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  • Heater Thermostat with Adjustable Hysteresis

  • Created: Aug 14, 2015

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In HVAC, a thermostat is the device which senses the temperature of the system and activates the output device so that it maintains a certain temperature. It includes a relay that is triggered by thermal conduction or convection. It switches off when the preset temperature is reached, and it switches on again when there is a significant change in temperature. The difference between the temperature at which the thermostat switches off and the temperature at which it switches on again is the hysteresis. Hysteresis prevents frequent switching on and off of the heater.

This particular circuit energizes the relay when the temperature falls, and de-energizes the relay when the temperature rises again. Replacing the PNP transistor (BC557) with an NPN transistor (BC547) will make the circuit operate the other way round. Voltage on pins 5 and 6 controls the temperature at which the relay is energized. Voltage on pins 1 and 2 controls the temperature at which the relay is de-energized, while resistor R3 controls the amount of hysteresis. The temperature at which the relay energizes can be adjusted from roughly 22ºC to 29ºC. The hysteresis is around 4ºC. On the other hand, the thermistor acts as the temperature sensor. Thermistors react quickly to changes in temperature which makes it sensitive to thermal currents. The circuit is designed for a 12-volt power supply.

This circuit is applicable but not limited to heater thermostats, and other similar HVAC equipment. It can also be used to control other devices. However, relays can be replaced with optoisolators for enhanced capability.



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