ATX PSU help

[USSEnterprise]

This may have been asked before, but I couldn't find anything. How do you make an ATX PSU power up if you don't have a switch, because you aren't building it into a case?

[RayB]

There is a wire you can "short" to ground to make it turn on.

[USSEnterprise]

thanks, but nevermind. I figured it out. I stuck a jumper from a disk drive over the two pins

[webgeek]

I know this is an old thread but it seems to come up a lot...

To ensure the power supply comes on when it gets plugged in, connect the green wire (power on) to ground (black wire). You can install a switch in there but I don't. The power supply has a switch under the AC plug in the back already. I just use that.

To get the power supply to give you constant voltage and not power down due to improper load, you install a 10 watt 10 ohm sandbar/wirewound resistor between +5 (red wire) and a ground. You will want to heatsink this resistor because it will get hot. I suggest zip-tying the resistor to the fan holes in the back of the power supply. This way the fan cools it off and it dissipates heat quickly.

To get the +12 volts, you use a yellow wire (if I remember correctly, check with a volt meter) and a ground. To get +5 volts, use a red wire and a ground. I suggest using a different ground for each of these steps as there are many available and it keeps things cleaner.

In my HydroCade thread on the project announcements forum, I post a link to a site that shows how to do all this. In the end, this technique has worked remarkably well for me. I'm really pleased with the results.

[JCL]

Just wanted to note that webgeeks power resistor is only needed if you are not using the PS for a computer. The motherboard provided a good load if you just want a PC that can be powered up with a powerstrip.

Good writeup wg!

[Luckydevil]

I'm not seeing the need for the resistor on the power supply in our application. I followed webgeek's link and the guy said he only saw a gain of .2 volts at best.

The system in my cab isn't complete yet, but I have tested it out using the power supplies and amps with absolutely zero problems of them powering down on their own or supplying low voltage.

Those guys are trying to squeeze every last ounce of power out for their applications. Just buy a decent power supply and this shouldn't be neccessary.

[USSEnterprise]

Meybe this should be made a sticky post, if it comes up so often

[Luckydevil]

http://www.quepublishing.com/articles/article.asp?p=30273&seqNum=6&rl=1

The link above explains it pretty good. From what I read it seems that if your power supply will work without a load on the +5v line then there is no need to add a resitor. In using a power supply for a car stereo amp the load on the +12v is sufficient enough for the amp by itself (assuming it is a decent quality power supply).

Correct me if I am wrong.

[webgeek]

The resistor is only needed if a power supply shuts down when it doesn't detect a load across the 5 volt line. I played with a few PS options and it seems like all the higher wattage ones did this. I suspect it's due to the increased heat and fire risk if something goes catastrophically wrong.

There are two parts to the link I listed. One of them shows how to apply a load to keep things running and the other shows how to use two smaller resistors to increase voltage. Unless you really need 12 volts then you can ignore the 2 resistor example.

[Luckydevil]

I'm using a pc power supply that supplies 34amps to the +12volt line and has no need for a load on any of the other lines to use the +12volt. Pretty cool looking and has performed great. It's only about $50 shipped also.

http://www.newegg.com/Product/Product.asp?Item=N82E16817182002

[webgeek]

I'm using this one:
http://3btech.net/chdufan650wa.html
It cost 30 dollars shipped and the resistor cost me less 80 cents for two of em.