Solid State Relay Space Heater Controller

The space heater controller pictured above uses a 25 amp Solid State relay, a dual op-amp and a few other parts. The circuit board was laid out on graph paper to work out locations for the holes. The hole positions were then copied to a bread board to be used as a template for drilling the holes. The bread board was then clamped on the top of the blank copper board as shown in the top picture and the marked holes were drilled using the bread board as a guide for the drill (no walking). The drilled board was then sectioned into isolated islands using a dremel tool and router bit as shown in the picture just below the breadboard. Be sure to wear a mask if doing this to avoid breathing fiber glass dust. The components were then soldered in place as shown in the upper right photo. The lower left photo shows the relay mounted to the heat sink, and the finished assembly is shown in the lower right photo. The assembly was then mounted on the inside bottom surface of the heater box. The LED power indicator, temperature adjustment pot and manual toggle switch are mounted on the top surface of the heater. The thermistor was mounted outside the heater box on the left side.


In operation, the desired temperature is set with the 10K pot on the left side of the schematic. The reference at pin 3 is about 4.5 volts so that when the temperature is above the setting, the voltage across the thermistor will be less than 4.5 volts which causes the output at pin 1 to move high to about 8 volts. The reference voltage at pin 5 is about 2.5 volts. The input voltage at pin 6 is half the voltage at pin 1 or about 4 volts which is higher than pin 5 and so the output at pin 7 is low, very close to zero and the heater is off. The 300K resistor from pin 1 to 3 provides a little positive feedback so the temperature must change a couple degrees before the heater will change state. This prevents the circuit from switching on and off too frequently when the temperature is very close to the setting.

When the temperature is below the setting, the reverse condition will occur with the voltage at pin 1 and 6 close to zero and the output at pin 7 will be about 8 volts which activates the SS relay and illuminates the LED. The thermistor used measured about 10K at 74 deg F and the pot setting came out to about 55 degrees with the pot adjusted in the center. The thermistor resistance changes about 350 ohms per degree so the temp range is about 15 degrees above and below the center of 55, or 40 - 70 degrees F. Higher resistance thermistors could be used with a larger value pot.

The heater can be activated manually with the momentary SPDT switch shown near the center of the schematic. The 1000uF cap at pin 5 is charged or discharged through the 300 resistor connected to the switch. When the cap is charged, the heater will remain on for about 10 minutes until the cap voltage falls below the 4 volt value at pin 6. Or, the heater can be switched off using the momentary switch to discharge the cap through the 300 ohm resistor.

The 9 volt power supply was made using a small AC line transformer, four 1N4001 rectifier diodes in a bridge configuration and a 2200uF filter cap. The transformer is the type found in portable radios. No regulator was needed since the load is very small and doesn't effect the supply voltage much.

Thermostat for 1KW Space Heater using SCRs

Below is a SCR controlled space heater circuit built in 2001 which recently became intermittent so I decided to replace it with the simpler solid state relay circuit above. Also, SCRs are much cheaper than solid state relays which run about $40 for the 25 amp units.

The heater element (not shown) is connected in series with two back to back 16 amp SCRs (not shown) which are controlled with a small pulse transformer. The pulse transformer has 3 identical windings, two of which are used to supply trigger pulses to the SCRs, and the third winding is connected to a PNP transistor pair that alternately supply 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 half cycle but only one conducts depending on the AC polarity.

DC power for the circuit is shown in the lower left section of the drawing and uses a 1.25uF, 400 volt non-polarized capacitor to obtain about 50mA of current from the AC line. The current is rectified by 2 diodes and used 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 first applied.

The lower comparator (output at pin 13) serves as a zero crossing detector and produces 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 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 maintain conduction.

The upper two comparators operate in 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 SCRs. 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 SCRs.

The comparator near the center of the drawing (pins 8,9,14) is used to allow the heater to be manually run for a few minutes and automatically shut 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 a 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 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.