Transistor switch circuit-self-control circuit principle

Can a reed switch be used to directly control the start and stop of a motor? Toy motors are commonly used power devices that can convert electrical energy into mechanical energy and can be used to rotate small electric fans, pump water from small centrifugal pumps, and other functions. Usually, toy DC motors have a low operating voltage. Although they can be started at 1.5 to 3V, the starting current is relatively large (1 to 2 amperes). If a reed switch with a contact load of only tens of milliamperes is used for switch control, its service life will be greatly shortened. Therefore, in automatic control circuits, electronic switches are often used to control the working state of the motor.

The triode electronic switch circuit is shown in Figure 1.

How to determine the VT base current limiting resistor R? According to the current distribution function of the transistor, a weak current IB is input into the base to control the collector current IC to have a strong change. Assuming that the VT current amplification factor hfe≈250, the collector current IC＝1.5A when the motor starts, after calculation, the base current IB required to make the transistor saturated and turned on is ≥（1500mA／250）×2＝12mA. In the circuit of Figure 1, the running current of the motor is about 500mA when it is unloaded. At this time, the power supply voltage (powered by two No. 5 batteries) drops to 2.4V, and the voltage between the VT base and emitter VBE≈0.9V. According to Ohm's law, the resistance value of the VT base current limiting resistor R＝（2.4－0.9）V／12mA≈0.13kΩ. Considering that when the VT IC is large, hfe should be reduced, and the resistance value R should be smaller, and 100Ω is actually taken. In order to make the motor start more reliably, R can even be reduced to 51Ω. When debugging the circuit, turn on the control switch S, the motor should be able to start automatically, and measure the voltage VCE between the collector and emitter of VT ≤ 0.35V, which means that the transistor is saturated and turned on, and the transistor switch circuit is working normally, otherwise VT will overheat and be damaged.

The heat automatic control circuit of automatic fire extinguishing is shown in Figure 2. This circuit is a heat control circuit by replacing the control switch S in Figure 1 with a bimetallic switch ST. When the candle flame burns the bimetallic strip, the strip tends to straighten, so that the switch ST is turned on, the motor starts, and the small fan blades rotate, aiming at the candle to blow air, and automatically extinguish the flame; when the bimetallic strip cools down, the switch is disconnected, and the small electric fan automatically stops, completing the automatic fire extinguishing procedure.

The magnetic automatic control circuit of automatic parking is shown in Figure 3. Turn on the power switch S, the toy car starts, and when it drives to the magnet, the reed switch SQ installed between the base and emitter of VT closes, short-circuiting the base bias current, VT is cut off, and the motor stops rotating, protecting the motor and avoiding large current discharge.

In the photoelectric automatic control circuit, a photoresistor can be selected as a photoelectric sensor element. Can the photoresistor be directly connected to the position of the control switch S in Figure 1? Usually, the light resistance of a photoresistor, such as MG45, is 2 to 10 kΩ when exposed to light, which is much larger than the resistance value of the bias resistor R. Obviously, it cannot generate the base current required to maintain the saturation conduction of VT. Therefore, it is necessary to use a transistor to amplify the current first, and then drive the switch transistor to work.

The photoelectric automatic control circuit is shown in Figure 4. VT1 and VT2 are connected in a similar composite tube circuit form. The emitter current of VT1 is also the base current of VT2. R2 is both the load resistor of VT1 and the base current limiting resistor of VT2. Therefore, when a weak current (0.1mA) is input to the base of VT1, the change of the stronger current of the final stage VT2 - the driving motor running current (500mA) can be controlled. VT1 uses a low-power NPN silicon tube 9013, hfe≈200. The same calculation method as before, when the two tubes are saturated and turned on at the same time, the base bias resistor R1 of VT1 is about 3.3kΩ, minus the light resistance of the photoresistor RG 2kΩ, the current limiting resistor R1 is actually 1kΩ. The photosensitive sensor can also use a photodiode. Pay attention to the polarity when using it. The negative pole of the photodiode is connected to the positive pole of the power supply. The photodiode has a directional selection for controlling light, and has a high sensitivity, and will not produce fatigue after strong light exposure.