My adventure with PSUs and Fans.

[Staz]

I recently bought 2 of these for a revamp of my water cooling setup. I had planned on using these with a fan controller, but could not find one that would handle the 27w.

So I decided to run them off of an external PSU and run them at 7v. The first PSU I tried was a 145w mATX PSU. I plugged it in and it blew up. Well it didn't explode, but there was a loud pop and blue sparks shot out of the side. Guess it didn't like sitting in my closet for 2 years collecting dust.

The next PSU I tried was a 150w U1 ATX PSU. I plugged it in and it didn't blow up so that was a good sign. I then used a jumper wire between the green wire and a black one to power up the PSU. Then I wired the fan to run 12v. Man that thing pushes some air, but is also quite loud. Then I tried 7v, The PSU shut down immediately and needed to be unplugged for about 15 seconds before it would restart. I tried at 5v and it worked fine but didn't put out enough air. I tried again at 7v and got the same thing as the first time.

I wasn't sure what was up but decided to try another PSU. This time it was a 230w AT PSU. I plugged it in and hooked up a fan to 12v the pushed the power button. Worked great. Then I tried 5v, same as before, the fan spun at 5v but just wasn't fast enough. As soon as I tried 7v I got the same thing as the ATX PSU. I was getting frustrated now. I looked around the house and found wall adapted at 9.6v, I tried that just to see how fast it would turn. It ran at an almost acceptable speed but the adapter was getting hot quick and it was still too loud. I called it a night, put the fans away and poured a glass of wine.

That night I remembered that ATX PSUs were switching power supplies and needed a minimum draw to work. So this morning I tried again. I hooked a 92mm fan up to 12v and it worked, then I tried another 92mm fan hooked up to 5v and it turned slowly. Then I hooked up the 172mm fan to 7v. It turned but ever so slowly and I could feel no breeze coming off of it. I unhooked the 92mm fan from 12v and hooked it up to 5v and the 172mm fan seemed to turn faster but barely.

Next I unhooked everything and hooked a 172mm fan up to 5v. It turned as before. Then I hooked up the other 172mm fan to 7v, it ran at the same speed as the 5v fan. Then I added a 92mm fan to the 5v line, this caused the 172mm fan on 7v to speed up slightly while the fans on 5v remained the same. Then I added more fans to the 5v but it wasn’t making that much of a difference, however there was a slight increase in the 7v fan every time I added more load to the 5v line.

At this point common sense told me that the PSU could not “absorb” current on the 5v line and that I would need a load equal or greater than (in wattage not amperage) than the load I was attempting to run at 7v. Seeing that my fans draw 2.26amps each the following formula should apply. 2.26 Amps x 2 (number of fans) x 7v = 31.46watts (total load I want to put on 7v) / 5v (load needed to make 7v line work) = 6.238amps.

I would need to put a load of 6.238 amps on the 5v line to make this work. That was totally unacceptable as I couldn’t think of anything drawing that much power. It will work with a PSU hooked up to a computer because of the motherboard and all of it’s components sucking up load at 5v, but this wasn’t going to work on a separate PSU.

Then it hit me, why not just run the fans in series. They draw the same load so they should split the voltage evenly. I hooked them up in series to the 12v line. They worked much better. Very quiet but still just a little to slow. Then I remembered the dedicated 15v 500w PSU that I had purchased for a peltier. It is adjustable from 12v to 18v. I hooked it up and hooked up the fans in series and tried it out. Worked like a charm. I think the sweet spot is ~13.5v.

I hope to soon have both these fans in a push me pull me set up running in series. I hope this offers insight to anyone who has read it and if you have any question or anything to add please feel free to reply.

Thnx for your time
Maurice

[Allan]

Hey Maurice, I'm glad it worked out for you! Say, if you have a digicam, some pictures of your "adventures" would be great.

[Staz]

Well I've got a camera, but I'm not sure what to take pictures of. A fan spinning at 12v looks much like a fan spinning at 6v.

I wish I had a pic of the blue sparks shooting out of the side of the first PSU, that would have been cool. I've got some 60mm, 80mm & 92mm fans I could take pictures of for size comparison to the 172mm fan. Also I could show the PSUs and how I had tried to wire the fans to them at the diferent voltages (on another forum this has alread been asked how to do that). But with my camera all the fans will appear stopped.

Saddly this will have to wait. I have been waiting for a job to go out and the weather has finally cleared. I leave for offshore for 2 weeks at 2PM today. But I will get on it when I get back.

[Daniel ~]

Blue sparks from PSU would differently = Front page!! ":O}

[Aedan]

Originally Posted by Daniel ~

Blue sparks from PSU would differently = Front page!! ":O}

I'll bear that in mind for the next time I do anything with a PSU!

[Aedan]

Switching PSUs need a minimum load to them for sure. However, if you're just running some fans off them, the 7V trick won't work, as you found out!

What happens is that the 12V from the 12V line is fed through the fan, and then onto the 5V line. However, there's no loading on the 5V line (IE, nothing acting as a load), so the voltage rises up towards 12V!

Most decently designed PSUs would shut down once the 5V line gets above 6V, in order to protect whatever's connected. Some PSUs have a 'crowbar' circuit as additional protection. This shorts out the 5V line once the voltage gets above 6V or so. The action of the crowbar circuit either shuts the power supply down, or if the PSU's gone faulty, causes something internally to fail (usually with a loud bang!). Either way, it protects the device connected to the PSU!

Seeing that my fans draw 2.26amps each the following formula should apply. 2.26 Amps x 2 (number of fans) x 7v = 31.46watts (total load I want to put on 7v) / 5v (load needed to make 7v line work) = 6.238amps.

In terms of your little difficulty with calculating the size of the load, you're doing it a bit wrong, but that's ok. If you ever need a large load to put on a PSU, a car headlight bulb (or brakelight bulb if it's not such a big load) is ideal.