Thread: Has any one had any luck converting a standard PC power supply to a DC to DC unitl?

Originally Posted by DeltaFX

Someone told me 240v AC was converted to 240 DC, used to feed PWM units to have 12, 5, 3.3.....

Can't be...

I seem to recall from my basic physics that a current (AC or DC) through a coil on an iron core generated an electromagnetic inductance. However ONLY if the magnetic inductance came from AC, could that be collected into another coil as current.

If the power supply in your home PC simply took 240V AC and rectified it to 240V DC and then used a DC power convertor to drop the voltage down to 12, 5, 3.3 then there would be no need for the big coil of wire or the heat it generates!

What will be happening is there is a big coil and a small coil on the same ferrite core. The big coil carries the 240V and the small coil the output voltage. Lets assume that the Small coil is a 10th of the size of the big coil - it would output 24V which would still be AC (this assumes a lot of things like no loss of energy etc etc. ) That would then be rectified through a group of diodes to give a DC voltage - which would basically be about 24V... ...this would then be converted down to the correct voltage. This is of course an over simplification:

1. Most AC computer power supplys have a 110V and 220V switch - this probably halfs the length of the AC input when 220V is selected.

2. Most AC-DC transformers will often have more than one voltage output - for the example I gave (which is theoretical - I have no idea what outputs a real PC transformer has)... might be 1/10th length (i.e. 24V) 1/20th length (i.e 12V) 1/40th length (i.e. 6V) 1/100th length (i.e 2.4V) each of which would then be voltage corrected through seperate circuits.

IN THEORY:

IF you knew what the DC voltage coming out of the regulator was then you SHOULD be able to feed a simillar DC voltage in at that point... If you were happy to play with casing off (not recommended) you could measure the voltage(s) and see what they need as inputs... ...but you'd almost certainly end up having to build a circuit to covert (some of) your 12v DC from the car battery to use on these inputs... ...and THAT defeats the point...

If anyone wants to prove me wrong and refer me to a web page that says you can use inductance coils to transform DC to DC then I'd be delighted to be proven wrong...

Calum

You guys have diliberated and theorized enough here about this and some of you have been right some not so right:P So here it is.
I am an industrial Electronics Technician for a Pharmacutical company, and Ive biult many power supplys (the sproggy mk2 a few times is pretty neat project) etc, and modified ATX supplys into efficient battery chargers a couple of times. They make a really good on demand 14v power supply (up to 15 amps!)

The 240 AC comes in (or 115ac if your in USA) and is filtered a bit, passed by a overvoltage device (usually a blue Metal Oxide disc) varistor, then fed into a bridge rectifier. The Nuetral is centre tapped in this arrangement, to make a plus and minus pair of power rails, stored in two capacitors. These are the two biggest caps you will find in any of these power supplies. THESE are the ones to make sure you dont touch, because they have + and - 150 volts DC stored in them, usually at 470uF, so between them, they have 300 volts and at a very lethal level of energy!!

The rest of the circiuts, are the main controller chip, this takes control of the voltage sensing, (which throttles the MOSFET's up and down to control the voltage and power output into the main transformer) and the protection circiutry.

There is sometimes a transformer operating at high frequencies (more efficient) between the 300 volts centre tapped supply, and the outputs, but the outputs are further controlled by seperate mosfets, to each output their respective voltage. 12, -12, +5, -5, and +3.3 etc. There is sometimes other transformers, etc, depending on how the engineer found it easiest at the time to get from A to B to get his job done. It sometimes depends on whats available in parts, and cost etc.

But usually in all of these, the 12v ouput is taken as a sensing voltage, and fed thru a resistor network to take off a fraction of the voltage and feed it into the controller chip, to control the voltage properly. I sometimes hack out that resistor and put in a variable resistor, so I can control the output voltage, and turn it into a efficient battery charger.

One such unit managed to feed out 14volts at 15 amps into a dummy load for hours and didnt even get warm. Amazing things really, when you compare them to the old way it used to be done (heavy mains transformers)

SO you want to convert one of these into a 12 volt DC unit eh? Would be easier to biuld something from scratch. The 240v version is all biult around making the voltages and currents needed to run a pc, from a 300 volt DC bus.... so the mosfet circiuts are not right for running directly off a 12 volt DC bus. Just feeding in 12 volts into the DC bus wont work, there just isnt enough power.
SO the next option is to rewind the transformer so it feeds more power in. Ok this will work, but it has to be operating at a high frequency (AC), so you have to attend to that too, ...oh wait, now you've just biult an inverter! See how its a waste of time?

Don't bother, and just either buy a Opus or make a sproggy. In my car PC (Nehemiah 1gz DVD rom and 80gig Hdd 512mb ram) Im actually using a ITX 200 watt plug into motherboard unit (the one thats about twice the size of a small box of matches) and have no problem with it at all.

I agree with stimps, this can be done, but mains powersupplys do not have the correct circutry to handle DC to DC. Many times the mains power transformer will have as it's secondary a single layer of foil as a secondary winding. Sometimes only fractional turns of mutistranded wire. Desinging the transformers in these type of circuits is probably the biggest challenge facing a designer. In my opinoin DC to DC is much better handled by high frequency inductors rather than transformers. All that said, if you have a junk supply to play around with, go for it. You may stumble on to something we haven't thought of.

Walt

Originally Posted by waltsongs

All that said, if you have a junk supply to play around with, go for it. You may stumble on to something we haven't thought of.

Yeah, just dont damage yourself with it, whatever that means