The schematic to the Kit i am currently using us on page #2 of this thread...
Here is a link to the data sheet for the thermistor that I have.
HERE
Its the "1,000Ω @ 200C" Model: GB2315R-3-200
SteveB: it must be some sort of proportional control. ON/OFF is unacceptable....
Rolf: Its not overkill and quite necessary as one of the requirements is that the heating system is that it needs to turn on, be able to deliver the desired heat(with decent precision) with in 5 seconds of operation signal, maintain the target temp range while the flow rate of the air changes
slightly, and immediately begin cooling when the operating signal stopped. I highly doubt a house thermostat can do that..
Also... I want something thats fairly simple and cheap that is home made. I'd like to be at least semi original.
So far I've accomplished a few of my requirements. My current model simply runs off of the PWM circuit and I've adjusted the power so that the once the air is flowing, the circuit is powered on, the target temp is maintained only because the air flow rate is constant, and when I power it down, the little element cools quickly.
What I have in my mind is just setting the power a little higher than i need to get to target temp.. then using the thermistor somehow to bring the power down to when the temp goes above 'target'. So I would have to add some kind of loop with the thermistor that, as the resistance decreases (increase in temp), the pulse width decreases.
So... what do your expert minds think?