Search all of my sites with Google

Tuesday, September 8, 2009

Melted Multimeter

I was riding along on the way home from work last night when the multimeter started reading really wierd, then suddenly the motor stopped. I was afraid I'd burned out the motor windings or something, but a quick check revealed the motor wasn't even hot yet.

I pulled over under a streetlight, and verified the circuit breaker wasn't popped, so I disconnected the DMM I was using as a voltmeter for the pack voltage, and metered across first the breaker, then the DMM I was using as a motor current meter, with the motor engaged and set (with the clutch disengaged) to half throttle. There was no voltage across the CB, so that's ok, but full voltage was across the DMM, so that meant something was wrong inside that.

As a quick fix, I just unbolted the wires running into it from the banana plugs on it and bolted the wires directly together, bypassing the DMM's shunt, and everything worked ok motor-wise.

When I got home, I opened up the DMM, and found this:

It's a little hard to see in that pic if you don't know what you're looking for, so here's a bigger one:

The solder holding the shunt to the PCB has actually **MELTED**, and the shunt has come completely out of the PCB thruhole at the top end.

Wow. Well, considering it's only rated for 10A, and even that is almost certainly a limited-time spec (can't remember the specific time, but this is the $2 Harbor Freight Centech DMM, so it's probably in the online PDF manual for it if you are just dying to know). With the testing I've been doing it's probably had 30-50A thru it for quite a few times for at least a second or three during acceleration from a complete stop, and had constant 6-11A thru it during cruising for at least several minutes at a time.

So I really wasnt' that surprised that it failed like this, only that it took this long to do it.

I soldered the shunt back on, and now it works fine again. :)

I'm thinking of taking the shunt out of that failed 30A panel meter and temporarily adding it to this one, if I can figure out if there is a way to recalibrate this meter to read correctly with the larger shunt (probably not).

If I can't, no biggie. I have some mA-scale panel meters that I can put that shunt in, instead, and mark the current draw on them based on my Fluke's readings thru the same circuit, then draw up a new faceplate in the computer, print it out, and put it behind the needle of the panel meter. It'll be accurate enough for my purposes, plus I love analog meter displays. :)

But for those of you using those cheap meters, be careful with the 10A range--they're not fused, so you'll melt the solder holding the shunt in, possibly burn the PCB, and maybe even melt your DMM's casing, if you do what I did and aren't lucky enough to have the shunt fall out before anything *REALLY* bad happened. ;-)

1 comment:

  1. I actually do have a couple of the Centech's that I must have gotten from Harbor freight as spares or something.

    As long as I can keep this DSL lite, posting seems better.


Alternate suggestions or improvements to anything that's been posted is very welcome, and extreme detail is preferred to brevity.

Keep in mind that unless you leave an email address in your comment, I haven't any way to reply to you except to reply to your comment here. That means if you want a reply, you'll have to come back to *this* blog entry and it's comments to see my reply to you, unless you leave some method of contact within your comment.