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
you then tweak the pot until the voltage on the collector is the rated
voltage, the voltage on the emitter will tell you what the current is. Else,
you can tweak the pot until the voltage on the emitter is such that it
reflects the rated current, based on the resistor value. The emitter will
follow the base voltage, hence the term "emitter follower." By doing this,
you eliminate the need for a high-power resistor, and, you have a wider range
of voltages and currents you can monitor. If you're concerned about the
transistor's gain, you can use a darlington pair.
Dick
----- Original Message -----
From: "Chris" <mythtech_at_mac.com>
To: "Classic Computer" <cctalk_at_classiccmp.org>
Sent: Friday, June 07, 2002 1:13 PM
Subject: Re: [OT] measuring DC wall wart
> >It does sound a _little_ high, but it might be OK. Try loading it (A 100
> >Ohm wirewound resistor would do, for example). See if that brings the
> >voltage down to nearer 9V.
>
> I tried a 100k Ohm since I didn't have a 100 Ohm, no effect. I don't have
> a bulb or anything handy to try with that.
>
> I think I am going to dig out the modem it goes to, and test the voltage
> with it hooked to that. That will at least give it real use load (the
> modem powers up just fine, it just fails to respond to all AT commands
> except for getting the firmware, and it won't attempt to go off hook or
> dial... I'm not 100% convinced the modem is really "dead" so much as it
> just doesn't want to co-operate).
>
> Thanks to all those that responded... I knew I could get some good
> answers here.
>
> -chris
> <http://www.mythtech.net>
>
>
Received on Fri Jun 07 2002 - 19:44:01 BST