Temperature Controller With Defrost Cycle

This temperature controller has a range of -50 to +150°C and permits defrosting. R7, VR3, and R8

set the controller"s trip point. SI initiates defrosting, S2 cancels defrosting. VR1 and VR2 set the defrost-temperature trip point. The LM134, IC1, is a thermal sensor. One section of IC3, a quad op amp, buffers the sensor"s output. The other section functions as Schmitt trigger and buffer for the normal-cycle circuitry and as a comparator for the defrost-cycle circuitry. you wish to control a heater rather than a refrigerator, omit the final inverter in Ql"s base circuit. You must select LM7805s that have outputs between 4.95 and 5.05 V, or the circuit might not work.

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