Post-turnoff cooling

[TaskMaster]

Originally posted by rth
In this case, the 2200 uF and 43 Kohm combo yields a 3 minute 5 second discharge time, so the fan will run for this amount of time after shutoff. Raising either the cap value or the resistor value will yield a longer cooldown period, if required. Vice versa is also true.

Don't you mean raising either the cap value or lowering the resistor value will yield a longer cooldown period?

[rth]

Originally posted by TaskMaster


Don't you mean raising either the cap value or lowering the resistor value will yield a longer cooldown period?

No sirree, bob. A lower resistor value will yield a higher discharge current, which yields a quicker discharge and shorter fan run time.

[TaskMaster]

Sorry - I don't know what I was thinking (actually I was still thinking backwards until I thought about a short or open circuit as the resistor)!

[Guy Grotke]

Now if it could just turn off the +12 power supply, then it would be perfect!

[Stocker]

Separate relay, coil in series with the resistor, relay kills the +12?

[rth]

For all practical purposes the 12v is off... think about all the wall warts in your house that never get turned off.

Separate relay, coil in series with the resistor, relay kills the +12?

Plausible, though relay coils draw a bunch of current so you'd need a hefty (gigantic, actually) capacitor to account for this. I just wouldn't worry about it, grid power is cheap and those little guys use very little with no load