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Build a Powerful 1300W Space Heater Circuit Diagram
Description
Below is a thermostat circuit recently constructed to manage a 1300 watt space heater. The heater element (not depicted) is connected in series with two back-to-back 16 amp Thyristors (not shown) which are controlled using a small pulse transformer. The pulse transformer possesses three identical windings, two of which are utilized to furnish trigger pulses to the Thyristors, and the third winding is connected to a PNP transistor pair that alternately supplies pulses to the transformer at the beginning of each AC half cycle. The trigger pulses are applied to both Thyristors near the start of each AC half cycle but only one conducts depending on the AC polarity.
DC power for the circuit is displayed in the lower left segment of the drawing and employs a 1.25uF, 400 volt non-polarized capacitor to obtain approximately 50mA of current from the AC line. The current is rectified by 2 diodes and utilized to charge a couple larger low voltage capacitors (3300uF) which provide about 6 volts DC for the circuit. The DC voltage is regulated by the 6.2 volt zener and the 150 ohm resistor in series with the line limits the surge current when power is initially applied.
The lower comparator (output at pin 13) functions as a zero-crossing detector and generates a 60 Hz square wave in phase with the AC line. The phase is shifted slightly by the 0.33 uF, 220K and 1K network so that the Thyristor trigger pulse arrives when the line voltage is a few volts above or below zero. The Thyristors will not trigger at exactly zero since there will be no voltage to maintain conduction.
The upper two comparators operate in the same manner as described in the "Electronic thermostat and relay" circuit. A low level at pin 2 is produced when the temperature is above the desired level and inhibits the square wave at pin 13 and prevents triggering of the Thyristors. When the temperature drops below the desired level, pin 2 will move to an open circuit condition allowing the square wave at pin 13 to trigger the Thyristors.
The comparator near the center of the drawing (pins 8,9,14) is utilized to allow the heater to be manually operated for a few minutes and automatically shuts off. A momentary toggle switch (shown connected to a 51 ohm resistor) is used to discharge the 1000uF capacitor so that pin 2 of the upper comparator moves to an open circuit state allowing the 60 Hz square wave to trigger the Thyristors and power the heater. When the capacitor reaches about 4 volts the circuit returns 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 pot.
Circuit diagram
