# Corsair PSU question

#### Rifter

##### Lifer
My 850TX seems to be self powered. The other day i turned my computer off to clean it, this was after some heavy gaming so it was warmed up pretty good. I unplugged all the wires and took off the side panel. My PSU fan was still spinning, which is fine im assuming it has a thermal sensor and wanted the PSU to be down to a certan temp even if off. The problem was it WAS NOT PLUGGED INTO THE WALL. So i figued great its got some energy in the caps and will stop withing 30 seconds and i can clean it, wrong it spun the fan for 7 min with no power, is that normal?

Ghost!!

#### v8envy

##### Platinum Member
I think we have the first documented case of a > 100&#37; efficient power supply.

But yeah, you can store enough energy to kill you in some of those monster capacitors. Spinning a fan for a few hours should be no sweat.

#### Stefan Payne

##### Senior member
Depending on the fan it may happen but not for 7minutes...

#### Tsavo

##### Platinum Member
I think we have the first documented case of a > 100% efficient power supply.

But yeah, you can store enough energy to kill you in some of those monster capacitors. Spinning a fan for a few hours should be no sweat.

110% efficient...sell it on Ebay for \$10,000,000.00

#### Rifter

##### Lifer
Yeah i guess caps holding power is good but i didnt expect that long, next time i clean my PC ill have to let it cool down first.

#### johnnydrinkawater

##### Member
There are usually two 330uF caps in there, in parallel, they are 660uF:

time constant = R x C = R x 0.000660

The time constant (according to wiki) is the time it takes a cap to discharge to about 36&#37; of it's fully charged state.

Let's assume it took 7 minutes (420 sec) for the cap to run out of sufficient power at 36% (this means that the fan requires at least ~4.5V from the caps to register any motion and that the fully charged value of the caps is 12V).

Then,

time constant = 420 sec = R x 0.000660

That means R = ~650k ohms is in series with the caps.

The point is to verify that this value is not out of the question, from a general point of view. However, I don't know what typical values are for PSU design. Of course, being a larger PSU, there may be more large caps in it, thus making the resistance even more typical.