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# Introduction of Three Magnetic Latching Relay Drive Circuits

Hello~everybody！Today we will introduce three relay drive circuits. Hope you like it.

### Magnetic holding relay drive circuit-1

The magnetic holding relay can keep the magnetic field injected by the last driving pulse in the electromagnetic coil inconvenient, that is, no driving current needs to be added during normal work, and only 200ms reverse pulse can be added when the contact state needs to be changed. No subsequent drivers are required. This greatly saves energy and reduces consumption. The circuit diagram is as follows:

The magnetic holding relay is controlled by P1.0 and P1.1 of AT89C52. When P1.1 is high, there is a forward current in the coil, and when P1.0 is high, there is a reverse current in the coil. The driving circuit consists of R21, R45, R47, R48, R49, R50, PNP transistors VT1, VT4, and transistors VT5, VT6, VT7, and VT8. L is an electromagnetic coil.

When P1.1 = 1 and P1.0 = 0, the transistors VT4, VT7, and VT8 are turned on, while VT1, VT5, and VT6 are turned off. The direction of the current flowing through L is + 12V → the E pole of VT4 → the C pole of VT4 → the B terminal of the coil → the A terminal of the coil → the C pole of VT7 → the E pole of VT7 → ground; the relay contact is turned on;

When P1.1 = 0 and P1.0 = 1, the transistors VT4, VT7, and VT8 are turned off, and VT1, VT5, and VT6 are turned on. The direction of the current flowing through L is + 12V → the E pole of VT1 → the C pole of VT1 → the A terminal of the coil → the B terminal of the coil → the C pole of VT6 → the E pole of VT6 → the ground, and the relay contact opens.

When P1.1 = P1.0 = 0, all triodes are turned off, and the coil has no current. When P1.1 = P1.0 = 1 is not allowed, because all the transistors are turned on at this time, the power consumption is very large.

### Magnetic holding relay drive circuit-2

The four transistors on the right form the bridge switch circuit, and the two transistors on the left are responsible for controlling the on and off of the bridge switch.

The general idea is cleared up, and the specific analysis is easy to handle.

When QD3, 5 is on 2, and 6 is off, the 3 terminal of KD1 is high potential and the 4 terminal is low potential.

When QD2, 6 is turned on 3, and 5 is off, the 4 terminal of KG1 is high potential and the 3 terminal is low potential.

When neither KK1 nor KK2 has a signal, none of the four transistors are conducting, and the bridge is in a high-impedance state and close to equilibrium. There are no voltages on both ends of KD1 (note that a 1M RD9 is connected and short-circuited the weak leakage current of the transistor).

### Magnetic holding relay drive circuit-3

The magnetic latching relay driving circuit shown in the figure above is driven by a special device BL8023, which has a simple circuit and low static power consumption. The recommended working voltage of the BL8023 in the circuit is 5-25V, the static power consumption is in the μA level, and the driving current can reach 400mA.

The trigger pulses output from the A and B output terminals of the front-end remote control receiving circuit are sent to the In1 and In2 terminals of BL8023 respectively. When the In1 terminal is “1” and the In2 terminal is “0”, the IC’s Out1 output is “1” Out2 output is “0”, the relay is closed at this time; if In1 terminal is “0” and In2 terminal is “1”, Out1 output is “0”, Out2 output is “1”, and the relay is open. When both the In1 and In2 terminals are “0” or “1”, the Out1 and Out2 output terminals are both in a high-impedance state, and the relay remains in the original state.

Compared with the existing electromagnetic relay, the only drawback of the magnetic holding relay is that it consumes more materials. In the long run, its environmental protection performance is better than that of the ordinary electromagnetic relay. The seismic performance is not much different from ordinary relays.