On 06/12/11 20:42, davygrvy wrote:
> On Dec 6, 1:27 am, > wrote:
>> On 05/12/11 21:32, davygrvy wrote:
>>
>>> Why any heater wire at all? What if I could just bolt a 15 amp
>>> (TO-3P) transistor right to the manifold itself and cook the metal
>>> directly? Hopefully there's enough material to grind a flat. Using a
>>> PNP transistor, collector is at ground potential so no need for any
>>> mica insulation.
>>
>>> https://www.facebook.com/photo.php?f...et=a.111195867...
>>
>> Remember, with the transistor fully on, there will be little voltage
>> drop across it. Therefore there will be little power dissipation. ...
>> Little heat .
>
> Not true when emitter to collector is right across the battery. 10A @
> 12VDC = 120W
>
>> you need to bias it half way between the rails to get the most power /
>> heat, out of it.
>
> not true at all without a load. I had nearly the same convo with a
> guy I work with. Like you, he normally thinks a transistor drives a
> load. What if it is the load and is operated as just a variable
> resistor?
>
>> It's early, so I might have missed something there ! ...
>
> paradygm shift
>
>> Like I said on FB, a plain old power resistor would be much simpler
>
> I'll post the heat chart from the simulator now so you can see how
> it'll work.
>
>> Rich
>

I must admit, my interest is audio. Different classes of output have
different heat characteristics.
Usually for a fully on transistor, the power/heat is dissipated in the
load. If you have no load ... then I guess you have to over current the
transistor to get the same max power, for the rating of the transistor ?

However you figure it , P = VI
If the transistor is shorted (fully on) then you only have the junction
voltage drop to put in the equation . Therefore you cant get the full
power if you stick to the max current rating .


It's been about 20 yrs since I did this thought process, but you will
still have to convince me I am wrong ! LOL

A simple resistor will give you a set amount of heat without any
controll needed - bar a thermistor circuit to switch it off. -