Thermistor project

[ed12309]

hey guys,
i have one year of digital electronics under my belt, but we never touched up on thermistors.
i wanna design a circuit that will turn on a heating wire at a certain temperature, maybe 35F, and it will turn off around 40. If anyone has some ideas on a circuit help would be great, thanks.

 

[ed12309]

im thinking of using just a bimetallic strip that will trip when it reaches a certain temp, i just want to make it a little more fail safe

 

[MikeMl]

Its all about the sensor. Get hold of one first, and then design the circuit around it. Ideally, you want it to have a resistance of ~10K at your set-point temperature.

Usually, the thermistor is one arm of a four-arm bridge. A comparator senses the voltage across the bridge. The bridge is balanced at the set point temperature.

Positive feed back around the comparator provides a slight amount hysteresis to prevent chatter as the sensor heats or cools through the trip point. The comparator output is used to switch the gate of a power FET to switch the load...

From an old posting:

 

[ed12309]

i have a 10k ntc thermistor at the moment

 

[Wp100]

Hi,

Well you can do it the analogue way with a bridge and op amp like a 741 / 324 etc. as Mike suggests, but as you say you are doing 'digital' I would assume you want to use a micro ?

Many Pic chips have ADC inputs to accept a thermistor that easily converts the temperature / voltage into a digital value so you can the program and control the output as you like.
There are many such circuits/ program codes in this site and on the web.

 

[ed12309]

so i should us a thermistor and a comparator and when it reaches the right resistance it will turn on, then i have a timer, and when the time runs up it will check the system again, to see if it matches resistance again or not, thats really simple, hey thanks alot

 

[ed12309]

Hi,

Well you can do it the analogue way with a bridge and op amp like a 741 / 324 etc. as Mike suggests, but as you say you are doing 'digital' I would assume you want to use a micro ?

Many Pic chips have ADC inputs to accept a thermistor that easily converts the temperature / voltage into a digital value so you can the program and control the output as you like.
There are many such circuits/ program codes in this site and on the web.

a micro would be nice but i would think using a thermistor a timer and a comparator wouldnt be that hard to do and it would be pretty compact.

 

[MikeMl]

Cool it in a refrigerator, which will be at about 37degF, and measure the resistance there. You will have to tweak the values in the four arm bridge so that the bridge balances near that temperature.

 

[MikeMl]

so i should us a thermistor and a comparator and when it reaches the right resistance it will turn on, then i have a timer, and when the time runs up it will check the system again, to see if it matches resistance again or not, thats really simple, hey thanks alot

No timer needed. As the environment that the thermistor is in heats and cools, the heating/cooling load switches on and off. The "timing" is done by the thermal mass of the environment that is being controlled. Think of a house thermostat, or your refrigerator. No timer in those...

 

[ed12309]

um

thanks i got it now