Transistor power rating(newbie)

[rmk]

Would someone clarify what the transistor power rating means. I have seen some apps where a transistor, used to turn on a solenoid valve, has a lower power rating than what is used to power the valve.<p>I think I am missing just what the rating means.<p>Rich

[haklesup]

It has to do with the voltage drop across the xistor and the load. Take this example:<p>Imagine a transiator (E- GND) in series with a coil with a 120ohm resistance. (C to load, B to control ckt).<p>If the power in the transistor is P=Ic*Vce. Bias it at 3V and 100mA (whatever hypothetical xistor you may need to do this), the power dissipated would be 300mW. Now take that collector current and put it through the 120 ohm load, use the other power formula P=I^2*R = 1.2W. Finally using V=IR we calculate the drop across the load to be 12V. So 12V + 3V =15V is what the supply would need to be.<p>1.2W is dissipated in the solenoid and 0.3W is lost to the transistor.