Re: Multimeter damaged when powered from regulated power suppy.


"Geogle"
> 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.


** You are wrong !

A portable DMM must only be powered its *internal* battery. It is critical
for human safety that the battery remain inside the meter and impossible to
touch while the meter is in use.

Forget the externally supply idea completely !!.

It is lethal to YOU as well as the meter.




......... Phil



.

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