thermal tracking circuit


2003-03-01 6:54 pm
I'm looking into constructing a thermal tracking circuit for my 350W BJT-based subwoofer amplifier and was wondering if anyone had any suggestions.

I have seen the LM35 and I figure I would run the output from the sensor to 2 comparators. One comparator circuit would be set at the threshold for turning on my fans and the other would be set at a threshold for shutting down power to the amp (turning off the power relay).

The best I can figure is that I'll need a schmitt-trigger circuit in here to slow down the output. I may also need something else to slow down the reaction of the circuit so I don't get the fan turning on and off continuously.

Any input would be appreciated!
What you would want is a comparator with hysteresis. That means that the two switching points are not equal. For instance, you would switch on the fans at, say, 50degrees, but you would switch them off at say 40degrees.
The way this is often done is to put some positive feedback around the comparator, which modifies the two switching points.
There will be a lot of examples on the net, I'm sure.

Jan Didden

Edit: like here, just a few pages down: