Ok here is my scenario:

Because our 2 gaming computers are pretty noisy, we moved them to another room in a closet. There are vent holes to the outside of the building, and now that the summer comes I want to add a small computer fan (12V 0.3A) to push the warm air out. The plan is that if one of the 2 computers is turned on the fan should spin.

What I have done so far... and it works:

I extended cables from both computers from the PSU (GND and +12V) to a little box.

I added 2x 1N5059 diodes in parallel on each +12V from the PSU going to the fan.

I know now the diodes can handle up to 2A, so one of them on each +12V line should have done the job.

As I said, it works as expected and does what it should, but, and this is my actual question:

I know diodes allow a very small amount of current in the other direction.

Do I need to be concerned that one PSU could break the other one in long term use?

  • \$\begingroup\$ No concerns . from leakage. but a 5W fan to reduce ambient heat from 1kW computer heat in a closet could be improved greatly. What is the closet temp rise? CPU temp, or Mobo temp rise? \$\endgroup\$ – Sunnyskyguy EE75 Apr 22 at 12:42
  • \$\begingroup\$ Too much numbers ;D We always left the doors open in the winter so we actually heat the room with the computers. But now in the summer i want to blow out the warm air, so that it does not get in the room. I did not really check cpu temps or something. As far as i know, both computers do not thermal throttle while gaming, that was all i wanted to know myself. \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 13:04
  • \$\begingroup\$ Check MOBO temp \$\endgroup\$ – Sunnyskyguy EE75 Apr 22 at 13:13
  • \$\begingroup\$ That is all i have on Linux: k10temp-pci-00c3 Adapter: PCI adapter temp1: +30.0°C (high = +70.0°C) (crit = +72.0°C, hyst = +70.0°C) \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 13:20
  • \$\begingroup\$ But that is not my main concern. The computers do not thermal throttle, but the ambient temperature of the entire room rises, when the closet doors are open. And that is the reason i want a small fan to blow the warm air outside the building \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 13:23

The small amount of reverse leakage current in the diodes is not going to be a problem for the power supplies. The loads on the power supplies will sink the leakage current to ground.

If you remain concerned you could do these things:

  1. Remove one of the paralleled two diodes since it is not needed. This will cut the leakage in half.
  2. You could add a load resistor on the supply side of your diodes to ground to assure that there is a path to ground for any reverse leakage when that particular supply is off.
  3. Just equip your vent with two fans instead of the one. These would be powered separately by each computer. This could be a wise move because when both computers are on you actually have more heat generation in the closet.
  • \$\begingroup\$ Thank you very much. I guess that answers pretty much my question. And thank you for the improvement ideas. \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 12:21
  • \$\begingroup\$ If you remove one diode then the output with the higher voltage will feed current back into the other. Never a good idea. \$\endgroup\$ – scorpdaddy Apr 22 at 12:27
  • \$\begingroup\$ @scorpdaddy - If you read the OPs question you will note that he has used four diodes in his connection box, two in parallel for each supply line to the fan, \$\endgroup\$ – Michael Karas Apr 22 at 12:32
  • \$\begingroup\$ @scorpdaddy: dont know if you got it wrong: I added 2 diodes on each +12V PSU line for a total of 2 x 2 = 4 diodes, where each 2 diodes are parallel. Just to double the ampere they can handle. \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 12:35
  • \$\begingroup\$ Ok. When I read the answer it sounded like removing the diodes on a branch. Perhaps re-word it? \$\endgroup\$ – scorpdaddy Apr 22 at 13:11

There is no chance that 1 PSU will feed the other PSU via the diodes. So no chance of break it! Did you put an fuse in place at the (+12V) FAN side just in case? You wil loose about 0.6V via the diode (see datasheet 1N5059) to your FAN but that will not be an issue I think. You can add an picture just in case so we can check it.

best regards, Erwin.

  • \$\begingroup\$ Thank you, yes I checked the voltage before, it's around 11.6V as expected on the fan side. I don't really care about that little voltage drop, there will be nothing else hooked up other than a fan, and it spins fine. I didn't think about a fuse... is it needed, or would it be better to have one? \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 12:30

I would have used the supply you took from each computer to control a relay, one relay for each computer will give total isolation (excepfor the shared power line) and thise relays can drive the same fan without problem.

So, no diodes or losses from diodes, but do need a power supply to the fan, but that is easy...

  • \$\begingroup\$ Maybe not a bad idea to isolate it completely, but I would need a 3rd power supply that is always turned on for this. I was thinking about something like that, but decided for the diodes. sorry. \$\endgroup\$ – Benjamin Gillhofer Apr 22 at 12:51

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