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Adventures in Engineering - Multi-meter? Moar liek MELTY-METER, amirite??
The wanderings of a modern ronin.

Ben Cantrick
  Date: 2013-01-30 23:03
  Subject:   Multi-meter? Moar liek MELTY-METER, amirite??
Public
  Music:KMFDM - Power
  Tags:  fail, failure as an electronics technician, smells like victory, the smell of burning polystyrene
Oh the lulz...

One our our techs at work was tasked with repairing and testing a very large (1,000 W) power supply unit. Meet the Sorensen DCS 20-50:


The front panel


From the side

Notice the enormous transformer center-bottom. That part alone weighs about 60 lbs. All told the PSU weighs maybe 90 lbs. The pictures don't do it justice, imagine it 20% larger than it looks here.


Never let the magic smoke out

His first idea was to run 5 amps through this little 28 gauge clip lead. Poor thing, it didn't stand a chance...


Second attempt - meter is measuring amps

He got smarter(?) the second time. Using a 0.5 ohm, 225 watt power resistor. Judging by the meters on the front of the supply, we have ~20A @ ~11V for a total of ~220 watts. (Less than a fourth of the unit's rated maximum...)

Unfortunately, he also wanted to watch the current in real time, and so left his meter hooked up in series with the power resistor!

Kids, your meters are not supposed to be able to run 20A continuous! They're only supposed to handle that kind of current for about 60 seconds, at most. He plugged the meter in, turned the PSU up to 11, and then just walked away... for half an hour! ("Need to do a proper load test.") When he came back, the smell of burning plastic was in the air and the meter had shut itself off in an attempt to stop the thermal overload...


Melty-meter!

Is that the outline of an HRC fuse melted into the back of the meter?! ;D


Open the meter up

Sure enough, the HRC fuse has melted itself several millimeters into the back casing!


Pry fuse off with screwdriver

"No serious damage." (!!!!)



Hm, that mA/Cx socket doesn't look so good...


Whoops!

On closer inspection we find the plastic ends of the leads that plug into the meter... have melted onto the inside of the meter!


Oh dear...

Attempts to remove the melted leads were... not entirely successful. ;]


Post-script: A fresh set of leads was plugged into the meter. The prongs that held the HRC fuse inside had melted off the PCB, but aside from the 20A current measurement mode, the meter appears to be working just fine! I tip my hat to you, Wavetek - that is one tough meter!
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Coinneach Fitzpatrick: Feed the kitty
  User: scarybaldguy
  Date: 2013-01-31 06:05 (UTC)
  Subject:   (no subject)
Keyword:Feed the kitty
That'll buff right out.
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Helvetica 'Foofers' Bold: Shiny Bubbles
  User: foofers
  Date: 2013-01-31 06:12 (UTC)
  Subject:   (no subject)
Keyword:Shiny Bubbles
A little WD-40, it'll be good as new.
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McKavian
  User: mckavian
  Date: 2013-01-31 14:07 (UTC)
  Subject:   (no subject)
Off topic, but could you please put this behind a cut? While interesting, the picture size is throwing off my LJ.
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Ben Cantrick: ronin
  User: mackys
  Date: 2013-01-31 17:20 (UTC)
  Subject:   (no subject)
Keyword:ronin
I should have thought of that.

Done!
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Alex Belits: photo-shortest
  User: abelits
  Date: 2013-02-01 03:41 (UTC)
  Subject:   (no subject)
Keyword:photo-shortest
Captain Obvious: What happened to the idea of trusting that big freaking metal wire spiral in a sane temperature range really has constant resistance, and measuring voltage instead of current?
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Ben Cantrick: ronin
  User: mackys
  Date: 2013-02-03 00:56 (UTC)
  Subject:   (no subject)
Keyword:ronin
The tech said that he got pulled away by a customer with a question, and didn't mean to leave the meter inline for that long. He was just going to measure it for one minute at the start of the test, run it for half an hour (without the meter), and then put the meter back in again for one minute at the end of the test to verify the current was still the same.

A likely story!
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