Author Topic: DMM help please  (Read 1429 times)

Offline tschmidlin

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DMM help please
« on: January 08, 2011, 02:15:19 PM »
What am I doing wrong?

I got out my DMM to trouble shoot something, but I'm getting odd readings.  I'm trying to measure the DC output of a 22.5V DC converter, so I set the DMM to read DC voltage and put the probes on the output (inside is positive for this one) and the reading doesn't change from the 30-60mv it reads when not connected to anything (it fluctuates).

So maybe the converter is bad, right?

Well, just as a sanity check I switched it to AC volts and checked the outlet.  It goes from something under 5 mv (again fluctuating) when not connected to anything, right up to 365 mv when I stick the probes directly into an outlet, one in each of the straight slots.

So what's the deal?  Bad DMM?  User error?  I've used this thing intermittently over the years so I sometimes forget what I'm doing, and I'm not the most electrically inclined person.  But I don't remember having this kind of problem in the past.
Tom Schmidlin

Offline kerneldustjacket

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Re: DMM help please
« Reply #1 on: January 08, 2011, 03:36:07 PM »
Tom, most DMMs have an internal fuse to protect them...perhaps that has blown. (it is not a fuse for the entire unit, which is why you'll still have a display)
Or, one of your probes has developed an "open," i.e. the wire inside has broken. Try this (if you haven't already): set the DMM to read ohms and touch the two probe ends together...you should read 0.000 ohms.
John Wilson
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Offline weazletoe

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Re: DMM help please
« Reply #2 on: January 08, 2011, 03:54:19 PM »
Without looking at the meter, it almost sounds to me as if you have the probes in the wrong place. When it says "oupput", are you sure that's not for checking ohm's and continuity? In other words
Outputting voltage" to get a resistance reading? That's sure what it sounds like.
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Offline kerneldustjacket

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Re: DMM help please
« Reply #3 on: January 08, 2011, 04:14:36 PM »
weazletoe has a good point; most meters use a separate input to measure amps...I've made the mistake of having the probes in the wrong inputs a few times over a 25 year career in medical electronics!! ;D
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Offline tschmidlin

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Re: DMM help please
« Reply #4 on: January 08, 2011, 04:41:17 PM »
Nope, they're not in the wrong inputs.  There's 2 red ones, one for amps, and one for everything else and up to 400 mA.

Anyway, thanks kernal, I'm pretty sure it's the fuse.  I did the ohm test and it reads O.F, so I pulled the probes and just stuck a wire into each of the inputs and it still reads O.F.  Sounds like it's the fuse to me, but it is opaque (ceramic).  I guess my project will be on hold until I can get another fuse.

Thanks again for the help guys.
Tom Schmidlin

Offline tschmidlin

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Re: DMM help please
« Reply #5 on: January 08, 2011, 04:59:44 PM »
Further proof - I took the fuse out entirely and it behaves as before :)

Looks like a trip to radio shack is in order . . .
Tom Schmidlin

Offline kerneldustjacket

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Re: DMM help please
« Reply #6 on: January 08, 2011, 06:08:12 PM »
Sounds like it's the fuse to me, but it is opaque (ceramic).


Ceramic fuses are annoying...you can only confirm good or bad by using an ohmmeter...and in your case that's a true catch-22!

Anyway, good luck...hope that sorts it out.
John Wilson
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Offline hokerer

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Re: DMM help please
« Reply #7 on: January 08, 2011, 06:29:36 PM »
Further proof - I took the fuse out entirely and it behaves as before :)

Looks like a trip to radio shack is in order . . .

Err, just stick a penny in it   :P
Joe

Offline tschmidlin

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Re: DMM help please
« Reply #8 on: January 08, 2011, 09:53:40 PM »
Sounds like it's the fuse to me, but it is opaque (ceramic).
Ceramic fuses are annoying...you can only confirm good or bad by using an ohmmeter...and in your case that's a true catch-22!
Yeah, can't use my ohmmeter to check the resistance of the fuse from the ohmmeter :)

Further proof - I took the fuse out entirely and it behaves as before :)

Looks like a trip to radio shack is in order . . .
Err, just stick a penny in it   :P
;D  I would, but it won't fit back together with a penny jammed in there.  Maybe half a penny . . .

I need to go get other fuses anyway, I blew a few in the Christmas lights before I found where the junction was getting wet.
Tom Schmidlin

Offline 1vertical

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Re: DMM help please
« Reply #9 on: January 09, 2011, 03:12:22 PM »
Uh...is there a battery inside? It may be gonzo...
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Offline tschmidlin

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Re: DMM help please
« Reply #10 on: January 09, 2011, 03:25:51 PM »
Uh...is there a battery inside? It may be gonzo...
Good idea, but that was the first thing I tried. :)
Tom Schmidlin

Offline kerneldustjacket

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Re: DMM help please
« Reply #11 on: January 11, 2011, 06:46:04 AM »

;D  I would, but it won't fit back together with a penny jammed in there.  Maybe half a penny . . .


A half penny? They have those in Englad, right?
Better book a flight to get one...while you're there you may want to do a pub crawl and sample cask ales.
John Wilson
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Offline tschmidlin

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Re: DMM help please
« Reply #12 on: January 11, 2011, 09:46:38 AM »
;D  I would, but it won't fit back together with a penny jammed in there.  Maybe half a penny . . .
A half penny? They have those in Englad, right?
Better book a flight to get one...while you're there you may want to do a pub crawl and sample cask ales.
A new DMM would be cheaper, but not nearly as much fun!  I replaced the fuse last night though, and it works as expected now.  Thanks!
Tom Schmidlin

Offline kerneldustjacket

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Re: DMM help please
« Reply #13 on: January 11, 2011, 09:50:26 AM »
;D  I would, but it won't fit back together with a penny jammed in there.  Maybe half a penny . . .
A half penny? They have those in Englad, right?
Better book a flight to get one...while you're there you may want to do a pub crawl and sample cask ales.
A new DMM would be cheaper, but not nearly as much fun!  I replaced the fuse last night though, and it works as expected now.  Thanks!

Fantastic! ;D Glad I was able to help.
John Wilson
Savannah Brewers League
Savannah, GA

Offline denny

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Re: DMM help please
« Reply #14 on: January 11, 2011, 10:20:30 AM »
Uh...is there a battery inside? It may be gonzo...

IIRC, the batteries are only used to measure resistance.
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