LM723 as a thermostat

This thermostat has been in service since January 14, 1980 and has never needed a repair. Even the aluminum electrolytic (capacitor) on the power supply has not failed!

The "real" one drives a triac for the output (short sighted of me to do that.) If you set the hysteresis with any resistance higher than 680k, it misses half of the cycles of the power line and causes a buzzing noise in my space heater, so I lowered the hysteresis from 2.7 Meg to 680k to stop the noise. Another method would be to add a common emitter transistor stage to increase the open loop gain of the LM723.

EDIT: All capacitor sizes are, "what I had laying around that day". You are allowed to calculate for different sizes and will probably end up with a less expensive design because what I had laying around that day was more than what was necessary for the job.

Update: The triac failed "open" on December 26, 2017. It seems to me that the failure was caused by temperature cycling. Three weeks short of 38 years with a 4 amp load? Seems like a good result.;)

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