Thermistor switch

367

03:58:32

I'm building a thermostat for a TEC-based minifridge I built last summer.
Using the 555IC and a thermistor, I plan to create a 30Minutes defrost cycle when the temp falls below 3℃.
The defrost cycle is implemented by switching the peltier (modeled as a 120W bulb) power from 12V to 5V*
The whole device is powered by an old ATX PSU rated for 300W, so I get  3.3V, 5V and a 12V rails for free.

*from what've I learned switching the peltier completely off will induce thermal stress on the TEC couples leading to shorter life for the device.


The 555 is powers a 6V relay that switchs off the 120W load.
The 555's trigger is connected to a voltage devider where the lower potentiometer+resistor simulates the thermistor and the upper potentiometer enables temp control.

I set the 555 cycle to ~5 seconds in the simummm..lation to make it presentable

published 10 years ago

thebugger

10 years ago

You needn't resort to such complicated techniques to make a thermostat. Just use a thermistor and the hysteresis from a Schmitt trigger. I think I've seen some circuits online. Just search for op amp thermostat circuits.

Sine_eyed

10 years ago

Maybe I missed something, but it seems that the relay won't trigger if the upper pot (tmp cntrl) is set to anything above 1%. So it doesn't have much "control" really. I was ready to give this project an official SineEyed Seal of Approval®!..

spammailbox

10 years ago

@Aine_eyed, you're right. I'll fix this soon.

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