Simple and practical thermostat controller circuit

Although this circuit is simple, it can indeed achieve control effects. See diagram for circuit. The working principle is as follows: ST is a WTQ-288 electric contact pressure thermometer. When the temperature in the thermostatic box drops to the lower limit, the pointer of ST contacts the lower limit contact, and the two-way thyristor is forced to conduction through R, turning on the heating. The power supply of device RL is turned on, so the temperature inside the thermostatic box rises. The ST pointer rotates and disengages from the lower limit contact. Although the trigger circuit is disconnected at this time, due to the phase-shifting effect of capacitor C, when the power supply voltage crosses zero, the capacitor current is not zero. In this way, when the power supply is reversed, it can still provide trigger current for the triac. And make the thyristor conductive. When the temperature in the thermostatic box rises to the upper limit temperature value, the pointer of the ST electrical contact thermometer contacts the upper limit contact, and the triac loses the trigger current and turns off when the power supply voltage crosses zero. At this time, the temperature of the thermostat box begins to drop. When the temperature drops to the lower limit, the bidirectional thyristor is triggered to conduct again, turning on the power of the heater RL, causing the temperature in the thermostat box to rise. This cycle achieves the purpose of constant temperature. The power of the thyristor can be selected according to the load size. When the power is increased, the capacity of the capacitor C should be appropriately increased to ensure sufficient trigger current.