Improved circuit diagram of 8050 transistor emitter drive relay

A 5V voltage output from the IO port of a single-chip microcomputer drives a relay with a working current of 400~450mA through the emitter of an 8050 transistor. The following is the circuit diagram:

Figure 1: Circuit diagram of using the emitter of 8050 transistor to drive a relay.

The above figure is a relay drive circuit diagram. The 5V drive signal output by the IO port of the microcontroller controls the operation of the relay through the emitter of the transistor VT. The relay cannot work with this connection method.

There are two situations:

1. Assuming that the relay is a 5V relay, when the IO port outputs a 5V signal to turn on VT, the voltage between the ce poles of VT is about 7.7V, and the working current of the relay is 400~450mA. Assuming it is 400mA, the tube power consumption of VT is 3.08W, and the Pcm of the 8050 transistor is 1W. At this time, the 8050 may be burned out after being turned on.

2. If the working voltage of the relay is 12V, when the output signal of the IO port is added to the base of VT, its emitter voltage is only 4.3V, that is, the voltage across the 12V relay is only 4.3V, and the relay cannot be attracted at all. If you want to use 5V voltage to control the operation of the 12V relay through the 8050 transistor, you can connect it according to the circuit shown in the figure below.

Figure 2: Circuit diagram of IO port controlling 12V relay through 8050 transistor

In the circuit above, the relay is connected to the collector of VT, and a protection diode VD is connected in parallel at both ends of the relay. When the IO port outputs a control signal, VT is saturated and turned on, and the 12V relay is powered on. Since the working current of the relay marked in the question is 400~450mA, the β of VT is required to be as large as possible to reduce the current drawn from the IO port. Here, a tube with β≥250 is selected, and the base resistor R is selected as a 1.3KΩ resistor, which can fully saturate and turn on VT. Since the Icm of 8050 can reach 1.5A, it can fully drive this high-current relay.