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In "Upgrading and Repairing PC's - 18th edition" by Scott Mueller on pg. 1278 he discusses multimeters. "You should only use a DMM (digital multimeter) instead of the older needle-type multimeters because the older meters work by injecting 9V into the circuit when measuring resistance, which damages most computer circuits.

A DMM uses a much smaller voltage (usually 1.5V) when making resistance measurements, which is safe for electronic equipment."

  1. Most DMM's that I've looked at have 9V batteries. Are they internally stepping down the voltage used when making these measurements?

  2. Wouldn't the concern of injecting 9V be true when measuring continuity as well?

  3. A little off topic, there is a fascinating way to test for laptop screen inverter failure, (http://www.fonerbooks.com/test.htm), is anyone aware of a safe DMM that is capable of this as well?

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migrated from serverfault.com Jan 15 '10 at 22:14

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When looking for a DMM have a look through the manual, which should state what voltage is used for resistance and continuity measurements. It it doesn't, just keep looking. –  John Gardeniers Jan 15 '10 at 22:14
    
Just to clarify, the "needle-type" meters are called analog multimeters, so when somebody refers to a digital multimeter (dmm), they are referring specifically to the digital style. I've never heard of analog meters being "unsafe" for any electronics, and theres nothing special in a computer vs. your average tv, dvd player, cell phone, etc. Analog meters may be preferable in some situations, but whatever style you use, quality is your friend. All of that said, Fluke DMMs are well regarded. –  Joe Internet Jan 15 '10 at 23:24

6 Answers 6

up vote 2 down vote accepted

I think my fairly generic DMM has a resistance measuring voltage of less than a quarter of a volt, and a continuity check voltage of less than one volt. The continuity range also tests diodes so it has a higher voltage to overcome the voltage drop of a diode. When measuring voltage it as a resistance of over 30 MOhms.

I think you will be fine with any slightly modern DMM. So long as you are careful where you stick it. :)

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I've used regular DMMs for years, they're perfectly safe so long as you don't get the very cheapest or really old ones - stop worrying :)

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Yeah, you can use normal multimeters.

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I think it matters far more that you know how to use a multimeter and what will happen when you use it, than what brand/model of multi-meter you use.

Even with the most expensive multi-meter you can cause damage to the equipment or the meter if you connect it the wrong way or with the wrong settings.

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True, but the question was not about brand/model but about how DMM's work and if anyone knows one that is safe (with the focus on the safe spec, not brand/model). If you're using equipment that can damage your PC there is no "right" way to use it. Thanks for the opportunity to clarify. –  Ssvarc Jan 15 '10 at 21:09

While true, this may be somewhat the wrong question. I almost never do resistance checks with my meter. I do mostly voltage checks, and for those I believe you want a meter with a very high internal resistance (going from memory here, since my reference material is all at home). Anyways, something new and expensive will be best. :-)

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I think this is on the right track... DMMs typically have a higher input impedance than analog multimeters, so they leak less current through the meter when measuring voltages. –  coneslayer Apr 21 '10 at 1:02
    
10 or 11 Megaohms (Mohms) are common impedances for DMMs. –  mctylr May 14 '10 at 20:30

Older analog multimeters may use the raw voltage from its battery supply (6-9V) because at the time 9V was pretty much considered low voltage (pre-TTL ICs) except in special circumstances.

Nowadays modern multimeters, which tend to be digital, and thus called DMM, use approximately 1.5V or less (~1.0 V) for resistance and continuity testing (which is really just a low-resistance test mode that often includes a buzzer).

I wouldn't assume that a modern analog meter is low enough voltage, without measuring it in resistance / continuity mode (via a second meter), because many I've seen are cheap low-quality meters that the low-cost manufacturer is the most important "feature" of them, so voltage regulators, or a extra Zener diode would cost more to add, so I would assume (before measuring) that is 5-9V in "meter-powered" test modes (not voltage or current measuring modes).

1.0 - 1.5 V is in generally low enough to not damage most electronics technologies (1.8 V is an increasingly common low-voltage power supply limit these days).

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