From left to right, the analysis is as follows: the level signal of the single chip microcomputer is input to an operational amplifier of 74LS245 from port 3, which shapes and amplifies the waveform, causes the optocoupler diode to emit light and the optocoupler transistor to turn on, thus achieving the purpose of signal transmission level isolation (because high voltage may enter the pin of the single chip microcomputer without isolation, and the single chip microcomputer cannot bear high voltage, generally within 5V).

Then the waveform is shaped and amplified by another operational amplifier of 74LS245, which pushes the switch tube TIP 122 to switch the circuit according to the input level signal of the single chip microcomputer, so that the current of 12V passes through the primary winding of 1: 1 transformer, and the secondary induced voltage turns on the silicon controlled SCR, so that the alternating current is heated by the heating device.

The diode connected in parallel with the transformer primary is a freewheeling diode, so when the TIP transistor is turned off by a signal, the current in the coil can be discharged through this diode (why should it be released? According to the characteristics of inductance) to protect TIP 122. The parallel resistance of the secondary pole is similar to the resistance of the primary diode, and it is also used to protect the thyristor from being broken down by the induced voltage of the inductor (if you don't believe it, you can try to remove this part of the circuit and see what happens! )

In fact, it seems a bit complicated for beginners, but in fact, a bunch of things just realize the function of a switch, but this switch is not controlled by people, but by the pulse signal of a single chip microcomputer.

To say that it can control the power supply is actually to control the on-off time of this switch. However, unlike manual control, pulse control requires switching once in a very short time (why is this necessary? It is because the transformer is turned on for a long time that it will not work. Why? Transformers can't transmit DC! Therefore, the switching frequency is generally very high, ranging from tens to hundreds of KHz. What should I do? That is to say, it is enough to make the time ratio of high level and low level different in one switching cycle. This is actually the duty cycle of PWM in the switching circuit, so that the on-off time of the power supply can be adjusted as a whole, thus realizing power adjustment.