[OT] measuring DC wall wart

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sat Jun 8 22:27:40 2002

Well, perhaps you can find out whether or not a given wall-wart can do what
the label says, in general, I've found they don't, even within 10%. With the
fixture I suggested, which takes about 10 minutes to build, maybe 30 if you do
it right, and I agree that a heatsink, in this case a big one, is indicated,
not because there are wall-warts that might damage the 3055, but because a
nice 5"x8" heatsink makes a handy platform to which to attach things, (TO-3's
are rated, typically for 80W, and wall warts aren't.) so you can use a fairly
large 10-turn pot with a vernier or the like, and a big P-clamp to tie it
down. You also get a place to which to screw the barrier strip you will need.
If you want to be really fancy you can even include individual binding posts.

As for replacing one component, a resistor, with three or more, I'd point out
that the resistor approach will, at best, provide an approximation with the
one and only wall wart in the universe, if they've built it yet, that actually
does what the sticker says. The three components I mentioned give you a tool
that will tell you what the voltage is at a given current, and what the
current is at a given voltage, either of which you can measure. If you have
lots of AC wall-warts ( of the 40 or so that I have in the box downstairs,
only about three are AC types.) you need simply put a pridge rectifier at the
collector side of your circuit and you're on your way. When you're done you
have a tool that will measure many different wall-warts, not just one. What's
more, you don't have to go looking for that one resistor that almost works.

Dick

----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <cctalk_at_classiccmp.org>
Sent: Saturday, June 08, 2002 6:12 PM
Subject: Re: [OT] measuring DC wall wart


> >
> > It seems to me that you can take a common TO-3 NPN transistor, e.g. the
> > 2N3055, or something of that sort, drive its base from a pot between the
> > positive and negative voltage, and put a low-valued resistor, e.g. 0.5
ohms or
> > so in the emitter-to-ground path, you have a programmable current sink.
If
>
> True, but why do you insist on over-complicating everything. You want to
> take a circuit consisting of one component (the resistor) that will
> work for AC and DC warts, and which is relatively robust agianst
> overloads and eeplace it with a circuit containing 3 components (at
> least), that only works for DC (and is polarity sensitive), and which
> can burn out easily if overloaded. That 2N3055 is going to need a
> heatsink as well...
>
> You can make power resistors from stuff you find lying about the house
> (old heating elements, etc -- heck you can even use a jar of salt water
> with 2 electrodes in it at a pinch -- electrolysis is a problem if you
> use it for any length of time on DC, but it will act as a power resistor
> for long enough to test a PSU). Not everyone has 2N3055s lying about. I
> do, but...
>
> -tony
>
>
Received on Sat Jun 08 2002 - 22:27:40 BST

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