Multimeter damaged when powered from regulated power suppy.

[Geogle]

I tried powering a cheap 7107 (LCD) based multimeter using a 9V(7809)
regulated powersuppy and an isolation transformer. When I tried
measring the mains supply in ac HV setting, suddenly the multimeter
stopped functioning - even the display stopeed functioning from then
on. Is there something fundamentally wrong with the setup ?
Thanks!

 

[Chuck Harris]

Geogle wrote:

I tried powering a cheap 7107 (LCD) based multimeter using a 9V(7809)
regulated powersuppy and an isolation transformer. When I tried
measring the mains supply in ac HV setting, suddenly the multimeter
stopped functioning - even the display stopeed functioning from then
on. Is there something fundamentally wrong with the setup ?
Thanks!

I'm not sure what this has to do with CAD, but yes there is a fundamental
problem with what you did. The electronics inside of the multimeter are
intended to remain floating from the test leads. The transformer you
used has no isolation shield, so it forms a capacitive coupling between
the voltage from the power line and the internal electronics of the DVM.
This causes the internal electronics of your DVM to ride up and down in
voltage with the AC mains. When you connected the test leads to the
power mains, you provided the capacitive coupling with a completed circuit,
and it put mains current through some sensitive circuitry in an unsafe way,
and poof some more electronics trash was created.

The lucky part is you didn't get hurt by touching any of the parts of
your home made power supply. The ground side was most probably at
full mains potential when you connected the black lead of your DVM
to the mains connection.

-Chuck