Converting a PC Power Supply

[Bill2k]

I just converted one today using some instructions from this web site: Converting a PC Power Supply

With about $15 in parts from rat shack, I now have a 12 volt/15 amp regulated power supply. I could have made it for $8 but I put an automotive power socket (cigarette lighter socket) on it too.

[CoolCat]

Just be careful what you hook up to it, as many OEM PC PSUs are low-quality junk that can easily destroy an expensive device with it if it goes. Also keep in mind that the power specs on generic PSUs are almost always overrated. They tend to rate them at high input voltages and at lower operating temperatures then normal conditions, giving them a higher output rating. Plus the cheaper manufacturers rate them at peak power output, not their true continuous output (which is always much lower than peak).

It will probably work fine with light loads, but don't try drawing alot of current out of it, as you might overload it and cause serious damage to whatever you hooked up to it.

[Bill2k]

Thanks for the heads up. I doubt I'll ever draw more than a couple of amps. I'm just using it to power an amplified speaker at the moment. The other uses will be for charging my Tom Tom, and the occasional bread boarding.

[K9WG]

Originally Posted by CoolCat

Just be careful what you hook up to it, as many OEM PC PSUs are low-quality junk that can easily destroy an expensive device with it if it goes.

... snip...

It will probably work fine with light loads, but don't try drawing alot of current out of it, as you might overload it and cause serious damage to whatever you hooked up to it.

I will affirm that. I have had two PCs damaged when the power supply went bad. Blew out the motherboard and the keyboards. Sure wouldn't want to hook up a $400 radio to one.

[N1SQB]

Bill, could you post the ratshack parts with catalog numbers that you used? I'm in a tinkering mood and would love to do one of these just for kicks. I have dedicated power supplies for the serious stuff. This would be great for small projects and things like that.

Manny

[Trprc]

I converted two PC power supplies over 10 years ago. Instead of using resistors as the false load to keep the PS running, I used a 12V automotive bulb, serves two purposes. I use them to power my RC airplane on board batteries chargers.

They are working up to now flawlessly. Do note that they put out around 11.8 VDC under load.

[mitaux8030]

There is another big problem with PC PSUs when used on transmitting radios. PC PSUs DC outputs have lots of switch mode harmonics on them. Feed these switching harmonics into a transmitter, and those harmonics will mix in the transmitter stage and cause all sorts of wierd & whacky frequencies to be transmitted. I've seen a CB pumping out quite a wide range of nasties from 24 to 32 MHz when powered from a PC PSU.

PC PSUs need some very thorough filtering before connection to a transmitter. It wouldn't hurt to do this sort of thing for a receiver connected to such a PSU either.

[gtriever]

Originally Posted by K9WG

...Sure wouldn't want to hook up a $400 radio to one.

Pop the top on a $5,000 Motorola Astro Spectra/XTL5000 consolette and take a peek inside. A computer PSU (with modified output connectors) powers the whole unit.

[K9WG]

Originally Posted by gtriever

Pop the top on a $5,000 Motorola Astro Spectra/XTL5000 consolette and take a peek inside. A computer PSU (with modified output connectors) powers the whole unit.

Maybe so but I guarantee you that:

A. It is not a $25 power supply from Best Buy

B. The $5,000 Motorola Astro Spectra/XTL5000 is much better protected then a $400 PSR-600.

Like I said I had 2 PC power supplies go bad and took out the mother board, keyboard, and mouse. To me it is not worth the risk when I can buy a $70 Astron.

[mitaux8030]

To be fair and add balance to my thoughts:
*PC power supplies are not filtered for RFI, because they're not meant to have radio receivers or transmitters attached to them. Good quality ones are better in this regard, but still not what I'd consider ideal for direct connection to a transciever.
*I've always held secret fears that a switch mode power supply can cause massive damage to its connected load. What happens if the switch mode controller goes silly and regulates the output at 150 volts? Or the switching transistors go short circuit and delivers a very nasty voltage for a very brief time to the load? But this has never happened to me yet. The only SMPS that has died on me was a Doss 40 amp supply, and the voltage never spiked, it failed 'cleanly'.
*In contrast, linear supplies that I previously considered safe - if something goes wrong the voltage won't go above the transformer voltage or there abouts - has caused more damage to my connected loads than any SMPS due to the pass regulator transistors going short circuit.
*One SMPS power supply that I've seen, specificly designed for radio use, was absolutely TERRIBLE for noise on its output. Another (the Doss above) needed ferrites to quieten it down.

So summary is: either SMPS or linear supplies can & do fail, and the better quality ones that bother to put in protection help to protect the connected load, but don't 100% rely on it; linear supplies are always quiet RF wise; and SMPS supplies have the potential to be very RFI noisy, but the better ones are pretty good. You get what you pay for!

[OCO]

If you're concerned about a PS going into overvoltage, build a simple crowbar circuit with a fuse . Tons of circuits for a simple Google of "power supply crowbar protection"....

[mastr]

Originally Posted by K9WG

Maybe so but I guarantee you that:

A. It is not a $25 power supply from Best Buy

B. The $5,000 Motorola Astro Spectra/XTL5000 is much better protected then a $400 PSR-600.

Like I said I had 2 PC power supplies go bad and took out the mother board, keyboard, and mouse. To me it is not worth the risk when I can buy a $70 Astron.

Obviously, you have never worked on an XTL5000 consolette.

[WJYSTV62]

You get what you pay for.
Well made switching power supplies have more than adequate filtering to power a transceiver. If the output of a supply was particularly noisy it would interfere with the operation of a computer. Some switchers do include crowbar circuits and/or other over-voltage and over-current protections. A linear supply with such circuits costs more as well. Commercial broadcast transmitter makers are using switch mode supplies. You think protections aren't in place to use them in a $400K transmitter?
Linear supplies can cause just as much damage. While they may be limited in output voltage by the transformer they could deliver their full current just like a switch mode supply could in the event a fault. However, the majority of switchers out there are not very well made. That $7 knock-off replacement Wii supply direct from China will spew out RF in the air and on the wires like some sort of spark-gap radio experiment.