Transistor heat levels

[AxelD]

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

 

[audioguru]

so the circuit is a lot more than just a simple speed controller.
The conduction of the Mosfet and hence the speed of the fans is determined by the voltage divider so a voltage regulator would keep the speed constant if the voltage changes.
It doesn't matter if the unregulated voltage rises when the motor speed is turned down. If the input voltage is constant then a voltage regulator is not needed.

 

[AxelD]

Im still not 100% sure i understand but thats probably because im a bit cloudy on some of the basics. Everyone please bear with me, your input is appreciated.

In terms of my circuit, what is the correct way to look at the resistance of the circuit. What "load" does the power supply "see"

Thanks

 

[audioguru]

The current in the circuit is the current of the fans plus a few mA from the 3.7k resistors feeding the base of the transistor.

 

[AxelD]

Thanks i think i understand now.

The voltage i apply to the base of the Mosfet ensures that the full supply voltage is available on the drain - i.e. the gate voltage i supply is much higher than the threshold voltage required for saturation. Because of this im seeing a change in transistor current as the supply voltage changes.

I will modify the circuit so that i get more or less 12v available on the drain irrespective of the changes in the supply voltage.

Thanks again.

 

[audioguru]

The threshold voltage of a Mosfet is the gate-source voltage that just barely turns it on. It needs a much higher voltage to saturate. It will saturate in your circuit because its current is very low.

The drain voltage goes to +12V only when the Mosfet is turned off. When the Mosfet is saturated then its drain and source voltages are about +0.7V which is the base voltage of the transistor.