Relay control circuit a

Relay control circuit a

Shown is the relay control circuit . There are many switches in the microwave oven. If the switch is turned on, it can provide power, and if the switch is turned off,
it can provide protection. There are many components in a microwave oven that need to be controlled, such as fans, light bulbs, food turntables, timers, etc. Most of the signals controlled by relays
    in the circuit are strong signals, and many of them have a voltage of 220 V. If a transistor is used directly to control the switch, the transistor will have to withstand a high voltage of 220 V, which requires the use of high-voltage-resistant transistors and high-power transistors. This will not only increase the production cost of the circuit, but also ensure the safety during maintenance. Because the voltage and current of the transistor are high, a heat sink must be added. The heat sink may be charged, and the three legs of the transistor may also be charged, which is very unsafe. By using relays, low-voltage signals can be used to control high-voltage signals.     As you can see from the picture, there is a coil made into an electromagnet inside the relay. When the electricity is energized, the electromagnet will move, thereby driving the contacts to open and close . The voltage on the contacts is isolated from the relay coil, so there will be no high voltage in the circuit controlling the relay coil, which is very safe.     The two circuits on the left and right in the figure have the same structure, but the control transistors used are different. The circuit on the left uses NPN transistors, and the circuit on the right uses PNP transistors. The polarities of PNP transistors and NPN transistors are different, so in a circuit using PNP transistors, the voltage must be applied to the emitter of the PNP transistor, and the ground must be connected to the relay end. The driving signal of the house is also opposite to what was mentioned before (NPN transistor circuit). If the transistor is turned off, the control signal in the circuit is high level, and if the control signal appears low level. The transistor will turn on, and 12 V will flow through the relay to control the on or off of the relay contacts. If a PNP transistor is connected to the circuit , the polarity of the designed protection diode must be reversed, because the polarity of the induced electromotive force generated by the coil also changes . When looking at the circuit diagram, pay attention to the model of the transistor. Different transistor models have different control signals , voltage polarity, and protection diodes .