Reconfigure ATX power supply for different outputs?

[IanB]

I have a reclaimed 200 W Mitac power supply based around the TL494 chip. Its designed outputs include 12 V 6 A and 5 V 22 A. I don't have any use for so much power at 5 V, but there are many things that will run off a 12 V supply.

Is it likely to be feasible to modify it somehow so that it has more power on the 12 V output and less on the 5 V, or is this basically a function of the transformer windings?

I haven't tried to map out the circuit yet (I know nothing about these supplies), but there is certainly a connection between the 5 V and 12 V outputs. The 12 V output starts out low at about 11.6 V, but the voltage increases when I put increasing loads on the 5 V output.

[Simon]

The simple answer is NO! yes the voltage depends on the transformer and everything is optimized for a certain voltage and a max current.

[IanB]

So then the best I can probably do is to tweak the voltage sense divider network to increase the 12 V level slightly to 13 or 14 V?

I've also read advice to keep a small load of a few watts permanently on the 5 V output to help the 12 V output along a bit. In my testing that certainly seems to have that effect, although it needs some effort to sink the heat. I'm thinking along the lines of a moderately powerful six volt bulb and doubling it up as an on/off indicator lamp.

[Simon]

From memory the supplies are regulated from the 5V output (or maybe the 12V) and the rest just tag along as the transformer has the required winding ratios. the 12V is just for running motors and other SMPS systems so is not critical. Some supplies have the 5V running off the 12v or more likely the 3.3V running off the 5V so they are tied in voltage and power output - Spec tables on supplies can be deceiving, read carefully.

[kaptain_zero]

I thought all ATX power supplies were switching supplies.... as in no transformer?!?!

[IanB]

I thought all ATX power supplies were switching supplies.... as in no transformer?!?!

Switching supplies still need a transformer to:

(a) Isolate the low voltage side from the mains;
(b) Transform the high mains voltage down to the low output voltages.

The transformer is a special transformer that works at high frequencies, which allows it to be smaller and lighter.

[Uncle Vernon]

I have a reclaimed 200 W Mitac power supply based around the TL494 chip. Its designed outputs include 12 V 6 A and 5 V 22 A. I don't have any use for so much power at 5 V, but there are many things that will run off a 12 V supply.

Is it likely to be feasible to modify it somehow so that it has more power on the 12 V output and less on the 5 V

Technically feasible, yes! Practically feasible no!
You have to ask will the time, parts and effort required equate to  substantially less than the cost of a readily available hi current SMPS. Your answer will almost certainly be no. PC supplies are always a bitch to mount and properly enclose in comparison with regular enclosed SMPS units. The parts for modification will cost more than a higher current 12 supply.

Sure if you want to achieve a proof of concept or because I could mod go ahead but it will never be your best option.

[IanB]

Changing tack a little, what kind of parts might be worth reclaiming from old power supplies?

There's an IEC socket and mains switch, some nice heat sinks, a big bridge rectifier. Looks like some Rubycon electrolytics if they are not worn out.

What kind of transistors do these supplies use? Are they only good for switching, or can they have linear applications too? I have not taken one apart yet to look at part numbers.

[ivan747]

Changing tack a little, what kind of parts might be worth reclaiming from old power supplies?

There's an IEC socket and mains switch, some nice heat sinks, a big bridge rectifier. Looks like some Rubycon electrolytics if they are not worn out.

What kind of transistors do these supplies use? Are they only good for switching, or can they have linear applications too? I have not taken one apart yet to look at part numbers.

Transistors are usually P channel MOSFETs. Specially IRF ones. You get an optoisolator of two, a bunch of low voltage caps, lots of coils, assorted wire, a fan, nice mounting hardware, specially for heatsink electrical isolation; an awesome, bendable and modifiable enclosure, lots of resistors, useless transformers (unless you are into SMPS design), some beefy caps on the high voltage side, some mains rated caps, and a handful of computer power connectors.

I think they are more useful for powering high power electronic projects, but don't expect your 12 bit ADC to work accurately without some heavy filtering. Separate digital, analog and high power circuits, preferably in different rails. The -12V is useful for powering op-amps, but add a 7905, and have a +/-5V supply. This will add some isolation from the high power stuff, which is likely to be powered from the +12V line. Take advantage of the +5V standby for turning the rest of the circuit on and off with a microcontroller. I am currently doing a project involving motor control, and ATX power supplies where the best value for money. And it has some control lines and a Power OK line. I am not doing ADC with this.

Ivan

[IanB]

On going through a computer graveyard, it looks like the first Mitac power supply I came across is a gem. It has a main power switch next to the power socket and there is lots of room inside the case for modifications. Here is what I did with one of them as a mini-project:



I installed a chunky load resistor on the +5 V rail to stabilize the supply (the white thing zip tied to the heat sink), added an indicator lamp for a little bit more load and to fill up the cable exit hole, brought the +5 V and +12 V rails to binding posts on the outside of the case, and tied off the unused outputs. I left the unused wires long and coiled them up in case I want to use them in the future.

The binding post terminals are covered with hot glue to insulate them and prevent the nuts coming undone and falling into the interior. All the wire terminations and connections are insulated with heat shrink. I placed some moulded plastic around the exposed mains terminals as extra insulation.

Rubber feet on the base complete the job.

This is an exterior view of the the back; the on/off switch can just be seen above the power cord:



Here is the front with the indicator lamp:



It's not a lab supply, but I figure it will work nicely as a utility high power 5 V and 12 V supply to have around the place.

[tycz]

You can change the outputs by rewinding the transformer. It's not nearly as difficult as rewinding a 50 hz mains transformer. There was an article on the procedure in Silicon Chip a few years ago.