Programmable Thermostatic Switch using AD22105

Rajkumar Sharma
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SKU: EL90113
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This project simply switches the system OFF or ON whenever the temperature rises at the desired set point. The desired setpoint can be set using a fixed resistor R6 or PR1 trimmer potentiometer. Use any of at a time R6 or PR1. You can calculate the setpoint resistor R6 using the following equation:

A relay is provided to switch ON or OFF the load. The relay can switch 7Amps/250V AC load. D2 LED indicates the relay operation. J1 provided to choose normally closed or open options for the relay. This project is built using AD22105 IC from Analog devices. The AD22105 is a single-supply semiconductor thermostat switch that uses circuit architecture to realize the combined functions of a temperature sensor, set point comparator, and output stage all in one IC. By using one external resistor or trimmer pot, the AD22105 can be programmed to switch at any temperature selected by the system designer in the −40°C to +150°C range. The internal comparator is designed to switch accurately as the ambient temperature rises past the set point temperature. When the ambient temperature falls, the comparator relaxes its output at a somewhat lower temperature than that at which the comparator originally switched. The difference between the switch and un-switched temperatures, known as the hysteresis, is nominally 4°C.

To combine and calculate the initial tolerance and thermal drift effects of the setpoint resistor R6 use the following equation:

R_MAX is the worst-case value that the setpoint resistor can be at T_SET. R_NOM is the standard resistor with a value closest to the desired R_SET. ε is the 25°C tolerance of the chosen resistor (usually 1%, 5%, or 10%). TC is the temperature coefficient of the available resistor. T_SET is the desired setpoint temperature.

After calculation, compare R_MAX to the desired R_SET from Equation 1. The required value of R_SET at a T_SET of 125°C is 5.566 kΩ. If the nearest standard resistor value is 5.600 kΩ, its worst-case maximum value at +125°C is 5.713 kΩ, which is +2.6% higher than R_SET, leading to a total additional error of −0.52°C beyond that given in Table 1 in the datasheet.