One of the zero-trigger thyristor temperature control circuits

In the thyristor AC voltage regulation system, according to the control method, there are usually two types: phase-shift trigger and thyristor zero trigger. The former
changes the voltage (or power) on the load by changing the conduction angle of the thyristor. The voltage (current) fed to the load
is a missing-angle sine wave with low power factor and contains many high-order harmonics, which will produce Radio frequency interference to the power grid and radio
can also cause malfunctions between thyristor devices. The more powerful the thyristor device used, the more severe the interference caused
. The latter uses thyristors as AC switches, triggering conduction when the AC voltage (or current) crosses zero, and
realizes power adjustment by controlling the on-off ratio. The voltage (current) obtained on the load is always a complete sine wave, thus The shortcomings of the former
method are avoided.
The circuit is shown in Figure 11-80. The temperature measuring element is a thermistor RL with a negative temperature coefficient.
    When the temperature is lower than the set value (the set value is determined by the potentiometer RP), the thyristor vl is turned off, vz, V. On, the electric
heater EH heats. When the temperature is higher than the set value, vl is turned on, V2 and V3 are turned off, and EH stops heating.