Temperature control circuit composed of TC620

How it works

The temperature in the south is high and lasts for a long time in summer. The power transformer in the transformer room runs continuously in a high temperature environment. Its own temperature rise is easy to exceed 65℃, resulting in damage. Therefore, a fan is often installed to cool it. Disconnect the manual control switch S2 of the fan operation in the attached figure, turn on the power switch S1, and put the control device into use. The 2nd and 3rd pins of the three-terminal integrated voltage regulator ICl output a stable DC 12V voltage, and the power indicator H lights up. When the temperature rises to the set upper limit temperature control point (self-set), the 5th pin of TC620 outputs a high level, so that the transistor V obtains a forward bias voltage and turns on. Relay K is energized and attracted, the normally open contact K1 is connected, and the bidirectional thyristor VS is triggered and turned on. The fan M is energized and operates to cool the power transformer, and the temperature gradually drops. After a period of time, when the temperature drops to the set lower limit temperature control point (self-set), the potential of pin 5 of TC620 changes from the original high level to the low level, causing the relay K to lose power and release K1 to disconnect. At this time, VS loses the trigger voltage and turns off, and the fan M stops rotating. When the temperature rises to the upper limit temperature control point again, the circuit repeats the above working process. The fan M only operates within the temperature range between the upper and lower limit temperature control points, and stops rotating in other cases. The operation of M finally realizes the automatic control of temperature.

Component selection and setting of upper and lower temperature limits

The current model of IC1 is LA7812, which can be used interchangeably with AN7812, LN7812, W7812 and other models; IC2 uses TC620XVOA model temperature control integrated circuit; VS is BTA type plastic-encapsulated bidirectional thyristor (5A1400V); T is a power transformer with a power of 8~10W and a secondary winding voltage of 16-20V; relay K model is JQX-4F; the remaining components can be selected according to the markings in the attached figure.

The specific value of the control point can be determined according to actual needs. The specific method is: put this circuit into the temperature control equipment (drying box/oven, etc.), when the temperature in the equipment reaches the upper limit temperature control point, adjust the upper limit temperature control potentiometer RP2 so that M just starts to rotate; when the temperature in the equipment drops to the lower limit temperature control point, adjust the lower limit temperature control potentiometer RP1 so that M just stops rotating. In this way, the upper and lower limit temperature control points are set, and later they can be replaced by fixed resistors according to the actual resistance values ​​of RP1 and RP2 at this time.