Doing A Simple Temperature Actuator Circuit

[ali3nz0r]

Hi everyone... i think this is a newbie question that you can solve with no problem but i need help doing a actuator.
Basically i need to heat a fabric, i'm using one pic16f877a and a transistor. My pic can give a maximum of 5V and 25ma as output, but i need 500ma flowing throw the fabric to feel some heating. The resistance of fabric is around 100 ohms. I bought the transistor "BC337" but i dont know if is the certain one. The schematic that i was doing was something like this:

««««««««««««««««««««««««5V
««««««««««««««««««««««««|
««««««««««««««««««««««««|
output (0-5V)---R=330ohms--- BC337
««««««««««««««««««««««««|
«««««««««««««««««««««R=100ohms
««««««««««««««««««««««««|
«««««««««««««««««««««««GND

The output values are around 0 and 5V, and i need a lineary response on the fabric (mA), like 0v -> 0mA and 2.5V->250mA and 5v->500mA
dunno if this will works, can you help me?

 

[Raza]

Hi,
Your diagram is explanatory enough. Secondly at what voltage you will operate the transistor, if as it appears you will feed 5 volts to collector and your load will be grounded in emitter circuit. If yes you will get only 44mA through the transistor on fully conducting (5-0.6/100=4.4/100=0.044) Ohms' Law I=E/R.

 

[ali3nz0r]

ty Raza, ok youre right so that 100ohms are from the fabric's resistance so i can't change it, and the 5v of the collector i can't change it either. So how can i do my schematic to have a linear response (volts on base/mA on emiter)?
Another thing that 330ohms resistance is for what? that is really necessary?
and how did you know that the drop of C-E is 0.6??

ty and sorry for my questions