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What are the principles and precautions of the relay drive circuit

                       
Update:26-03-2021
Summary:

The relay drive current generally needs 20-40mA or more […]

The relay drive current generally needs 20-40mA or more, and the coil resistance is 100-200 ohms, so a drive circuit must be added.

1. Transistor Driven Relay

When the NPN transistor is driven: when the base of the transistor T1 is input high, the transistor is saturated and turned on, and the collector becomes low, so the relay coil is energized, and the contact RL1 is closed.

When the base of the transistor T1 is input low,prepayment meter shunt the transistor is cut off, the relay coil is cut off, and the contact RL1 is cut off.

The PNP transistor drive circuit is not currently used, so it will not be introduced here.

Transistor T1 can be regarded as a control switch. Generally, VCBO≈VCEO≥24V is selected, and the amplification factor β is generally selected between 120 and 240. . The resistor R1 mainly acts as a current limiter, reducing the power consumption of the transistor T1, and the resistance value is 2 KΩ. The resistor R2 reliably cuts off the transistor T1, and the resistance value is 5.1KΩ. Diode D1 reverses freewheeling to suppress surges. Generally, 1N4148 can be selected.
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According to the input and output characteristics of integrated circuit driver 2003, some people call it "driver", "inverter" and "amplifier" for short. Now the commonly used model is: TD62003AP. When the 2003 input is at high level, the corresponding output port outputs low level, the relay coil is energized, and the relay contacts are closed; when the 2003 input is at low level, the relay coil is de-energized and the relay contacts are disconnected; in 2003 The diode has been integrated to play the role of reverse freewheeling, so it can be directly used to drive the relay.

The method of judging the quality of 2003 is very simple. Use the DC gear of a multimeter to measure the input and output voltages respectively. If the input terminals 1~7 are low level (0V), the output terminals 10~16 must be high level (12V); Conversely, if the input terminals 1 to 7 are high level (5V), the output terminals 10 to 16 must be low level (0V); otherwise, the driver is broken.