Can you damage a passive circuit measuring things with a multimeter?

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Hi, and yeah, I know the term "passive circuit" is so general the answer is going to be yes. I'm just thinking about some passive circuit that has capacitors or coils, that could be charged even if the circuit is disconnected, and you could short something by error and damage it.

What I really mean with a "passive circuit" is that it's not charged at all, capacitors and everything is "dead", no energy stored. Could you damage some components by simply the energy you give with the multimeter?

Somewhere, a few years ago, I read that, for example, you shouldn't measure the resistance of a speaker coil, specially in small headphones, because you could damage it. I've been doing that many years, hahaha, to check if headphones are balanced, to check if they are broken, to check I repaired them correctly... but I am actually curious about if that's a myth or if there are components that are so sensitive you could damage with a multimeter.
 

danadak

Joined Mar 10, 2018
3,577
Broadly speaking yes you can damage some circuits.

Qualifying small junction devices, using the diode resistance measurement switch position,
its possible to damage junctions in small geometry transistors, like JFET gate, some bipolars,
like RF small signal devices.

If you accidentally try to measure current and improperly connect probes to a circuit that
shorts part of the circuit with the internal shunt again you can burn out some transistors,
diodes.

Regards, Dana.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Thanks!
What about headphones? Have you ever heard you shouldn't measure the coils with a DMM?
 

rsjsouza

Joined Apr 21, 2014
201
When I was a kid I damaged several of my dad's LED displays (FND560 for the oldies) by measuring them with his analog VOM in the lower ohms scale (x1). The analog VOM put out 3V at a significant current and it was like testing matches out of a matchbox: every segment that tested ok (lit up) was being burned in the process.
That VOM could kill a small 8 ohms speaker as well.

If you have a charged capacitor, measuring it in the current range may physically damage its terminals or the probes or even blow a fuse on the meter.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Thank you so much.
Now I'm curious, how much energy does a DMM introduce into a circuit?
What voltage does a DMM use?
 

Dodgydave

Joined Jun 22, 2012
8,381
Most modern Dvms use 3V batteries, older types use 9V, the Diode test puts out 2V, the Ohms test puts out full supply with current limit.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
Thank you!
So, let me get this right. An electronic component will die due to heat related damage, in other words, too much current flowing, right?
That is, there are way too many coulombs per second, that creates heat and the delicate component can die because the heat can damage an inside tiny resistance, cable, bridge something or simply burn it. Is that correct?
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
295
I don't really understand or think that's what happens. If you put a LED under "massive" 5kV, but just 0.00001C are available, what happens is that there will be just a very few electrons flowing, but they will flow very fast. I guess, at the end, the correct measure is W, J/s, that's I believe what really tells how a component behaves. In other words, if you put that LED under 5kV, but there' just one electron flowing, then, even though it's going so fast, since it's only one, it won't generate enough heat to kill anything. Correct me if I'm wrong.

Say a bullet is 1A. If it's under 230V, it will totally kill you. If it's under 1V, it won't kill you at all, it will be like throwing you a bullet shot by a hand. Same if you shot a bullet with a massive accelerator, like 5000V, but the bullet is thin as a wire. It won't kill you either. What matters is the overall energy really stored in the shot. A big rifle bullet won't kill you if it's shot with a water gun toy. A tiny rice sized bullet won't kill you even if it's shot with a huge incredible gun.
 
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rsjsouza

Joined Apr 21, 2014
201
Yes. The charge present will determine the damage done. For example, an electrostatic discharge does not kill you because it has very minimal charge (in Coulombs), but these can reach tens of thousands of volts that can break the insulating air gap and the skin.

The case of my dad's VOM was that it came from another era. Despite its voltage was just 3V, its current limiter was sized for a large value, thus being fatal to certain semiconductor junctions.
 

panic mode

Joined Oct 10, 2011
1,782
one of my first multimeters also had secondary battery (15V) for measuring high resistance ranges. the 15V battery was not common so i never used that range... but i really liked that meter... lost it while moving... :oops:
 

dl324

Joined Mar 30, 2015
8,877
one of my first multimeters also had secondary battery (15V) for measuring high resistance ranges.
Thanks for reminding me to take the battery out of my first analog meter (RCA). It had a 22.5V battery used for some resistance ranges. Fortunately it never leaked.

I used to keep a cheap Radio Shack analog meter in a couple of our autos because they don't need batteries to measure voltage.
 

atferrari

Joined Jan 6, 2004
3,447
Most modern Dvms use 3V batteries, older types use 9V, the Diode test puts out 2V, the Ohms test puts out full supply with current limit.
Hola Dave
As per what I know, diode test is based on passing a constant current which, in at least two of my DMMs, is just 1 mA. I recall measuring it once.
 
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