Current Flowing Indicator.

[Mark R]

You might have already seen it, but for some more general information about op-amps, some anorak has written a reasonably good article. It has a bit of an audio bias and doesn't say much about the power supplies, but it might be helpful anyway..

http://www.markhennessy.co.uk/articles/op-amps.htm

Hope this helps, and sorry it's a bit of a ramble!

Mark

I must say the anorak that wrote the article in the link is very good, what a most informative link that is!

cheers Mark

[Skywave]

Joe - if you're having problems with using an op-amp. for this, you might be better off with an LM311 voltage comparator (one of my favourite i.c.s).

LM311 tech. datasheet: [attachment=6138]

Al.

[Joe]

Certainly worth looking into, Al.

I've had a re-wire and now have it configured thus:-

   

The LED is still permanently lit, though I THINK I know why: off load there's a drop of 3.1mV across Rs, which I assume to down to the current flowing though the 390Ω resistor and 4.7kΩ pot. This raises to 26mV on load at 25mA.

The principal seems sound; it just needs tinkering with to lower the sensitivity a bit.

- Joe

[Nick]

The principal seems sound; it just needs tinkering with to lower the sensitivity a bit.

Could always just put another 1R resistor in parallel with Rs...

[Joe]

I THINK I've got it:-

   

I thought about lowering the value of Rs, Nick, but that's the only 1Ω resistor I've got, and none smaller.

Off-load there's 1.5V across the LED and on-load 1.7V which is enough to light it. A high intensity type would be better but the only ones I've got are white which I'm not keen on as indicators.

The next question will be if a properly soldered-up version works.

- Joe

[Skywave]

Joe - I've just re-read your post #18. if I may just shoot off on a tangent for half a mo' (this is relevant, bear with me, please), is your requirement for this whole PSU project only for charging NiMH AA-size batteries?

Al.

[Joe]

No, Al. It's really a general purpose bench PSU which gets used for battery charging amongst other things.

- Joe

[Mark Hennessy]

Hi Joe,

Looking good - sounds like you're getting there, and understanding what you're doing, which is the important thing...

I've just noticed how high your UNREG supply is, and that's pushing things a bit for a 741. I'd be tempted to install a resistor in series with the supply pin to drop this down a little - perhaps 1K or 2K2 - maybe a bit more. You'd need to put a capacitor between pins 7 and 4 of the op-amp - perhaps 10uF would be OK here.

Alternatively, a Zener diode might be better - a 10V or thereabouts should do. This has the advantage of being pretty constant in light (sorry!) of the changing current consumption caused by the LED being on or off.

If you don't get much joy playing with the hysteresis resistor, adding a resistor between +IN and ground might work. Perhaps try 100K... It's just to set up an offset that corrects for the fact you have that voltage across the sense resistor...

Of course, you could consider adding current metering to your PSU...

[Skywave]

Joe -if I may just shoot off on a tangent for half a mo', is your requirement for this whole PSU project only for charging NiMH AA-size batteries?
Al.

No, Al. It's really a general purpose bench PSU which gets used for battery charging amongst other things.
- Joe

O.K.; thank you. The idea I have is not relevant then; tangent terminated.

Al.

[Joe]

That's a shame, Al - we go down some very interesting tangents as a rule.

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.

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.

I'll have fiddle with a 100kΩ resistor across the +IN and ground as well.

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.

- Joe