Computer supplys for charger power question

[J57ltr]

Hey guys I have been using an atx power supply for years and it has worked fine until now. It always only put out about 11.8V. I am using a 10 ohm 5W resistor to load the 5V source.

Anyway it just died on me and I gave it a once over and just decided to use another since I have several. After a few that were no good I found a good one. I modified it to work but I still only get 11.88V. What is the proper way to increase the voltage to about 12.5?

Thanks

Jeff

[BrianG]

There are a few ways you can boost the output, with different levels of complexity:

1) Use a resistor to load the 5v and/or 3.3v line with ~1A. This usually just makes the 12v line more stable rather than boosting it though. Whether to load the 5v or 3.3v line depends on the design, a simple experiment on each quickly shows which one though.

2) Some supplies have an internal PCB mount pot you can tweak the output. Most don't. If there is one, note the current position, then adjust it while monitoring the output. If the output doesn't change, then the pot is used for something else, so put it back where it was.

3) Some supplies have a smaller wire that senses the voltage, like a feedback line. This wire usually is attached to the rail it is monitoring at the motherboard connector. You can use a couple resistors to trick this wire into seeing less voltage than there is, which causes the output to increase to compensate.

4) If you are really good, you could reverse engineer the PWM circuit and figure out how to trick the feedback pin on the main IC to output a higher voltage.

In any of these cases, you can usually get a solid 12-13v out of the supply, but anything higher tends to cause the supply to shut down. Your mileage may vary.

[J57ltr]

Thanks Brian I didn't think about loading the 3.3V line.

I'm not wanting to boost it just get it to 12V. I am going to go look up the colors to know the function I had it somewhere.

The smaller with is I guess the brown that is connected to the orange at the large Molex connector?

Jeff

[BrianG]

Usually, the 5v line is the one to use, but it can't hurt to try the 3.3v line too.

The "feedback" line wire color will be the same color as the rail it is monitoring (yellow for 12v), just smaller - if it exists. And they are separate wires, but are tied together at the motherboard connector - not a separate pin in that connector.

[J57ltr]

Well you are answering faster than I can find the onfo. lol

This is what I have:

Power to jacks= All the 18 gauge yellows + / an equal amount of Blacks -

10 ohm 5w resistor (Do you think I need to use a 5ohm resistor for a 1 amp load?)

Red
Black

Switch

Green
Black

On the large Molex:

There is a small gauge gray crimped to an 18 ga Orange inserte into the same terminal. (On my old powersupply these wires were soldered inside the case when I converted it)

Other wires are:

Blue -12V ?
small gray
multiple Orange
small purple

[BrianG]

Hmm, it might help to snap a picture to be sure.

I had a supply with such a feedback wire scheme (Antec 330w "true power") and got great results from such a low power supply. This post talks about this type of supply.

[J57ltr]

Here are a couple of pics. The only wire connected together is the brown and an orange. (orange is 3.3V right?) all the other wires are each connected to their own terminal. I'll take it apart at work in the morning. Is it normal for the 12V source to be less than 12V?

We use a small switching power supply on one of our controllers and and we have to set the +5 to bring the +/- 24V to be within spec.

Jeff

[BrianG]

If hat brown wire is a feedback wire, it's for the 3.3v line, which won't do much for the 12v line unfortunately.

And yes, it's pretty common for the voltages to be a bit under/over their values. IIRC, the ATX spec allows for +/- 5%.

The only thing I could say is to load the 5v line and see what it does, and/or open her up and see if there is a pot in there you can fiddle with.

Oh, and if you do end up loading the 5v line, make sure the power rating of the resistor is at least 4x the power needed because it will get quite hot otherwise. If you do load it with 1A, that means you'll need a 5 ohm resistor rated for 20w (or two 10 ohms in parallel, each rated for 10w).

[J57ltr]

I did find a pot, it's gooped up but nothing I can't adjust. It's really tight in this supply I am going to have to mount the terminals and switch on the outside. this thing has a 120mm fan on it. I was thinking of moving it to the outside but I can route the wires cleaner on the outside.

The reason I wanted to raise it to 12V is if I am charging at around 6 amps the input voltage sags to about 11.2. The old supply was 14A. This one is 15A so they are about the same.

Do you think that only having a .5W load is enough for the +5V supply or should I bring it up to 1A?

Thanks for the info, I'll pick up some different resistors tomorrow.

Jeff

[BrianG]

Hmm, you should be able to get that goop off without wrecking anything. That might be the thing you need to make this work. If you can get it to work, I would shoot for a voltage closer to 14v because the higher the voltage is, the less current (and losses) your charger will pull for a given load.

0.5A might be enough, I just personally like using a solid 1A. Only way to tell is to test it. Try it with just 0.5A load and see how the 12v behaves. Then try again with the 1A load. Then just use whichever you prefer.

[J57ltr]

Ya the goop is no big deal it's that green bubbly sticky stuff that just peals right off, you know what i'm talking about. I'll give it a try I need to clean up the wiring so I'll test as I go.

On my other power supply I wasn't thinking when I gota the resistor at a local old school surplus electronics store and found some awsome screw in case 25W resistors, but wasn't thinking that it was a 1 ohm instead of a 10. I hooked it up and the output was right about 12.5V, but the case was smoking hot. Amazing what 25W will do. Lol. When I switched to a 10 ohm the output dropped to 11.7 or so. I have some 2,4,8 phm resistors laying around.

Thanks

Jeff