Resistor Power Rating

[paul23]

This is a question I really should know the answer to, but I'm not sure, so thought it is best ask

I am using a Microchip fan controller IC in a circuit I'm working on and I am not 100% sure on the power rating for the resistor Rsense.  The fan motor is 12V 0.1A so 1.2W.  The voltage drop across the resistor is 900mV so I think a 1W resistor is more than adequate, but I'm not 100% sure I am correct and if I should add the 1.2W from the fan and use a 2W resistor instead.

Thanks

[grumpydoc]

The voltage drop across the resistor is 900mV

The datasheet has a table of nominal fan currents vs sense resistor which suggests about half that. At 500mA a sense resistor of 1ohm is suggested - dissipation in the sense resistor would be then be 0.25W, at 100mA 4.7 ohms is suggested giving a dissipation of 47mW

HTH.

[paul23]

Thanks, it was more the method than actual numbers I was confused with.  I tested with a 10 Ohm resistor (the smallest I have available) to try and prove them method and that's why I had 900mV.  So I am right in my original question then and the power dissipation of the resistor is just the voltage drop across it + the fan current + the 4.7 Ohms for the resistor?  I don't include the 1.2W from the fan?

[suicidaleggroll]

The power dissipated in any device is the voltage drop across it multiplied by the current through it.

The 1.2 W from the fan is being burned off in the fan, not the resistor.

[paul23]

Thanks that is exactly what I thought, I just wanted to make sure.