Blown DMM fuse

[Russ]

Hello

   I recently gave a friend a ($40.00) Commercial Electric DMM. She was using it for breadboad assignments. She says that she blew its fuse. Is that possible when conducting 9 volt DC Resistor experiments? (9v)

Thanks
Russ

[Ian.M]

Yes. 
Assuming its https://www.homedepot.com/p/Commercial-Electric-Auto-Ranging-Digital-Multi-Meter-MS8301B/206177758, its only got a 400mA fuse for its lower current ranges and they are on the same socket as voltage and resistance.   If you accidentally select the mA range, even in passing and the circuit resistance is under 20 ohms at 9V, odds are the fuse will pop.   Even a humble Alkaline PP3 is likely to be able to briefly supply over an Amp when shorted, and could pop a 400mA fuse.

[Paul Moir]

Sure.  The datasheet for a 9V Duracell says it has an internal resistance of 1.7 ohms, so potentially it can blow a 5A fuse.  Some can deliver more current than this.  What size fuse was it?
Looks like Ian.M answers that!

[iainwhite]

This is one reason I prefer multimeters that have a separate input jack for the Amps range(s).

[Russ]

This is one reason I prefer multimeters that have a separate input jack for the Amps range(s).

   

   I agree. I believe that the meter has a separate jack.

Russ

[Russ]

Yes. 
Assuming its https://www.homedepot.com/p/Commercial-Electric-Auto-Ranging-Digital-Multi-Meter-MS8301B/206177758, its only got a 400mA fuse for its lower current ranges and they are on the same socket as voltage and resistance.   If you accidentally select the mA range, even in passing and the circuit resistance is under 20 ohms at 9V, odds are the fuse will pop.   Even a humble Alkaline PP3 is likely to be able to briefly supply over an Amp when shorted, and could pop a 400mA fuse.

Ian

Thanks. I had to order a 400mA replacement fuse for her DMM. In the meantime, can I replace it with a 500mA fuse?

Thanks
Russ

[drussell]

   I agree. I believe that the meter has a separate jack.

It has a jack for the A range but the mA is on the same jack as resistance/voltage. 

That set-up is terrible, even a veteran electronics wizard can end up accidentally changing ranges past the mA setting while connected to something when they're not thinking about it.  It is an even worse set-up for a beginner.  Having extra fuses on hand would definitely be a good idea! 

[Ian.M]

Thanks. I had to order a 400mA replacement fuse for her DMM. In the meantime, can I replace it with a 500mA fuse?

Absolutely *NOT*.  You could temporarily fit a 250mA Fast blow fuse, but if it isn't a ceramic body HRC type the meter is then unsafe to use on mains voltages (if it was ever safe for that to begin with) so, while the temporary fuse is in there, tag the meter:

DO NOT USE on 120V AC MAINS
or any voltage above 50V DC
Tag only to be removed by Russ

[Russ]

   I agree. I believe that the meter has a separate jack.

It has a jack for the A range but the mA is on the same jack as resistance/voltage. 

That set-up is terrible, even a veteran electronics wizard can end up accidentally changing ranges past the mA setting while connected to something when they're not thinking about it.  It is an even worse set-up for a beginner.  Having extra fuses on hand would definitely be a good idea! 

Thanks. 👍

Russ

[Russ]

Thanks. I had to order a 400mA replacement fuse for her DMM. In the meantime, can I replace it with a 500mA fuse?

Absolutely *NOT*.  You could temporarily fit a 250mA Fast blow fuse, but if it isn't a ceramic body HRC type the meter is then unsafe to use on mains voltages (if it was ever safe for that to begin with) so tag the meter: "DO NOT USE on MAINS".

Thank you. She is only using it for 9v breadboard assignments at the time.

Thanks

[Ian.M]

Yes, but it just isn't worth the risk of leaving a meter lying around that claims to be rated for mains use, but you've modded so its now unsafe, without tagging it.   If you are the only person with access to it one can risk being a little sloppy, as you cant exactly sue yourself, but as soon as *anyone* else is involved, its foolish not to mark it, and if J. Random 3rdParty has access to it, if you cant restore it to exact original safety specs, it needs to be secured out of use till you can or till you smash it and bin it.

I'm assuming you aren't a single undischarged Chapter 7 Bankrupt (so essentially judgement-proof), with no personal ethics.

[Russ]

Yes, but it just isn't worth the risk of leaving a meter lying around that claims to be rated for mains use, but you've modded so its now unsafe, without tagging it.   If you are the only person with access to it one can risk being a little sloppy, as you cant exactly sue yourself, but as soon as *anyone* else is involved, its foolish not to mark it, and if J. Random 3rdParty has access to it, if you cant restore it to exact original safety specs, it needs to be secured out of use till you can or till you smash it and bin it.

I'm assuming you aren't a single undischarged Chapter 7 Bankrupt (so essentially judgement-proof), with no personal ethics.

Better yet, I will replace the meter and take it out of circulation until the proper fuse arrives.

Thanks
Russ