If you want to measure the voltage at the rated current, you have to provide a
current sink that's set up for that current. That's easily done with an
isolated negative supply and a current mirror. If you really want to do this,
contact me off-list and I'll provide detailed specifications.
Dick
----- Original Message -----
From: "Loboyko Steve" <sloboyko_at_yahoo.com>
To: <cctalk_at_classiccmp.org>
Sent: Friday, June 07, 2002 11:33 AM
Subject: Re: [OT] measuring DC wall wart
> Been my experience that a wall wart under no load (or
> any transformer under no load for that matter) will go
> a LOT higher than its 'rating'. I'd put, like, a 500
> ohm resistor across it (if it's less than 1/2 watt,
> might get "a little warm"!!) and measure it again. BTW
> a surprising amount of equipment has negative tip. If
> you aren't sure about the equipment I'd take a meter
> and look for ~0 ohm from a ground to the "ring" of the
> power connector. Badly made equipment didn't have ANY
> protection from reverse polarity.
>
> I like the RS adapers; just about the most
> useful+reasonably-priced thing in the darned store.
>
>
>
> --- Chris <mythtech_at_mac.com> wrote:
> > This is off topic, since the wall wart is only a few
> > years old, but there
> > are some great electrical people here, so I am
> > asking because I know
> > someone will know the answer.
> >
> > I have a wall wart to a modem. I got the modem and
> > wart in a box of used
> > stuff (ooh, some of which was more than 10 years
> > old, so at least there
> > is a reference to on-topicness).
> >
> > The modem failed to work from the day I got it.
> > Today, someone that has
> > the same modem asked if I had a power supply they
> > could have. I planned
> > to give them mine, since I have no real use for it.
> >
> > But before I mailed it to them, a voice went off in
> > my head telling me to
> > stick it on a meter first. It's label says it is a
> > 9vDC, 400mA Pos-tip
> > wart. But when I hook it up to a meter, I get 15v DC
> > off it.
> >
> > That seems a little high, even for something under
> > no load. Should I
> > safely assume the wart is no good, and point the
> > person elsewhere (Rat
> > Shack sells a usuable unit for only about $14, so it
> > isn't like the
> > person has no other choice). Or is it normal for a
> > 9v supply to put out
> > 15 volts? Any that I have measured in the past have
> > not been THAT far off
> > (I was expecting to maybe get 10 or 12 volts, but
> > not 15).
> >
> > If this thing has been pumping too much voltage,
> > that might explain the
> > dead modem I have (or the modem could have been used
> > as a soccer ball,
> > who knows, it was dead when I got it). I just don't
> > want to send the
> > person a bad supply, and have them fry their modem
> > when they try to use
> > it.
> >
> > -chris
> > <http://www.mythtech.net>
> >
>
>
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Received on Fri Jun 07 2002 - 13:55:33 BST