Instantaneous water heater automatic constant temperature experimental circuit diagram

I have been using the instantaneous water heater at home for many years, but every time I take a bath, I feel troubled. The water temperature control of the water heater is divided into 9 levels. The larger the number from 1 to 9, the higher the temperature. It should be easy to use. However, the water flow can also control the water temperature, and often causes the temperature to be out of control. For example, the water pressure decreases and the water volume becomes smaller. The heat cannot be taken away by the water immediately. The temperature (protection) switch inside the water heater will activate and you will be scalded. It is very uncomfortable to be tortured by cold water after a while.

In order to change this situation, I added a simple temperature control circuit, which can control the heating time of the original circuit, that is, the duty cycle. The effect is satisfactory, especially the temperature (protection) switch inside the water heater never operates again, which is very comfortable. .

The temperature sensor in the circuit uses an old-fashioned thermistor. Since the resistance is a bit small, a germanium diode is added for this purpose.

The main components used in the water heater circuit are: ULN2003AN, 7805/7812, BTA41600B, JQX-15F, HMS87C1202A, MOC3021.