'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 30,000F 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? ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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In an earlier contribution to this discussion,

Not really! A car battery sounds like an obvious choice - just connected to a trickle charger to keep it topped up.
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Roger Mills wrote:

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.
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ITYM 13.8v.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

You may well be right..
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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.
http://shop.ebay.co.uk/merchant/circuitspecialistseurope?_nkw v+power+supply&_sacat=0&_trksid=p3911.m270.l1311&_odkw=&_osacat=0
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In article

http://shop.ebay.co.uk/merchant/circuitspecialistseurope?_nkw v+power+supply&_sacat=0&_trksid=p3911.m270.l1311&_odkw=&_osacat=0
Oh 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.
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On Mon, 03 Aug 2009 09:43:09 +0100, "Dave Plowman (News)"
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.
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Yes - they are regulated. Just being ultra careful. Of course you'd see 17 volts plus in a car if the alternator regulator failed.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Mon, 03 Aug 2009 11:04:49 +0100, "Dave Plowman (News)"

That one isn't ACTING like it's regulated :-)
What about an old computer psu?
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On Mon, 03 Aug 2009 09:43:09 +0100, Dave Plowman (News) wrote:

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.
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Strangely the twin winding one is slightly smaller than the single one - despite being 10 amp as opposed to 100VA.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Those transformers are the wrong voltage. Try a 9v one, you'll be about right on the dc side.
NT
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Dave Plowman (News) wrote:

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.
<grabs random one from shelf> 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.
--
Ron



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Typo..

^12v rail.
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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.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

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...
--
R

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It's a bit of a 'whatever' ;-)
I've spent some time Googling on upping the voltage and not really found a step by step guide which suits my PS. And of course I've no circuit for it.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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In the end it proved simple to modify an ATX PS. But Googling on it threw up the most enormous amount of red herrings - some involving near complete redesign of the 'low volt' side.
The PS I have is relatively new - about 4 years old - and uses a SC6105 supervisor, so only the one IC in the entire device. Altering the voltage to the error input - pin17 - by a simple resistor change upped the output to 13.8v. Which is within the voltage tolerance of the overvoltage protection.
The PS I have differs considerably from the circuits given in the SC6105 spec sheet, though.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Mon, 03 Aug 2009 09:43:09 +0100, "Dave Plowman (News)"

Computer power supply?

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