'12' volt power supply.

I need a fairly large 12 volt DC power supply. About 8 amps peak. I already have a regulated 5 amp one but that struggles for the purpose I need. It needs to have an output of say between 12-14v at full load. And be made from bits I have lying around. ;-)

I have two suitable transformers - both toroidal. One I'd guess is a low volt lighting one marked 230v primary, 11.8v 100VA secondary. But with a bridge rectifier and 30,000µF of smoothing gives 14.6 with a 55 watt load. And somewhat over 17v with a small load.

The other transformer is an electronics type with two primaries marked

115v and two secondaries marked 12v 5 amp. Would that likely have any better regulation?

I'd like to keep the max voltage within what would be expected in a car - and the device takes a very small current with the 'engine' stopped.

The ideal would be to limit - rather than regulate - the peak voltage from the first transformer - but most circuits I can find are actual regulators. Any suggestions? Other than just use a car battery? ;-)

Reply to
Dave Plowman (News)
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Not really! A car battery sounds like an obvious choice - just connected to a trickle charger to keep it topped up.

Reply to
Roger Mills

Such stuff is so cheap now as switched-mode chassis power supplies, it's not really worth making - unless you need something rather unusual, or enjoy making.

The briefest scan of ebay found this supplier, but there's plenty more selling odd items or secondhand at still lower prices.

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Reply to
RubberBiker

If you have one handy that would seem a very good idea :-)

Neither of your transformers will have any regulation unless you add some. But maybe you don't actually have a problem. The 14.6v under load and 17v unloaded is not vastly above what's found at a car battery terminal when it's charging. And maybe the device you want to run (are you going to reveal what it is?) has some regulation of its own.

Reply to
Laurence Payne

Find an ex CB radio power supply..plenty on Ebay, or you can buy new for sensible money.

Designed to replace a 12v battery, typically 13.4v output.

Reply to
The Natural Philosopher

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indeed. But takes a while to arrive. I knocked this one up in a couple of hours for free, using bits from the junk box.

Reply to
Dave Plowman (News)

Yes - they are regulated. Just being ultra careful. Of course you'd see 17 volts plus in a car if the alternator regulator failed.

Reply to
Dave Plowman (News)

Try it and see? Too many variables. I'd expect the lighting toroid to be specced for a nominal 12v at full load and engineered down to a price. Hence it being rather high when off load. The electronics one may well have better regulation but the only real way would be to try it.

The two 12v windings at 5A each is 120VA, connect in parallel but make sure you get them in phase with each other. The primary in series of course.

Reply to
Dave Liquorice

Those transformers are the wrong voltage. Try a 9v one, you'll be about right on the dc side.

NT

Reply to
NT

That one isn't ACTING like it's regulated :-)

What about an old computer psu?

Reply to
Laurence Payne

ITYM 13.8v.

Reply to
Dave Plowman (News)

Strangely the twin winding one is slightly smaller than the single one - despite being 10 amp as opposed to 100VA.

Reply to
Dave Plowman (News)

How about any old junk PC power supply? They can be got for free from any old junk CPU chassis from your local recycling centre.

This one claims 15A on the 15v rail.

To make it power up, jumper the thin green wire on the motherboard plug to one of the black ground wires next to it.

Reply to
Ron Lowe

Typo..

^12v rail.

Reply to
Ron Lowe

Computer power supply?

Hmm... Getting the several hundred volt spikes you often find on car electrical circuits might be difficult :-)

Reply to
Peter Parry

Dave Plowman (News) formulated on Monday :

The usual way would be to series regulate the voltage, but neither unit has the voltage 'headroom' to allow a series regulator to work. The only other alternative is parallel regulation, which means the regulator system basically shorts the surplus current (voltage) - doing that would mean running the thing flat out all the time, turning lots of energy into heat - or at least any that wasn't used usefully. Not at all advisable.

Adding rectification and some smoothing always increases the voltage beyond the basic AC output voltage of the transformer by x 1.41, assuming no load. To be able to series regulate, most regulators need as input voltage at least the intended output voltage + 1.5v as measured on full design load.

CB power supplies rated 8 to 10amp are only really suitable for a very short intermitant load at that rating. 8 to 10amp I would only rate at around 3amp continuous load.

Were I needing something similar, I would be looking at converting a PC power supply. Either connect the high current 5v outputs of three identical PC PSU's in series to provide 15v (5v + 5v + 5v) - four x

30amp forward biased diodes in series, will then drop that down to around 13.6v.

Or take one PC SMPSU, find the voltage feedback network and adjust it so that the 5v output provides your 13.8v. There are several web pages on the net which document doing either of the above.

Yet another method which I have used several times is to convert ex-telecomms 50v SMPSU's to output 13.8v, using a similar process to the above. Last time I checked you could buy surplus 50v 50amp jobs for £10 to £15.

Reply to
Harry Bloomfield

Yup - I'd forgotten about those. And have one which was replaced on spec on a PC which shut down at random. And continued to do so after the PS was replaced. Only slight snag is the 12 volt rail is slightly under - I'd have preferred the 13.8v you get in a car. For soak testing.

Yup.

Reply to
Dave Plowman (News)

I did have a play using a 2N3055 and zener as a series reg but without success. It would regulate fine at 14 volts on a light load - with a small bias resistor - but increase the bias current to switch 8 amps and the volts came down to under 12.

Yup. I was trying to remember the figure for using two diodes and a centre tap. Smoothing isn't a problem. But I've a feeling the DC achieved that way is less than the nominal AC voltage.

Looking at Ebay some actually say this.

Interesting - I'll do a search. The one I've tried gives 11.8 on my test load.

Right. Plenty food for thought - thanks.

Reply to
Dave Plowman (News)

You may well be right..

Reply to
The Natural Philosopher

Ah, well.. There's the DIY element.

Rip it open, and find the zenner or whatever which sets the voltage, and throw a couple of diodes in series to drop a further 1.5ish volts giving the controller a 13.5ish reference...

Reply to
Ron Lowe

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