1KW Space Heater SCR Thermostat Circuit Diagram

Description

Below is a thermostat circuit recently constructed for managing a 1300 watt space heater. The heating element (not shown) is connected in series with two back-to-back 16 amp Silicon Controlled Rectifiers (SCRs) (not shown), which are controlled via a small pulse transformer. The pulse transformer contains three identical windings; two are utilized to supply trigger pulses to the SCRs, while the third winding is linked to a PNP transistor pair that alternately delivers pulses to the transformer at the beginning of each AC half cycle. The trigger pulses are applied to both SCRs near the beginning of each AC cycle, yet only one conducts contingent upon the AC polarity.

DC power for the circuit is depicted in the lower left corner of the schematic and utilizes a 1.25uF, 400 volt non-polarized capacitor to yield approximately 50mA of current from the AC line. This current is rectified by two diodes and then employed to charge a pair of larger low-voltage capacitors (3300uF) which provide roughly 6 volts DC for the circuit. The DC voltage is regulated by the 6.2 volt zener diode and the 150 ohm resistor, in series, to limit the surge current when power is initially applied.

The lower comparator (output at pin 13) functions as a zero-crossing detector, generating a 60 Hz square wave synchronized with the AC line. The phase is shifted slightly by the 0.33 uF, 220K and 1K network so that the SCR trigger pulse arrives when the line voltage is a few volts above or below zero. The SCRs will not trigger at exactly zero since there will be no voltage to sustain conduction.

The upper two comparators operate in a similar manner as detailed in the “Electronic thermostat and relay” circuit. A low-level signal at pin 2 is produced when the temperature is above the desired level and inhibits the square wave at pin 13, preventing triggering of the SCRs. When the temperature drops below the desired level, pin 2 will move to an open circuit condition, permitting the square wave at pin 13 to trigger the SCRs.

The comparator near the center of the drawing (pins 8,9,14) is utilized to enable the heater to be manually operated for a few minutes and subsequently automatically shut off. A momentary toggle switch (shown connected to a 51 ohm resistor) is used to discharge the 1000uF capacitor, thereby causing pin 2 of the upper comparator to move to an open circuit state, allowing the 60 Hz square wave to trigger the SCRs and power the heater. When the capacitor reaches about 4 volts, the circuit reverts to normal operation, where the thermistor controls the operation. The momentary switch can also be toggled so that the capacitor charges above 4 volts and shuts off the heater if the temperature is above the setting of the potentiometer.

Circuit diagram