Method/way to NOT peg Analog meter?

Thread Starter

Beetle_X

Joined Nov 2, 2012
52
Hi, I want to start using a analog meter again if a DMM is not necessary. Could the meter be zeroed out in the center of the scale to help prevent the meter from being pegged if hooked up wrong? I assume that just doing a 'slight scratch' on the conductor to test the polarity would help as well.
 

Ohmlandia

Joined Mar 2, 2020
30
Centre zero meters are available, but you can't usually make a conventional meter behave in a non zero way (unless its an active device, such as a VTVM). Just quickly prodding the test point to find out its polarity isn't really such a good idea either. Put the meter on its highest range and work down until you see the needle move, then correct polarity if necessary. In most cases, you should be expected to know the polarity in advance!
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
52
Wow a vacuum tube volt meter! What could be cooler? lol. It's too bad that a DMV is so much more practical. I will do as suggested.Thanks!
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
52
I just had a idea.What if the AC setting was used instead?The diodes would only let the meter move one way or not move at all.o_O
 
If you use a diode, you would lose a diode drop anywhere from 0.2 to 0.7 V.
With more work, OP-amp circuit you can build a "precision rectifier".
A full wave bridge on DC will drop 2 voltage drops and it would always read correctly.

Resolution and voltage range is needed for a better answer.
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
52
For low voltage circuits of mostly below 12 volts,I would like a hack to protect the meter. After the polarity is determined the analog meter could be used as normal on the DC setting. Lol. I have only a moderate amount of electronics knowledge so I threw the question out in this thread. I'll have to look at a VOM circuit. Intuitively I think it would work. Both the AC and DC meter parts have what looks like a voltage divider to 'stack up' a resistance value but the AC one has a rectifier too. As long as the AC circuit has the same resistance as the DC circuit I don't think it can damage the meter.:rolleyes:
 

Ohmlandia

Joined Mar 2, 2020
30
Yes you are right, you could use the AC ranges. But then you won't be able to detect the polarity anyway! And, because of the inherent errors already mentioned by other posters, the AC ranges are usually less accurate and maybe don't go down to low FSDs. And let's not forget that the manufacturer may have included some overange protection features in the design anyway. One of these would be a diode across the movement (or 2 back-to-back diodes) to limit the deflection voltage.
 

Thread Starter

Beetle_X

Joined Nov 2, 2012
52
I'm thinking that if the needle does deflect while being connected on the AC range ,the polarity would be correct and THEN the DC range could be used.If the needle does not deflect then the probes could be reversed.

This seems like a easy hack while fumbling around wiring trying to find out what's what. The desire to do this is motivated by sticking with a analog meter as much as possible and doing mostly auto work where just a matter of if you have power is the concern.
 
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Thread Starter

Beetle_X

Joined Nov 2, 2012
52
I just got a VOM in the mail today so I tried it out with my idea. If the polarity is reversed the needle does not move. If the polarity is correct , the meter reads about double as it does in the DC range. Using the highest range momentarily works very well but I guess if this is not possible the AC range can be used for DC to figure out the polarity.
 
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