volts? If so I would say your new meter is FUBARd. Radio Shack will exchange it won't they?

I once destroyed a meter by accidentally measuring battery voltage with the leads plugged into the current ports. Something in the meter blew to protect the fuse. The fuse in the Fluke I bought will interupt 2000 amps and costs $8 so I never use the current function any more because I am a forgetful, ham fisted idiot.

Most meters use Mean Average Deviation to get AC readings. The AC is rectified and then averaged (low pass filter). You can then multiply by the factor that would convert the full wave rectified sin wave average value to what the Root Mean Squared value would be. Only expensive meters (>$400) have the signal processing capability to do true RMS measurements and the are limited to like 10 kHz.

The peak function on most moderately priced meters holds the peak sample that comes out of the A/D converter. Since most meters use a dual slope A/d (slow but accurate) this is sampled at a pretty low rate. It would hold the peak average AC reading not the 60 Hz peak.

StevoRocket · StevoRocket's Garage

I knew that.

Not!

Paul T · 07-07-2006, 03:04 AM

Rick, I'm wondering if I also had the leads hooked up wrong when I measured the ac. I replaced the fuse on the new meter last night and got a nice 12.5v reading on the car battery - I'm not trying the ac! So, I guess I must have fried the fuse.

Thinking the same issue with the old meter, I replaced that fuse also but no dice - still get the same wacky readings so I'm thinking that one is toast. Thanks for the advice and explanations - some of which I even understand.