audioguru,
There is a switch just above R1. When the switch is closed, the mosfet turns on and C1=33uf is charged. When the switch is opened, the capacitor discharges and keeps the mosfet, and hence the fans, on for about another 3-5 mins.
I have C2=100uf to ensure that the fan doesnt stall at slow speeds. When power is first applied, the parralel resistance across the drain of the mosfet is low and the base current to the transistor is high irrespective of the Pot setting. This ensures that for about 1s max voltage is applied across the fans to kickstart it.
I dont quite understand why you say that the voltage doesnt matter. Can you possibly explain further?
Thanks
There is a switch just above R1. When the switch is closed, the mosfet turns on and C1=33uf is charged. When the switch is opened, the capacitor discharges and keeps the mosfet, and hence the fans, on for about another 3-5 mins.
I have C2=100uf to ensure that the fan doesnt stall at slow speeds. When power is first applied, the parralel resistance across the drain of the mosfet is low and the base current to the transistor is high irrespective of the Pot setting. This ensures that for about 1s max voltage is applied across the fans to kickstart it.
I dont quite understand why you say that the voltage doesnt matter. Can you possibly explain further?
Thanks