Re: Multimeter damaged when powered from regulated power suppy.
- From: "Geogle" <v2geo@xxxxxxxxxx>
- Date: 25 Jan 2006 23:16:34 -0800
Phil Allison wrote:
> "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.
>
I wanted a cheap alternative to a digital volt meter for measuring AC
Mains voltages, and wanted to keep it on for hours together.
It was to be used to monitor the mains supply, not for any other
purpose. Since nobody
is going to use it as a multimeter, I guess there won't be any problem
regarding personal safety. Keeping the personal safety apart is there
anything electrically wrong with
the setup ? Would any additions to the analog ( measurement ) inputs
help keep the instrument from damage ?
Thanks!!
>
>
>
> ........ Phil
.
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