Current Flowing Indicator.

[Skywave]

. . . and this was only to provide a simple at a glance thing - I wasn't expecting it to prove so complicated.
- Joe

Ah! But that's the rub, isn't it? What appears to be complicated to one is quite simple to another: only practical experience and private study is the long, winding path from the former to the latter condition. I've about 50 years of experience 'doing things' with electronics (as a hobby and professionally) - and still have a lot to learn. The one motto that I have learnt in this game is always to employ the 3 Ps: Patience, Persistence & Perseverance.
Keep at it Joe - you know you'll get there in the end - and be a bit more knowledgeable as a consequence.

Al.

[Joe]

Exactly, Al. I could have paid out for a moving coil 1A ammeter which would have been much easier but I wouldn't have learnt anything of had as much fun.

Cheers

Joe

[Mark Hennessy]

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...

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.

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

[Joe]

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...

A darned good analogy, Mark. I see myself more an electronic paramedic than a fully-qualified doctor though.

The proper version of this arrangement works:-

   

I can't call it finished yet as the 1Ω sense resistor is only a ½ or maybe even ¼ Watt one - I can't tell just by looking. Even I can do the maths to know that 1A² times 1Ω equals 1 Watt. When I get one of those I'll also get and try a ½Ω one.

The unregulated voltage was higher than I'd thought and the 25V smoother was looking very upset so I've changed that for a 50V one. It quickly drops on load.

Thanks for the help, everybody.

- Joe

[Skywave]

Hi Joe - so apart from the relatively small issue over the sense resistor, the job is finished: I knew you'd get there in the end. So, perhaps it's time to show us the complete cct. diag. of the entire PSU?

Al.

[Joe]

What are you trying to do to me, Al - you've seen my draughtsmanship.

I will have a go at drawing it out though. I'd never bothered before as it's based on the example circuits on the LM317 data sheet rather than being designed by me in any meaningful way.

- Joe

[Joe]

OK, Al: here it is:-

   

I think it's accurate.

Now to hide from Dr Mark before he starts on me about lack of de-coupling.

- Joe

[pwdrive]

That bridge looks wrong?

Lawrence.

[Skywave]

What are you trying to do to me, Al - you've seen my draughtsmanship.
- Joe

And quite adequate it is too - as we can see from the full drawing that you've now sent in - which I haven't seen before. Primarily, I asked about the full cct. diag. since I was a bit concerned about your comment, re: "the unregulated voltage quickly drops on load" and thought perhaps I could offer some advice.

Al.

[AlanBeckett]

Lawrence,

You beat me to it

Alan