Questions concerning thermistors

[undefined]

I've got some rather simple questions concerning thermistors. First, in viewing spec. sheets I only see one resistance value. What is that value telling me? In some of the specs there is something called a b-constant. What is that? Does a temperature actuated circuit demand an amplifier to take the small changes in resistance and produce larger voltage fluctuations? or can you just use them directly in series with a zener to make a temperature actuated switch? Basically I know little more than the general idea that they vary their resistance with temperature. Any answers, information, etc. would be much appreciated. Thanx for your time and patience with my ignorance.

 

[wwtog]

Most applications use the dR/dT----> dV/dT about a nominal value to drive a differential amp for proportional control, Or use the voltage drop and a windowed comparitor w/hystrerisis for on-off type control.

 

[undefined]

Yea, all I need is an on-off type deal.

 

[wwtog]

If you have access to an national semi analog circuits handbook, there is a lot of good examples, otherwise, radio shack sells these small and really nice, circuit design handbooks, I think op-amps and comparators are in the same book, good designs that work and can be built quickly.