[SOLVED] What blew my Fluke 179 multimeters fuses?

Hello Arduino Forum :-)

Recently a friend of mine gave me a Fluke 179 multimeter. I tested it out right away and found out that it wouldn't measure amps, I opened it to find out the two fuses was blown. The originally installed in the multimeter was: "Buss Fuse DMM-44/100" and a "Buss fuse DMM-11A" - I've tested them both with my continuity tester and found no connection across them...

Right away I ordered these: Buss Fuse DMM-11A http://www.ebay.com/itm/331072212283?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

Buss Fuse DMM-B-44/100 http://www.ebay.com/itm/380792668022?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

I asked the seller about the DMM-B-44/100 as I didn't know if it would fit because the original one didn't have the same model number, he told me it should fit, so I brought it and received it today. I tested it with my continuity tester before installing it, and it all seemed right.

I was going to test it right away, I moved the red probe to "400 mA" connector and selected the mode called "mA Hz" on the multimeter. I tested it with a small DC motor 12V which used 350mA measured with my power supply. As I connected the multimeter in series with the DC motor the screen on the multimeter showed the same 350mA as my power supply did. I've tested it a half minute and then it suddenly didn't show any amperage use at the multimeter, I couldn't get it to show amperage use again, even though my power supply still showed 350mA. I took the fuse out and tested it with my continuity tester to find out there was no connection...

I don't know much about the Fluke 179 but is it really possible that a dc motor using 350mA killed the fuse? - isn't there some kind of a cut off circuit to prevent this from happening?

I hope someone has an answer as I would like your opinion on if I should by a new fuse, or something else in the multimeter is broken, causing the blowed fuse. If you need any details just tell me :-)

Thank you very much Best regards JohannesTN

isn't there some kind of a cut off circuit to prevent this from happening?

Yeah... the fuse.

JohannesTN: isn't there some kind of a cut off circuit to prevent this from happening?

Yes, it's called a "fuse".

I always wanted to know why multimeter fuses are always buried behind several screws and layers of plastic casing. Wouldn't it make more sense to just put them on the outside or behind a little flap?

Crap... But shouldn't it be safe to measure the 12v 350mA DC motor, without blowing the fuse?

Wouldn't it make more sense to just put them on the outside or behind a little flap?

Too easy...

What value are the fuses? (admittedly I was too lazy to follow the supplied links). If the motor's current exceeds the fuse rating than after a while, the fuse will blow. The delay depends on the fuses characteristics (like slow blow or fast blow) and by how much you are exceeding it's rating.

Also keep in mind that motors have a higher start up current than running current. Any load on the motor will also make it's current consumption rise.

Motors briefly draw their stall current when they start up, and that could be many times the free-running (no load) current. In your case, it may have been a couple of amperes. To test motors, always start with the high current range of the multimeter, usually 10 or 20 amperes.

UnoDueTre: the DMM-B-44/100 has a value of 440Ma and a voltage rating (DC) of 1kV and as far as I can read it is a fast blow fuse

As I said the motor I tested with is a 12v 350Ma type. I guess that in theory it should be okay to measure the amperage used by the motor with that fuse as it didn't at any moment exceed the fuse rating (as far as I can tell), or am I wrong?

Jremington: So this means that it is quite likely that the motor did blow the fuse, and that it isn't some kind of abnormality in my multimeter blowing the fuse?

thank you :-)

One way to tell, assuming that the other fuse is for the high current scale (normally 20A), use that to measure the motor running and leave it on for some 20 minutes or so.

Even though both the DMM and your PSU show 350mA, they are quite slow and current spikes caused by the motor will not show up. As another test, get a 450mA, 600mA and 1A fuse, wire them is series (one a time) with your motor connected to your PSU and see which one/s blow and after what period of time (DMM not connected).

As another test, get some 450mA, 600mA and 1A fuses

Which answers your earlier question..

isn't there some kind of a cut off circuit to prevent this from happening?

Just put smaller fuses in front of the bigger ones then the bigger ones won't pop 8)

JimboZA:
Just put smaller fuses in front of the bigger ones then the bigger ones won’t pop 8)

Haa haa, true, forgot about that.

@JohannesTN

Just to clear up any possible confusion, when I said use higher current fuses, I meant stand alone and not in your DMM.
It would be very unwise to replace the recommended fuses with different ones.

UnoDueTre:

JimboZA: Just put smaller fuses in front of the bigger ones then the bigger ones won't pop 8)

Haa haa, true, forgot about that.

Takes a civil engineer to figure this crap out....

Your assuming is right, the other fuse is rated for 11A 1000V. My own conclusion from your writings is that I should have tested the motor on the high current setting first, to be sure not to kill the 400mA fuse, which I didn't... Since I didn't do that, it is likely the current spikes caused by the motor, that killed the 400mA fuse? - Am I right?

I guess I have to buy a new fuse then, and from now on measure by using the high current setting even though the motor is meant to be 350mA type :-)

JohannesTN: it is likely the current spikes caused by the motor, that killed the 400mA fuse? - Am I right?

My money would be on that, yes.

Okay, I'll buy a new fuse, and remember not to do this again...

Thank you to all of you for helping me out this quickly :-)

No problem. We didn't want you to worry unnecessarily and possibly blow a fuse. :)

Haha, looks like I already did ;-)

In future put small cheap fuses in front of the expensive big ones.....

I will, thank you :-)

That 350mA rated current is going to be at a specific load. Increase the load on a DC motor, current increases.

And as pointed out, when first started a DC motor draws the stall current until it spins up. On a 350mA rated motor, that could be a couple of amps. Even short spikes of current that high will probably blow a 440mA fuse.

Which brings up another point - it isn’t wise to measure current so close to the maximum of the meter like that. Note that you must move the leads to an entirely different jack to measure in the 10A range. So the <=400mA inputs are running nearly at their limit with something rated to draw 350mA. What if it draws 15% more? You are over 400mA.

Especially with a motor. But just about anything above the level of an LED draws a big pulse of current when first powered up. Motors spin up, or capacitors charge, or light bulbs must heat up to operating temperature.