Hi,
I am new to element14 so not completely sure that I am posting in the correct place, but I have a technical question.
I am designing a heating control system using a PIC, using a number of thermistors and coils of nicrome wire to heat up various parts of a machine. I am looking for a method to switch on the nicrome wire coils - I understand that this could be done with a transistor but am unsure of exactly how and what components to use?
Thanks in advance for any help,
Edd
Hi,
Without more details on the coil and how it is to be powered, it's difficult to give you a complete answer, but assuming that you are using a relatively low voltage dc power supply for the coils, then a suitable N-type MOSFET arranged as below should do the trick.
What is the resistance of the coils?
What is the supply voltage?
What is the max current drawn?
Answering the above will help you choose an appropriate FET.
Well it should just be ohms law
Volts = Current * Resistance
If it's a 5 volt supply and an approx 5 ohm load then it will draw approx 1 Amp.
Power = Volts x Current ~ 5 * 1 in this case, so 5 Watts.
So a FET that can handle 5W @ =>5V (ideally a bit more) and turn on at Vgs 5V or less should be fine.
One thing I am wondering though is if the resistance of the heater wire will drop as it gets hot, I suspect it will, so you may need to introduce a resistor in series with it to limit the current, but you will need to experiment abit to see how this goes.
If you want to learn more about MOSFETs, this is a good place to start http://www.irf.com/technical-info/appnotes/mosfet.pdf
