IFlySlowPlanes

I decided to give k3nv's "How_to" a try, having recently acquired two supposedly unused old AT-style PSU's from a local computer repair shop. One was a 250W unit and the other was just 200W, but I was hoping to get some use out of them. Well, the 250W unit was DOA, so that left the 200W unit.

But I guess I did something wrong. Nothing disastrous (no sparks, smoke, or shop destruction)! But instead of getting 12VDC across the terminals, I'm only getting around 10.95, and my Triton isn't happy with that. The only thing I did differently from k3nv's instructions was that I could find no PS-ON wire with my unit, so I omitted that step (seemed like it was acceptable procedure based on other how-to's I've read on on the subject elsewhere, but I'm probably wrong!). The unit powers up fine, the fan runs, but as I said, not enough juice. I know a 200W PSU isn't ideal, but it was all I could get at the time. The other 250W unit I got was DOA. Am I hosed, or is it possible that this 200W PSU can still be made to work? I thought that 12V was almost a given, but only the amperage would vary depending on how powerful the PSU was. One other note...I used 4 yellow (+12v) wires and an equal number of black (common) wires. Still...there must be one thing I'm missing, although I can't for the life of me figure it out.

And here I was feeling so proud of myself for finishing a project!

bwalker25

some cheaper not so high quality powersupplies wont put out 12v on the 12v rail. As such a decent powersupply of say around 300 - 350w would be plenty and should be good enough to put out 12v. I dont think it is anything that you have done that you are only getting 10.95v on the 12v rail, its just the quality or lack there of, of that powersupply. All I can say is try again

Rodney

I believe you have to put a load on the 5 volt bus in order for the 12 volt bus to come up to full voltage. Put a 5 ohm 10 watt (or higher wattage rating) on the 5 volt bus and you will find the 12v bus will come up to spec.

Andrew

The AT style PS does not have a soft PS_ON wire (green), but is hard switched on the AC side with a double pole single throw switch. Although the majority of the AT powersupplies will power up when switch on, some will not. However, none will produce full voltage on the 12v rail without loading the 5v rail. Secondly, the resistive load will greatly stabilize the 12v side when the rail comes under load. This is true for both the AT and ATX PS.

I would suggest you read thru the TIPS section of [link=http://web2.murraystate.edu/andy.batts/ps/powersupply.htm]this page[/link] to determine if your supply is sufficiently loaded or not. The general progression of resistive loading is 10 ohms, 5 ohms, then 2 ohms on the 5v rail (red). Use the highest ohm rating that will bring your 12v rail to the necessary level, but do not drop below 2 ohms since the marginal gains are insignificant and the associated heat and power loads can become excessive.

How did you determine that your larger supply was DOA?

IFlySlowPlanes

Andrew: That tip about the resistor is undoubtedly what I needed. One more quick trip to Radio Shack tomorrow and I should be set! As for determining that the larger PSU was DOA...I guess what I should have said is, when I initially plugged it in and turned it on, the fan wasn't running. Figuring that it probably only ran when temps got higher in the case, I proceeded with the conversion anyway, using the same procedure as with the smaller PSU. Should have probed it before all that I guess to see if I was getting power at the switch or elsewhere on the board, but I didn't have a multimeter at that point (only went out and bought one after I realized I needed grommets for the binding posts on both units anyway, and found a cheap digital one at Home Depot). Anyway, once I had wired up the binding posts on the 250W unit, I tested them and was getting a zero voltage reading. Didn't bother to test elsewhere, but I will. It occured to me that it could just be a bad switch, which I could probably replace easily....anything beyond that and I'm out of my depth. It has been an intersting exercise in any case!

Dave

Andrew

Dave --

If you will load the 5v rail on the DOA PS, you might find it is OK. Many supplies, more ATX than AT's, appear to be bad when they only need to be pre-loaded to function.

Hardly any of the AT's had thermally controlled fans, but within the past two years, the number of ATX supplies with PWM thermal control has been increasing steadily. Even with thermal control, the fan will not completely stop, but continue to run at a low noise rpm.

Be sure none of your binding posts are grounded to the case -- this includes both the positive and negative posts. Most supplies I've tested isolate the case ground from the DC ground, but in any case, positive posts should not touch the case or the overload circuitry will kick in.

Keep us posted on how your conversion goes.

redgiki

Rather than a resistor, I just purchased an automotive light bulb to load the 5V low side of my PSU. Works great! I bought a double-filament, and ended up using both filaments to bring it up to the level at which my Triton would work.

So now I have a really reliable 12V power source, and a really nifty blue light that (given it's designed for 12V and running at 5V) will probably run forever...

Red Scholefield

My Feedback: (9)

I'm glad to see at least one has figured it out. Using an automotive light bulb, preferably one with dual elements as the practical solution to loading the 5 Volt buss. Remember there are two contacts on the base of the bulb, jumper these two together and connect to the 5 volt line, and run a black lead to the base of the bulb. Sometimes these are aluminum so you can't solder to them. In this case just buy a bulb socket from the same place you got the bulb. These sockets fit nicely into the hole where the bundle of wires came out of the power supply, tack solder to the power supply case.

Andrew

The turn signal lamp is an easily adapted workable solution to pre-load -- any automotive supply and most Walmarts will carry the 1157. Running at 5v, you can also expect a long life and if you install a twist-lock socket, replacement is ultra simple. Plus, it makes a nice Power_On indicator.

For various reasons, I use power resistors, but the substitute of a lamp is perfectly acceptable.

k3nv

I hope you didn't cheat and used solder It is a solderless mod remember? hahaha. I never converted an AT PSU so I wouldn't know in detail what is needed. However that resistor/bulb thing should do the job. I read that in all of the other PSU conversions online. I've never had to do it for my conversions.

Let us know how it goes... Just in case I have to convert some old AT's. In my neck of the woods, AT's are hard to find! I only came across used ATX's.

IFlySlowPlanes

I decided to test your theory, and what do ya know! The 220W unit came to life! At least sometimes it does. Seems to work fairly randomly. Unfortunately, when it DOES work, it doesn't even produce as much DC voltage as the 200W unit did BEFORE adding the load resistor. If I understand you correctly, substituting a 5 ohm load resister might help? I tried adding a 12v light to the 5v rail in addition to the 10 ohm resistor, so I know it can't be right to load it up more. I'm such an electrical noob!

Anyway, step by step I'm getting there! The smaller 200W unit is producing around 11.3 to 11.5v now, which seems to be enough for the Triton to be happy.

Thanks for all your suggestions and assistance!

IFlySlowPlanes

I crimped everywhere except on the load resistors (but only because I ran out of butt joint connectors...or whatever they're called!). Those I did solder.