power question

[scsn]

Hi,
We are using a power transistor to generate heat to a small chamber, see attached. It works well but I was wondering : With 24v input and with a 12v on the emitter (we have a 6.8R 50W resistor), what is the wattage through the transistor? Is it :
I=12/6.8= 1.76A, so power= (24 (collector) - 12 (emitter)) * 1.76 = 21W ?

And if we reduce the voltage on the collector to 18v the wattage would be 10.56W? Is this correct?

unfortunately we cant place the resistor onto the small chamber, as the design for a transistor was made prior to looking at the job, which is a pitty as heats up a lot..

Thanks
Scott

 

[bountyhunter]

Power dissipated in transistor is: ( VC - VE ) x (IC)

plus:

VBE x IB


where: IB is IC divided by trasistor current gain

 

[Diver300]

Why is the resistor there at all? I guess you need some sort of current control or limiting, but I would have thought that there would be ways of doing that with much less voltage drop across the resistor.

I used to make ovened crystal oscillators, where the crystal was in a small copper oven that that was held at a constant temperature by a transistor. There was a small resistor to help the current control, and a separate transistor limited the current when the voltage across the resistor hit 0.7 V. With a 12 V supply, and a maximum of 0.7 V across the resistor, there was only about 5% of the power lost in the resistor. Your circuit seems to lose about half the power in the resistor.