Re: Multimeter damaged when powered from regulated power suppy.

Chuck Harris wrote:
> 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

I accidentally posted it to this group, my intention was
electronics.basics group. My apologies.

> 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 tranformer I used is a step down one 9-0-9 seondary one, which is
used in normal 9V regulated supplies.

> 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.

Do you mean to say it is difficult to power the meter from a step down
regulator, and measure the mains at the same time ?

>
> 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.

Since I used a normal step down transformer, I don't see the danger.
Please correct me if I am wrong.

Thanks!!

.

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