More 723.

The Dark force of dance batzman at gist.net.au
Thu Jan 22 13:41:06 CET 1998


Y-ellow Y'all.
        'n' thanks t-ya all. I'm still no further advanced but that's mainly
because Robbie, the guy from Derringers, decided he'd have a go at modifying
some HTML on Derringers site. So two days later I think I've got it all
working again and if nothing else, he's down on bended knee and offering to
be my personal bottom wiper.

Paul Perry offered me a less elegant solution by using a dif amp and an
opto. I think that's probably the only real solution off hand. This was
gonna be a quick project. :( In the original PSU they'd done something
similar. However they'd used a transistor as a cro-bar and if you
over-loaded the system it would simply blow the fuse. The problem was that
for some unknown reason the cro-bar kept triggering and blowing the fuse.
Frustrated with this I thought I'll just rebuild the whole thing.

The only reason I was using 723s is because just prior to this I'd designed
a little 3 volt reg to run my walkperson off of. Anything below 5 volts is
hard to attain. Flushed with the success of that, the fact that I've got
quite a number of 723s lying around, and the fact that I remember a big
thread on here concluding that 723s make the most stable regs, I thought
I'll just quickly knock together a couple of quick regs. Never say "It's
just a 5 minute job."

However I realized that hoping for adjustable current sensing was a bit
futile. I was thinking that I could get it to mimic the behavior of other
supplies but in fact all it would do anyway, is turn off when the current
was exceeded. To mimic the sag of other PSUs would require something far
more elaborate.

Out of interest, I'm using a 2n3055 and a 2955 as the series pass
transistors. By my figuring you should be able to draw 3 amps at least
through these guys. As long as they are on suitable heatsinks and your
transformer is up to it.

I would have built two separate supplies and linked them in the middle.
using two positive 723 based regulators. But unfortunately the PSU I chose
to butcher uses a centre tapped transformer. I looked at ways of doing that
with a center tap but it looks not possible. Or at least not practical or
advisable.

I'm in a hurry so I guess I'll just put a fuse in and not worry about
current limiting on the negative supply for now. And next time I'll pick on
something my own size that won't fight back.

In the mean time thanks for all your help. I'll see if I can get something
that will look at those TIF files. I'd really like to dig into Anders's web
site.

Thanks once gain.

Be absolutely Icebox.
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