I'm working a little project that I need help with. I'm new to electronic circuit design and would appreciate any direction I can get.

I need a 120v receptacle that will switch off when a certain temperature is reached via a thermistor. I would like the circuit to get it's power from 120v volt supply. The end project will have a small box with a wire leading to the thermistor. The box will plug in to a 120v outlet. A 120v accessory will be plugged in to the box and will turn on and off when the thermistor reaches a certain temperature.

Thanks

S

 

first off, you agree that an isolation transformer supplies the sensor/control circuit. From there, you compare a voltage reference (variable if so desired) against your thermistor, then switch either a relay or solid state switch with isolation.

If it's simply on/off, then a comparator will work, but if you want some kind of closed loop heat/cool circuit, you may want to consider a PID capable controller.

turn on and off when the thermistor reaches a certain temperature.

Do you mean blink at some rate when temp reached?

 

No, not blink. Just turn off the 120v receptacle when 85 degree water temperature is sensed and keep power off until it drops below 85 degrees.

Thanks

Scott

 

No, not blink. Just turn off the 120v receptacle when 85 degree water temperature is sensed and keep power off until it drops below 85 degrees.

Thanks

Scott

Here's a link to a Omron doc. It's got some pretty good background info about control requirements/ sensor types. It might help you understand the requirements for a simple controller circuit.

http://www.google.com/url?sa=t&sour...-BG-swwvMwb7Nar8g&sig2=GH4O-dO6vEzAgXrJQg9wQw

 

I use these quite often at work, you can often get them locally for around $55

The output is a simple SPDT relay that's totally open ended on all contacts so you can switch anything with it:

http://www.google.com/search?hl=en&...fai=CxZ1y8NC7TNeGIpSEygSsidnUCQAAAKoEBU_QSBXS

http://www.johnsoncontrols.com/publ.../temperature/electronic2/a419singlestage.html