A relay control circuit

Posted on Mar 28, 2007

A relay control circuit It shows the relay control circuit. There are many in the microwave switches, such as the power supply switch is turned on can be achieved, switch off p

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rotection can be achieved. In the microwave oven there are many components to be controlled, such as electric fans, light bulbs, food turntable, timers. In the relay control signal circuit which mostly belong to a strong signal, many of which are 22V of voltage. If the direct control using a transistor switch, then the transistor must withstand high voltage 220 V, which must use high voltage transistors and power transistors. This circuit will not only increase production costs, but also security overhaul has also been guaranteed. Because the high voltage of the transistor, current, it will inevitably need to add heat sink, the heat sink may be charged, the three legs of the transistor may also be charged, so it is very unsafe. Relays can be achieved by using low-voltage signal to control the high signal. It can be seen from the figure, in a relay coil is made of electromagnet, when energized electromagnet would be action, which led to open contacts close. And contacts on the relay coil voltage is isolated, the control relay coil circuit does not have the high-pressure, very safe. Left and right in the figure on the circuit structure is the same, but uses different control transistor, using the left side of the circuit are NPN type transistors, the right side of the circuit using a PNP type transistor. Polarity PNP transistor and the NPN transistor is not the same, so the use of PNP transistor circuit, the voltage to be applied to the emitter of PNP transistor, connected to the ground to relay this end. House of the drive signal also (NPN type transistor circuit) stated before the contrary, if the transistor is turned off, the circuit control signal is high, if the control signal appears low. Transistor will be turned on, and then will flow through the relay 1V to control the turn on or off the relay contacts. Circuit in the connection of PNP transistors, polarity protection diodes designed also be reversed, because the coil generates an induced electromotive force polarity is changed, and when looking at the circuit diagram, pay attention to the model of the transistor. Transistor models, the control signal, the voltage polarity protection diodes are not the same.

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