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DMM question

DMM question

DMM question

(OP)
I just done a dumb thing telling myself not to while doing it, I fried my DMM!

The story before the question;
I think I have a current drain on my truck battery so I installed a disconnect and planned to measure current while I pull fuses to locate the problem. Even going through a STAR (stop, think, act, review) I connected the leads with the DMM set for mAs across the battery. Even the leads got too hot to touch!

Luckily I used a radio shack 22-811 and not my Fluke. Opened it up and did not see any damage. I was surprise when I ohmed the amp circuit and it was not open, BUT I was surprised to find it 0.00 ohms! (as per my Fluke). I checked the Fluke's amp circuit with both with itself and the RS at 100 ohms.

SO if someone has a similar RS DMM, would you mind checking the ammeter resistance?

I don't have a "calibration" source for current. the RS ammeter seems to work when I check by going across a "C" battery (but flashed 7 amps)

RE: DMM question

Check the current of a small bulb with both your 22-811 and with your Fluke. Ammeters generally have very low resistance. The lower the better.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: DMM question

This is a perfect application for a clamp-on ammeter, rather than an in-line ammeter.
I doubt your DMM can pass more than 20 Amps before blowing the fuse. More like 10 or less in most cases.
Battery currents can go from tiny to 100A depending on what the battery is being asked to do.
Fluke makes a versatile and accurate clamp-on DMM, which I have, and would reach for to solve this problem.
I've tried other clamp-on meters but got garbage readings (Amprobe and Agilent).

STF

RE: DMM question

Clamp meters are great, Rogowski coil units are even better, provided they're used on the correct application.

Double check the function of the clamp meter if you're looking to buy, not all of them can measure DC (certainly, none of the Rogowski coil stuff can either).

RE: DMM question

Yes, and even the Agilent DC clamp probes can't read DC either!

For those who are curious, it's a simple test you can do before driving away from the store parking lot. Pop the hood of the car, leave the ignition off, turn everything you can off, but turn the headlights on. Clamp around the Positive wire on the battery. Expect a constant DC current. Wait a few seconds... current must not float around. Now turn the high-beams on - DC current should go up. Now turn all lights off - current should go nearly to zero. It doesn't have to be perfectly zero - but make sure you CAN zero the meter somehow. If the meter fails this test, return it, and if necessary demonstrate to the disbelieving store "expert". Especially if it's the store expert who clamped it around the extension cord of his point of sale computer.

STF

RE: DMM question

(OP)
I posted in the hobby section since this is something I tried to do at home and thus limited to things around the house. Making up a known wattage bulb and pigtail is a great ideal for me to check calibration, but I don't have the bulb for the one socket and pigtail i found.

the RS meter has a "unfused" 10 amp connector. Hooking across a 12VDC car battery could easily hit 100amps. I realized as soon as I plugged the leads in the meter I did wrong and quickly pulled them out, but I still expected something should have burnt open in that second.

the meter does have a 500mA fuse for the volt/ohm and it got popped. it looks like the ammeter shunt is a loop of heavy wire. the only visible damage I saw was the green varnish did blister on the connector board

as for a clamp on, it would be nice but I don't have one. and I will be looking for ~1mA

I had performed this battery drain test on another truck and found the radio amp would not shut down, thus it fuse is pulled since I don't listen to radio anyway

RE: DMM question

(OP)
Got an email reply from Radio Shack Customer concerning the amp shunt resistance..."they don't provide technical support."

I did not get a chance to make a known current loop to check calibration BUT I did proceed to check the battery drain on the truck with the supposedly minimum current.

the GOOD NEWS...Both the RS and Fluke showed ~20mAs.

I was extremely focused this time as my Fluke 8060A has been with me through out my career.

Thus the Radio Shack DMM is very robust! (Now back to finding the drain on the other truck)

RE: DMM question

You can get a DMM for less than $5 at Harbor Freight, which will work very adequately for your application. Quite often they have coupons for free DMM's.

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