rackmount case

No. This was so in the past, but today the motherboard has a local voltage converter that takes +12v and converts it down to the voltage the CPU and RAM needs.

It is this change that required the extra +12v connector that you see on ATX and later systems.

Reply to
Rob
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If that what you need, try a mainframe box: keeps the pies warm for that midnite operator feast. Just be careful not to spill gravy down the boards when getting the pies out. (A true event BTW - I think the box contained an S360/40 or similar).

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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie

There's no output transformer in a switch-mode PSU, so the 12v rail will probably be fed from a parallel switch-mode supply, albeit one with a lower current rating. It may have a capacitor across its o/p to deal with disk start-up spikes.

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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie

Power drill, metal too thick for my hand nibbler, hand saw and a lot of tidying with a file. I'm not set up for steel, most of my work is in thin aluminium...

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Reply to
Stan Barr

there most definitely is.

otherwise the possibility of putting mains voltage on the 0v would be extreme.

If you look inside any PSU you will see one large toroid (generally) That's the output transformer.

the 5V is used to derive the feedback to the switcher. The 12v lines are not directly regulated.

In essence the SMPS cosniste of a mains rectifier and smoother, with associated RF suppression stuff, that feeds ~400V (European) into the swticher, which these days is a chip, feeding usually a pair of mains rated MOSFETS that chop the primary of the output transformer. The main winding is spilt into a banlanced output with taps at approcaximately

5vbm and 212v balanced. Thats means that +-5v and +-12cvv are all part of te dsame syste, and te + and -5V are in general used to regulate the switcher. the 12v takes care of itself, because the transformer is tight coupled enough to not sag too much under the 12v loads.

here is a simpler circuit

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The mains shit is top left, the output transformer is top right, the actual one here uses power transistors driven by driver transformers, which dates it a bit.

the chip and ancillary stuff is bottom and center left.

Look at the circuit diagram I posted., It is perfectly clear how it is all done, and its is nothing like what you describe.

Nor indeed is there any reason to do it 'your way'

Mind you, I don't suppose my 20 years designing analogue circuitry including switched mode power supplies counts for anything against your superior OPINION.

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Reply to
The Natural Philosopher

?????????

So what about the additional 12V connectors for GPU and CPU? Some server motherboards use only 12V, with everything else run off regulators on the motherboard.

That's old.

Or cook the rectifier diodes, more likely.

Are you sure about that? I've read otherwise. No, I can't be bothered to find and cite reliable sources, I'm not that interested.

You think a plain empty rack box is cheaper than a server case (which you'd have to modify heavily anyway)?

Reply to
Rob Morley

No, a PC style SMPS has a single transformer that has separate windings and rectifiers for each output voltage.

However, the ratio of current capabilities of the windings has shifted over time. At first, the system was mainly powered from +5 and the +12 was only for disk motors and auxiliary voltages. Then +3.3 was added to power the CPU and RAM, and the power of +5 decreased. For a while there often was a second power connector with

+3.3 and +5 to satisfy the ever increasing power demand of CPUs. In those days a PC power supply could supply 40A at +5.

Later, with the appearance of even lower voltages for CPU and RAM, the motherboard got a hefty switchmode regulator of its own that is powered from +12 and regulates down to +1.8 or so. So now the output capability of +12 increased a lot, +3.3 and +5 are no longer so important.

Similar to the aux +3.3/+5 connector, as the standardized connector had 4 leads for +5 and only one for +12, there was an extra connector for +12 (with 4 leads), and later the standardized connector was extended by 4 pins, one for each voltage. There also is an aux +12v connector with 6 pins for power-hungry video cards now. Again, these are powered from +12 rather than +5 and regulated down to the required voltage on the board itself.

All in all, PC power supplies become less and less useful as heavy-duty

+5v supplies as needed to power many PIs. They may provide only 5A at +5.

Of course dedicated +5 supplies are still available, although at a higher pricepoint.

Reply to
Rob

the principles havent changed. sure its FETS not rannies..

possibly. those can be uprated :-)

people who are proved wrong often lose interest.

no I think given the volume its more likely more expensive, which is why I recommended a server case. That handily also comes with a power supply.

But then what do I know. I only spent ten years designing cases for the electronics I designed to go inside them.

But since you aren't interested in the benefit of that experience, I'll f*ck off now and do something more productive.

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Reply to
The Natural Philosopher

Nope - not in all cases.

I happen to have a 200w dual input voltage switch-mode PSU, thats old enough to be compatible with an IBM PC-AT case and with +/-5v, +/- 12v optputs, sitting on my bench, so I whipped its lid off.

The only transformer in it is at the 250v end of the PCB. Its quite lightweight, probably 30mm x 30mm x 50mm and most likely used to convert

110v to 240v if '110v operation' was selected.

There is also a roughly 25mm diam toroid in the low voltage section but, as this has equal numbers of turns on each (heavy gauge) winding, so is certainly not doing any form of voltage conversion, but is probably part of a low pass filter to keep any PSU cruft from getting onto the motherboard.

The whole point of switch-mode power supplies is that they replace the traditional expensive, heavy high current transformer and associated diode bridge and smoothing capacitor with a much smaller, cheaper and lighter piece of electronics. Otherwise, why bother with all that complexity?

Look inside the cigar-lighter plug of any eBay cable for charging USB devices in a car and you'll find a tiny switch-mode PSU circuit board. There's no transformer or toroid involved.

supply-200w.jpg

That would appear to be one of a range of possible circuits, but its apparently not the same as what's in my Task SMPSU and definitely nothing like the switch mode 12v to 5v converters.

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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie

I could take that as a challenge ... or not. :-) I don't mind being wrong, if it matters to you.

Reply to
Rob Morley

No, this is the central transformer of the SMPS. The 110/240 selection is made by a tricky circuit with diodes and two capacitors, that operates as a voltage doubler when run on 110 and as a rectifier when run on 240. It provides the ~300v DC that supplies the switcher.

The transformer has several windings that provide the output voltages, and regulation is done on only one of them. The current ratio of the outputs is determined by the design, and it has shifted over time as the PC developed. See my other posting.

Reply to
Rob

No that is the isolation transformer. It is a bit misleading to use the term "output transfomer" for it IMHO.

That's not quite the same as a mains powered SMPSU. You need the galvanic isolation in a mains powered device.

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Cheers 
Dave.
Reply to
Dave Liquorice

OK. I read TNP as saying that a transformer was an essential component in a switch-mode PSU. I merely used that as a readily accessible counter- example of a transformerless SMPSU.

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martin@   | Martin Gregorie 
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Reply to
Martin Gregorie

Yup, been there, done that, had the blisters.

The thing that takes most time these days, is often setting everything up. Something like that, once set up, it would just be a case of sticking a slot drill in the collet and running round the periphery. You'd finish up with radiused corners of course and you'd still need to get out the file if you wanted them square.

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Reply to
Stuart

Modern switch mode power supplied consist of a non-isolated off-mains power supply (with voltage doubling if set for 115v) followed by the switcher driving the primary of the *only* power transformer in the supply--providing isolation of the outputs--followed by rectifiers for the various output voltages (only one of which is used for regulation feedback) and filters. That's it.

-michael - NadaNet 3.1 and AppleCrate II:

formatting link

Reply to
Michael J. Mahon

You should probably have an angle grinder, just in case ...

Reply to
Rob Morley

On Sun, 18 Aug 2013 17:11:00 +0100, The Natural Philosopher declaimed the following:

USB is 5V (for USB 2, 5V and up to 0.5A to be distributed among four ports; USB ports above 0.5A capability are "special")

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	Wulfraed                 Dennis Lee Bieber         AF6VN 
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Reply to
Dennis Lee Bieber

On Sun, 18 Aug 2013 17:50:44 +0100, The Natural Philosopher declaimed the following:

And is likely a 1:1 isolation transformer, not a voltage conversion transformer.

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	Wulfraed                 Dennis Lee Bieber         AF6VN 
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Reply to
Dennis Lee Bieber

I never said it was an essential component of any arbitrary SMPS, I said (for safety reasons) it was a mandsatory component of a mains PC PSU.

You need the isolation, and it is CHEAPER to do it at X KHz where a small toroid can be used, than a 50Hz which requires a LOT of expensive iron and coper.

Ergo the design is standardised on a HV rectifier/smoother that is chopped by hexfets and applied to a transformer - usually a toroid.

If the design is good enough, it will handle any input voltage from about 48V DC up to 250VAC as an input. Not many are outside of Mil spec tho..

output windings are whatever you need to get the range of output voltages required.

At high chopping rates the windings may be very few turns indeed. That's why FETS are the in thing. Much faster switching times means less iron in the toroid = cheaper.

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Reply to
The Natural Philosopher

no.

Otherwise you end up with two smpses, one to do the isolation and one to do the rest

too pricey.

Look again carefully.

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Ineptocracy 

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Reply to
The Natural Philosopher

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