09-11-2012, 04:34 PM
(09-11-2012, 02:37 PM)Joe Wrote: The datasheet I've got quotes the maximum supply voltage as either ±18V or ±22V, depending on the version. I'd been planning on some sort of dropper as there's now plenty of head-room but the clue as to value is appreciated.
OK, that's a bit higher than I'd remembered for a 741...
Quote:I'll trust you on the 10µF cap; there's already a 3600µF one across the rectifier and the wires aren't very long but it'll certainly do no harm.
Probably only needed if you do use a dropper.
Quote:An ammeter is the obvious solution but I've always been OK connecting a multimeter up if I've needed to know the current and this was only to provide a simple at a glance thing - I wasn't expecting it to prove so complicated. That said, a digital meter would have needed it's own power supply and I'd odds-on have blown it up.
When giving advice about fixing radios, I usually emphasise the importance of monitoring the current drawn by the set. It's like a doctor using a stethoscope...
But, starting simple and building up is a good way.
My first PSU didn't have voltage or current monitoring, and it was pretty rubbish. My next effort used an LM317T to make a variable supply, and a 7805 to provide a fixed 5V supply. The mains transformer came from a crappy '70s music centre that had two windings; one for the audio circuitry (~20) and one for the dial lights (~6V). Pretty nice, and even the box was OK for a homemade effort. But still no metering.
Next was still based on an LM317T (plus big transistor to give more current), but included digital metering. I got an LED module from somewhere, and decided it was time to make use of it. I honestly can't emphasise how useful this was, and would encourage you to consider it. If not for this PSU, then perhaps as a future project. After all, you can never have too many bench power supplies (actually, I've lost count of mine!), and building each one is good experience.
Incidentally, you can see this PSU here: http://www.markhennessy.co.uk/workshop/2...c_kit2.htm
It was clearly made in the case of a Marconi counter - I was given 2 broken ones, and made one good one from the both. With hindsight, I wish that I'd persevered with fixing them both, but to be fair I was only 16 at the time! I'd never strip a fixable bit of gear now, but I think it was worth it when I think back to the lessons learnt at the time.
The beginnings of the next PSU are visible above in that picture, and it is detailed here: http://www.markhennessy.co.uk/dualpsu/index.htm - it's still working well, and the only occasional trouble I get comes from using a certain type of edge connector onto Veroboard tracks on the rear of the front panel. One day I'll replace the arrangement with something better, but as it only stops the front panel buttons from working, it's not too big a deal.
None of the above is intended to be an exercise in "showing off", rather it's just to show that we all started somewhere...
Cheers,
Mark
