Keep the PSU fan spinning after you turn the computer off

[370forlife]

Re: Keep the PSU fan spinning after you turn the computer off

to cool the power supply when in standby, those two transistor circuits like to make some heat when not in use.

[everell]

Re: Keep the PSU fan spinning after you turn the computer off

When the computer is plugged in, but turned off, the 5vsb in the power supply is still running. This produces heat in the power supply, even though the computer is turned off. End result is that the heat cooks the capacitors in the power supply while the computer is off.

How much heat depends on the power supply circuit design, parts layout inside the power supply, and available air flow inside the computer case. The heat problem is worse for some power supplies than others. For instance, the Bestec ATX-250 12E has a reputation for killing mother boards. The problem is due to 5vsb circuit heat failure.

The circuit in this thread uses only one fan for cooling whether power is on or off. Earlier I had demonstrated a modification using one fan when power is on and another fan when power is off. Attached photo is a Bestec ATX-250 12E with two fans. This power supply will no longer be a mother board killer. It has a modified 5vsb circuit and a separate fan for cooling the 5vsb.

[Rulycat]

Re: Keep the PSU fan spinning after you turn the computer off

Thanks for the explanation. Are good brands such as Antecs affected by this?

EDIT: More specifically the TruePower Trio (seasonic)

[Wizard]

Re: Keep the PSU fan spinning after you turn the computer off

To vent out the heated air from slowly cooling down warm parts, this also serves to help you to cool room off quickly for the night when PC is turned off so it is not a radiator for 20-30 minutes without the cooling help.

This circuit cut out fan automatically after a certain time.

Cheers, Wizard

[370forlife]

Re: Keep the PSU fan spinning after you turn the computer off

EDIT: More specifically the TruePower Trio (seasonic)

Probably not, depends on the age of the power supply and what wattage.

[etnietering]

Re: Keep the PSU fan spinning after you turn the computer off

They're all right. Seasonics are good, so they probably don't need something like this, but I believe it would be beneficial to any power supply. When the power supply is on, it's dumping out anywhere between 40W and several hundred Watts of waste heat, mostly through the heatsinks. When you turn the computer off, the airflow over the heatsinks (from the fan) stops, but they're still very hot. Since there's no airflow anymore, the heat stored in the heatsinks can radiate out and bake all the other components, perhaps even making them hotter than they were when the computer was running. Also, the 5v standby circuit is still running, generating heat of it's own.
The power supply I put my circuit into was an Antec, but it's not because I particularly thought it needed it, it was just easily accessible, and the one I picked.

[riotpack]

Re: Keep the PSU fan spinning after you turn the computer off

Think of this as no more than a question, but when the computer is running everything settles down to a standard operating temperature (which may change depending on computer loading) usually 35 degrees Celsius - 45 degrees Celsius cabinet temperature.
When you shutdown the computer all the components will contract because they are cooling down and this continuous thermal stress eventually kills components.
Now if you are feeding cool air over the hot components once they are no longer dissipating heat to cool them, then aren't you in fact exposing them to more thermal stress and thereby reducing their life?

[stj]

Re: Keep the PSU fan spinning after you turn the computer off

keeps the standby circuits cool.

[riotpack]

Re: Keep the PSU fan spinning after you turn the computer off

But its only on for a couple of minutes once the computer is turned off isn't it (the fan in this design) ? So that would result in the standby circuits (Estimated around 5 watts) going from hot to cold and back to warm when the fan finally turns off?

"R2 and C5 control the length of time the fan runs after you power down - with these values it runs between 2 and 2.5 minutes. I know there are probably better, more efficient ways to do this, but I used the parts I have on hand. I know that PSU fans are 12 volt, and some won't run on only 5 volts, but this runs then at 12 volts (minus the diode voltage drop) until the PSU turns off, and then the 5 volts kicks in. Withe the two fans I tested this on (one 80mm and one 120mm) the 5 was enough to keep them spinning."

This is what I read.

[Scenic]

Re: Keep the PSU fan spinning after you turn the computer off

the fan never stops on this design.
keeps running off 5VSB all the time..

[riotpack]

Re: Keep the PSU fan spinning after you turn the computer off

Then why not just use this? - I know it's messy; I just quickly drew it up.

[Wizard]

Re: Keep the PSU fan spinning after you turn the computer off

Put transistor on ground side of fan's, and feed the base via 200-500 ohms to limit current via with diodes or zeners is not meant to be used as current source. Need a pull down resistor of around 1K or so on base as well.

This way this is done correctly.

Cheers, Wizard

[stj]

Re: Keep the PSU fan spinning after you turn the computer off

how you do it will actually depend on how the psu fucks with the fanspeed.

some will have constant 12v and mess with the ground wire, others will mess with the 12v and keep the fan grounded.
these are best because ones without a constant ground are almost impossible to use the tacho output for monitoring.

[370forlife]

Re: Keep the PSU fan spinning after you turn the computer off

EDIT

Whoops, wrong "running the fan while psu is off" thread.