rackmount case

Fair enough. I said "I read [you] as saying...".
OK, but those DC-DC devices I mentioned accept 12v and output 5v, however I don't remember seeing anything recognisable as a transformer or even a choke on the (very small) PCB and no components off it either, so presumably there must be a way of handling the voltage conversion without a transformer. The same remarks apply to the Maxim DC-DC converters.
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martin@   | Martin Gregorie 
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Martin Gregorie
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Err, USB 1.0 is 5V at
Reply to
Martin Gregorie
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Unfortunately, higher chopping rates and faster switching times can also mean that a lot more interference is created, potentially spoiling radio reception anywhere near some digital devices! The price we pay.
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David 
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Reply to
David Taylor
To operate efficiently, they always include a choke. It is possible to convert from 12 to 5 volt without it, but it is inefficient to do so.
Reply to
Rob
it is almost possible to do it in the case of step down using simple chopping but you will still probably need a smoothing choke. in tte scale of step up you an use charge pumps bit its woefully inefficent. or a buck convertor whch is a transformer with one winding :-)(
But I guarantee you there will be some component in there that is probably toroidal , bigger than a thumbail and with wires on it, somewhere. uialy there are hal a dozen., but the rest are RF filters.
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Reply to
The Natural Philosopher
you should see what semiconductor mains rectifiers used to do to AM radio..
At least with HF its a small choke and a small capacitor...again thats way PSUSs come in metal boxes with every single wire equipped with some kind of HF choke.
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Reply to
The Natural Philosopher
Rest assured, for any but very small amounts of power, there is an inductor.
At the high switching frequencies of these DC-to-DC converters, several microhenrys is enough, and it won't look much different from a resistor.
-michael - NadaNet 3.1 and AppleCrate II:
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Reply to
Michael J. Mahon
Nope. You'll never find a 50/60Hz transformer in a switching power supply. It would be quite heavy and a waste of iron.
It is the high frequency "power" transformer, whose secondaries are the output voltages.
As has been suggested earlier, instead of guessing, look up the schematics of a few switching supplies and see for yourself.
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Michael J. Mahon
actually that way pure chopping works: but the choke is necessary to smooth the output.
If you dont care about smoothing, you can omit the choke, or if there is something inductive you are driving.;.
'brushless' DC motors are in fact controlled by three synchronised SMPSs synthesising what amounts to three phase variable voltage and frequency outputs. They use the motor windings as the chokes.
e.g.
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the three phase variable voltage/frequency stuff is done my 6 FETS on the right, and the chopping login in done by a microcontroller on the left. The big capacitors smooth input voltage as even though its from a battery, the wire inductance is enough cause instability if they are not there.
The essential principle of step down switched conversion is to interrupt current flow to derive essentially a 'lower' voltage than was there, in a relatively lossless manner. That's is when V is high across the switch I is zero and vice versa.
For step up, you need to store energy on something so you 'charge' it up in one part of the cycle and 'release' it at some higher voltage. inductors and capacitors both work, but it gets ugly with capacitors inductive transformers are the ideal solution, allowing different voltages to be tapped off a secondary. The amount of power that can be transferred is proportional to the actual mass of magnetic material times how much flux energy per unit volume it can hold, times the frequency of operation. I.e. size in inversely proportional to operational frequency.
and because in the limit step down is simply one different case of step for a transformer coupled SMPS, MOST step down SMPS will use one as well. BUT they don't HAVE to.
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Reply to
The Natural Philosopher
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Didn't find too much of a problem with 50 Hz silicon rectifiers, to be honest, except triacs and related devices. SMPS and HF radio are much more of an issue - there seems to be a very devious and deceptive practice of testing /with/ interference suppression components fitted, but supplying the units /without/ those components - to save a few pence.
I also noted that an un-boxed Raspberry Pi, with its 5V 2A SMPS also radiated quite a lot of interference. A nice metal rack-mount case for my 4, soon to be 5, RPi cards starts to sound like an attractive idea!
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David 
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Reply to
David Taylor
5V is only required for USB ports and disk drives, The machine core runs largely off 12V (which is regulated down to whatever voltage the CPU core, ram, and bridge chips use)
-12 is used for serial and very little else (I once encountered an ISA-bus VGA card that used it)
yeah, regulation of +5 is typically better than on +12
-5V where available, ATX-3 omits it.
yeah, rack cases are fairly cheap compared to blank enclosures.
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Reply to
Jasen Betts
there's a transformer, but it's not really "output"
not with it connected directly to the ground wire.
No, that's the output choke.
the main transformer is usually located between the switch heatsink and the recifier heatsink, it's typically a ferrite E-I core transformer.
theres also usually a smaller transformer for 5V standby and possibly another even smaller one for feedback
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Reply to
Jasen Betts
what else is it then? it takes chopped 400 V and turns it into the various supply rails that form the output...
in the UK that would immedieately trip your mains earth leakage trip; No part of any mains circuit is to be connected to earth. Except at the supply incoming point where either the neutral is a coax outer buried underground, or the neutral is earthed via a bloody big spike in the ground, or sometimes both.
AFTER that the neutral and live are absolutely definitely and legally totally isolated from earth, and failure to do that will trip the RCD. As will too many RF interference capacitors across the live/neutral and earth.
isolation from the mains is a mandatory part of any LV equipment.
In short it is utterly totally impossible and illegal to generate an LV supply that is still directly connected to the mains and have it both earthed, and run, or have it at all and pass any safety standard.
There is ALWAYS an isolation transformer somewhere .
In PC supply it happens to be the output transformer, because that's the most cost effective way. make the one transformer do both jobs. Isolation and step-down.
I think i've seen both . Es and Is were more common at lower switching frequencies. These days toroids will do as well and are a tad cheaper. Unless the block in the bottom centrer is a potted EI this looks like all toroid..
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sometime, but not always.
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Reply to
The Natural Philosopher
er no. Not the average older ATX motherboard, which can take over 10A at 5V. OK newer ones do it differently, but the average power supply doesn't know that.
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Ineptocracy 

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Reply to
The Natural Philosopher
Leaving all the detailed electronic nit picking aside, is the conclusion that a PC case and PSU provide an ideal environment for a Pi?
In which case, why not a frame in a drive bay which gives external connectivity and uses the internal PSU?
network it up with the PC and you have a Pi workstation.
Cheers
Dave R
Reply to
David.WE.Roberts
No, it is not ideal A modern PC supply does not provide most of its output power at +5v (older ones do!).
And I was not really looking for a bulky PC case. I am looking for a 1U high and maybe 20cm deep case, similar to a switch.
Reply to
Rob
Dennis Lee Bieber Inscribed thus:
More likely a coomon mode choke !
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Best Regards: 
                        Baron.
Reply to
Baron
Could you simply cheat and leave out all smoothing on the assumption that its irrelevant if there's a fairly meaty Li-poly battery between the charger and the PDA/satnav/phone electronics?
As I said, I don't remember seeing anything like that but its been a while. OTOH the PCB was about thumbnail width, so its something I'd not overlook. Unfortunately, I can't easily take a look right now: the converter is in a Maplins metal box behind the panel in my H.201 Libelle, which is locked in its trailer and 45 miles away.
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martin@   | Martin Gregorie 
gregorie. | Essex, UK 
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Martin Gregorie
If you just happen to have an old PC around it's a (sort of) OK solution, b ut for a new build no, it's a rubbish idea.
Farnell & RS both do cheap (< £1 per watt) single output 5V PSUs that will be silent, more efficient and much smaller. They also both do a wide range of reasonably priced boxes and cases to suit any use and pocket. e.g. 300x300x 50, top, bottom, back & front separately removable, all aluminium £45
Whatever you use, you'll need to make *some* holes, so why not start out wi th none (the wrong size and in the wrong places).
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W J G
Reply to
Folderol
About quarter of a century ago, I did notice a 200W AT psu which relied upon a small (3 to 5 VA) 50Hz transformer to provide the startup bias supplies to the HT switching transistors. I don't think that was a common strategy though, even way back then.
Apart from that very early PC/AT PSU, I don't recall observing any other such PSUs and modern smpsus only use a ferrite cored HF stepdown transformer to both provide the mandatory isolation and the bulk of the voltage conversion.
Any other ferrite cored toriodal transformers you might see in a PC PSU are most likely the common mode rejection choke in the mains filter cct (which also provides sufficient leakage to double up as the series inductance elements of the balanced Pi LPF cct).
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Regards, J B Good
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Johny B Good

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