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Computer Power Supply Converted for Ham Use

Ham Power Supply from old PC

  12V (or 13.5v) PSUs with enough wattage to supply a decent ham mobile can be expensive.  OK, Maybe not quite prohibitively expensive but it is certainly more fun to spend money on more interesting gadgets like a new radio or antenna than something with so simple a function as to just sit there and supply a fixed DC voltage. Besides, this is what got me into ham radio, the opportunity to build or modify things.  I had already mounted a car radio in the drive bay of an old IBM PC and powered it by the PC's original power supply when I was in high school.  When I saw all the articles on the Internet about building a ham supply from a computer power supply I had to go for it.  This same method is good for powering non-ham equipment such as car stereos, GPS units, travel chargers or anything else which is meant to go in a car.

   Standard computer power supplies produce 3 voltages, +5, +12, -12 and sometimes -5 and/or +3.3V.  It's not as simple as just hooking the radio up to the +12V output.  There are a few tasks involved in this conversion

   I have a pretty large collection of junk computer parts so my first step was to pick my victim.  I could have gone with the highest amperage power supply I had available but I was concerned with getting the full 13.5V out.  The power output of computer power supplies are controlled by resistor. There may be separate resistors for each voltage or just one which controls them all.  Sometimes it's easy and these resistors are variable.  If they are not variable they can be recognized because they are usually the wire-wound type as these are more precise.  The websites I found suggested looking for a wire-wound resistor, replacing it with a pot and adjusting until you get the right voltage.  Then they suggested replacing the pot with a fixed resistor of whatever resistance the pot was at.  I did not want to deal with this, partly because I didn't see any obvious wire-wound resistors in the first few supplies I took apart and also just out of laziness.  I checked all the spare supplies I had and only one already used a variable resistor  so this is the one I used.

Turn-signal bulb used to load the "5V" side of the PSU

   At this point I had my supply, it was open, and I was looking at the voltage adjustment pot but I couldn't set it yet because I couldn't turn it on.  I still needed my 5V load.  I had decided I was going to do this with only the parts I already had, I didn't want to buy anything.  That turned out to be a little more difficult than I had imagined.  The recommendations I found on the Internet varied but most involved a resistor of about 8-10 ohms, 8-10 watts.  I couldn't find anything in my junk pile which was a 10watt resistor let alone at such a low ohm value.  Most of the common ones these days are 1/4 - 1/2 watt. I may have been able to make it out of resistors in parallel but that would have taken way too many to be worthwhile.  I did find one person who suggested using a car turn signal bulb.  He said it wouldn't get too hot and would last for years at the lower voltage but it would provide the necessary load.  I was still worried about heat though, plus I was worried it wouldn't be enough resistance as the spare bulbs I had on hand registered as shorts to my ohm meter.

   Convinced I needed a resistor and unwilling to buy one I attempted to make my own out of pencil lead.  First I used the leads form a refillable pencil but they were to thin and would immediately burn through.  Next I carefully removed the lead from a #2 pencil with a razor-blade.  These too burnt through until I tried 5 of them, about 2 inches long each in parallel.  I went through a few pencils as the pieces were easily broken. This got slightly warm to the touch but seemed to work OK. For my testing I had the lead pieces in alligator clips.  I needed a more permanent mounting solution.  To mount my pencil lead resistors I decided to glue them onto a piece of leftover perfboard with superglue.  Then I was going to twist stiff copper wire around the ends, through the holes in the perfboard and solder the twists shut.  That almost worked but I squeezed a little too hard and shattered the first couple of lead pieces.  At this point I was out of patience and done.  It was time for another solution.

   I decided to take another look at the light bulb idea.  Some time ago I had bought a package of turn signal bulbs when the problem turned out to be in my wiring.  I still had those available. I didn't have anything to mount them in though.  Although I wasn't supposed to be spending anything on this project I went to the autoparts store and looked for a socket.  They were all far to big to fit inside the supply and I didn't want the bulb out in the open where it could be broken.  Instead I went back to my earlier solution for mounting the graphite.  I super-glued the plastic base of the bulb to another piece of the same perfboard.  This was one of the bulbs with the wires that wrap around the bottom of the base to make contact with the socket.  I unwrapped the wires, twisted them through the holes in the perfboard to keep them in place and soldered one of the 5V wires and a ground wire from the supply to it.  It worked.  I was still a little concerned that the resistance should be higher than what the light-bulb had to offer.  It always registered as a short to my ohm meter.  Still, I couldn't believe it really is a short because that should burn out the fuse when it's used in a car.  After a bit of researching light-bulbs on the Internet I found that they do start out as a short but the resistance quickly rises as the filament heats up.  Since my ohm meter uses a much smaller amount of electricity to check the bulb it never gets past being a short.

PSU voltage adjustment trimpot

   Now that I had my load I still needed to find a way to mount it inside the power supply.  Still concerned about heat I wanted it to be near the fan and not touch anything.  If possible I thought it would be nice if it could have a little bit of shock absorption because if the filament breaks I will have to unsolder the bulb and break the superglue which holds it to the perfboard, or waste another piece of perfboard. What I came up with was to mount it on a stiff piece of wire. I drilled a hole in the perfboard which I used to bolt on a piece of copper wire I had laying around.   I also drilled a hole in the side of the power supply, and bolted the other side of the copper wire to that.  Now all I had to do was to bend the wire until the bulb was in a position where the hot glass wouldn't rub against any of the wires or other parts inside the supply.  It fit quite nicely in the open area in front of the fan which also helps take care of the heat which I was concerned about.

   Next came setting the voltage.  At first I tried just hooking my volt meter to one of the 12V outputs and turned it up to see how far I could go.  At around 15V the power supply would turn itself off.  Next I set it to the 13.5V which I was actually looking for.  This seemed to be good but I found that when I hooked a radio to one of the other leads and transmitted into a dummy load the voltage dropped quite a bit. I ended up setting the supply to output about 14.5V when there is little to no load.  Automobile voltages aren't necessarily all that dependable and anything made to use in a car should be able to handle this.  It now drops to about 13 volts when I transmit.

   Some people have done this and found that there is a noticeable hum in the output, some have not.  A common suggestion is to place either a large value capacitor in parallel with the output or a choke in series.  I did not hear any hum in my receive so I tried listening to my transmission with my HT while transmitting into a dummy load.  I did not hear any hum this way either so I decided not to add any kind of filter.  Later a friend of mine told me he could hear hum while I was on the air so I will probably revisit this and add both.

Anderson power poles on PSU

   The wires coming out of a computer power supply are made to supply smaller amounts of power to the various parts of the computer rather than just having one output to a radio which may use more power.  Many have recommended unsoldering the wire from the PSU and soldering on a heavier gauge.  I opted to keep the original wires but I twisted them together so that I have two positive wires and two grounds going to each connection.  You can attach binding posts or any kind of connectors you like to your new power supply.  I chose Anderson Power Poles for mine as I am attempting to standardize on these throughout my shack and car.

   My next task was to replace the power switch.  It was typical AT power supply switch, a DPDT push-button dangling on the end of a chord with it's terminals exposed waiting to shock someone.  At first I was going to just drill a round hole in the front of the supply and install a toggle switch.  I was hoping to leave as few extra holes in the front of the supply as possible though.  I noticed that the existing hole was a rectangular notch at the top of the panel that looked pretty similar in size to a rocker switch I had.  It turned out I did have to widen the hole a bit.  This was an easy task with a large pair of tin-snips which I had.  Then I was just able to slide the rocker switch into the hole.  When I put the top back on the supply, this would hold the switch in place.  Now I had my power switch without adding any new holes.

Rocker switch mounted on PSU

   Many people have made some very nice faceplates for their power supplies using copper clad PCB blanks.  I decided not to do this as I was eager to put my supply to use and get on the air.  Also, my power supply sits inside a shelf where it is not very visible and I would rather save my PCB materials for something else.  I did however add some stick on rubber feet to the bottom which I had lying around.  At least it will not scratch the shelf.  Recently a friend gave me an almost new power supply which is broken.  Some of the wires had rubbed against the sharp fins of his CPU heat sink and shorted out.  It's otherwise brand new and was made to be seen in a case with a big window and has a very glossy paint job.  I am going to take it apart and see what is broken.  If it isn't an easy fix I may gut it and move my ham supply into the case to make it look a bit sharper.

UPDATE - 8/24/09

   This seems to be the most popular page on my site so I guess I should keep it up to date!  The other day my computer died.  It didn't just die, it smelled like smoke bad.  I really am not in a position to replace it or rebuild it with new parts right now but I have quite a few "junk" computers, both my old ones and gifts from friends whom were upgrading.  While collecting parts I came across some "Silencer" fans which I bought from PC Power and Cooling some years ago.  These are really nice fans, they have ball bearings so they don't wear out quickly but better yet they make almost no noise at all.  I decided my PC was kind of loud and these fans were from an old server project and no longer doing me any good where they were.  I replaced as many fans as I could with the quiet ones.  While doing this it came to mind that my ham supply was really loud.  It made sense as it came from a cheap PC to begin with so it probably had a cheap fan.  Also, the fan was turning extra fast because I had turned up the power.  I decided this was a good opportunity to revisit that project.

   I saved my last quiet fan for the ham supply.  I thought I would use the -12V side of the supply for the fan since it wasn't doing anything else and was surprised to find that this is already how it was wired to start with.  I also decided to add a resistor inline to counteract the extra power since I had the voltage turned up.  Using the very scientific method of WAG and trying one resistor value and then another I came up with 120 ohms as a good value.  This slowed the fan enough to get the ultra-quiet operation I was looking for while still blowing a good amount of air.  I measured the airflow with another highly accurate labratory method.  I pointed it at my face and compared what it felt like.

   Unfortunately I see now that I didn't have any pictures of the back of the supply.  This was one of those where the fan guard was simply stamped into the metal of the case itself.  I wish I had a "before" picture to show what I mean.  I never liked those. I don't think they let as much air through plus the air pushing by them tends to make more noise.  I cut this out with a dremel to make a large open hole for the fan.  Then I bolted on a wire frame fan guard which probably came from another broken power supply or some other computer part.

   While I had everything apart I just couldn't leave well enough alone.  I decided it was time to make this thing look nicer.  I considered moving it into my friend's broken power supply case which I wrote about previously but I decided to save that for another project.  That case is quite a bit larger plus it already has two fans in it.  I didn't want to use two quiet fans for the supply as I wanted as many as possible in my PC.  Laying around I happened to have some leftover spraypaint from a project I haven't written about yet.  It was black Rustoleum Hammered Finish paint.  I recently "discovered" this stuff and I really like it a lot!  The hammered finish has a texture which looks quite nice plus it helps to hide any imprefections in my not-so professional paint job. 

  I definately recommend replacing one's fans with quiet versions for any computer or other muffin fan project.  The fans I used are "Silencer"s from http://pcpowerandcooling.com but I bought those years ago and it would seem they only want to sell complete power supplies now, not simple fans.  Another site I almost bought from back then is http://quietpc.com.  They have some interesting looking stuff and I hope to replace the rest of the fans in my computer with their products one of these days. I'll probably write something here on the site about that experience. No, I'm not affiliated with either of these companies.

3 Comments - Add Comment

Ken Mayes - 2014-09-18 13:55:08 - Reply

Am I correct in assuming that you only use the 24 pin main power connector in this project?

Leif - 2014-09-19 23:03:28 - Reply

Actually, I think I just cut off all the connectors, kept all the 12V wires plus an equal number of ground wires. They all attached to the same place on the circuit board so it wasn't like they were separate circuits to try to spread the load.

Marcos - 2016-01-03 11:01:05 - Reply

Saludos, quiero saber, si la lampara es a 12Vdc, y si esta conectada a los 5V, y si usted uso algun potenciometro para aumentar el voltage a 14.5Vdc. Gracias