5
\$\begingroup\$

There is a DMM (not a cheap one, so its worth to repair it), whose leads where put into a 230V AC socket on the frequency measuring range. The meter did "bzzzt" sound and stopped showing anything.

Neither of its two fuses (1A and 10A) got blown (I checked with a resistance meter, both of them have less then an ohm).

What element would usually get fried in such situation? Is there any chance to repair it, by replacing some elements? Maybe its a common accident, so someone might know what usually gets burnt in such situation.

It was an auto-ranging DMM, the setting it had when in contact with 230V AC was:

  • Frequency: 100Hz - 10MHz ±0.05%

EDIT, more details:

I opened it, and it looks like there are three transistors (or maybe something else with 3 legs?) on the board fried, and nothing else looks like fried. They are however fried to the extent that it is not possible to find out what markings where on them, so I don't know with what kind of transistors (or whatever this is) I can try to replace them?

All the three fried elements are about 2.9 x 1.3 mm in size, are SMD with three legs, look exactly like SOT-23:

SOT-23

Two of them are next to each other, and their descriptions on the PCB are: TP1 and TP2. The third element is in a different region of the board, and is described TC on the board. These are the only discrete transistor-looking-like elements on the board.

I would be very grateful for some wild guesses with what elements, with what parameters to replace them. Are they transistors or maybe voltage reference? Its worth to do it, even if it takes me several tries to find the right one.

The DMM was a Digitek DT80000.

\$\endgroup\$
3
  • 2
    \$\begingroup\$ You could help a lot by providing model number of the multimeter. \$\endgroup\$ – AndrejaKo Apr 8 '11 at 10:58
  • \$\begingroup\$ @AndrejaKo: provided link and description of what is fried. \$\endgroup\$ – miernik Apr 8 '11 at 11:06
  • \$\begingroup\$ I suspect it's irreparably shagged. \$\endgroup\$ – Tom O'Connor Apr 8 '11 at 14:52
5
\$\begingroup\$

Small parts like transistors don't stop the 230V. It's very probable that most of the 230V went into the controller and that that is fried as well. This would explain why the meter doesn't do nothing anymore. Having the controller replaced may cost you more than a new DMM (if they replace it at all, you'll probably get a replacement DMM). This is a custom IC which you won't find to do it yourself.
It's impossible to identify the fried parts without a schematic if they don't have readable markings anymore. (It's even impossible to identify good parts without marking with absolute certainty.)
TP1 and TP2 probably refer to test points rather than components; TP is normally not used as a prefix for components.

\$\endgroup\$
4
\$\begingroup\$

If the display isn't working, it's probably damaged beyond repair as the chip will have been zapped as well as the other devices on the input. It's a bit surprising that it failed if it is a good quality instrument, it should have been protected against such mishaps. A typical Fluke DMM will take 1000V on all ranges, IIRC.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.