In this transistor circuit diagram, will the battery voltage flow back to the microcontroller through the transistor?

sky999

In this transistor circuit diagram, will the battery voltage flow back to the microcontroller through the transistor? [Copy link]

 

Please advise, in the following transistor circuit diagram, will the battery voltage flow back to the microcontroller through the transistor?

tagetage

The circuit diagram is wrong.

In addition, the 3.3V I/O cannot control the 4.2V voltage to conduct through the PNP transistor.

maychang

The motor in your case cannot rotate because there is no return path to the negative terminal of the battery.

The battery voltage will only flow back into the microcontroller through R2 and R1.

sky999

maychang posted on 2024-9-7 09:56 The motor of the original poster cannot rotate because there is no circuit to the negative end of the battery. The battery voltage will only flow back into the microcontroller through R2 and R1.

Sorry, the main picture is wrong. The actual situation is like this. Won't the battery voltage flow back to the IO port through the transistor?

sky999

tagetage posted on 2024-9-7 09:53 The circuit diagram is wrong. In addition, the 3.3V I/O cannot control the 4.2V voltage to conduct through the PNP transistor.

Um, the main picture is wrong, but the 3.3V IO can normally control the 4.2V transistor to conduct, as long as E>B>C.

tagetage

sky999 posted on 2024-9-7 10:13 Sorry, the main picture is wrong. The actual picture is like this. Will the battery voltage not flow back to the IO port through the transistor?

This circuit is so simple, you can figure it out by experimenting yourself.

maychang

sky999 posted on 2024-9-7 10:13 Sorry, the main picture is wrong. The actual picture is like this. Will the battery voltage not flow back to the IO port through the transistor?

If the motor is driven by external force and its speed is higher than when it is directly connected to the battery, the motor will become a generator, the Q1 collector junction will be forward biased, and the voltage generated by the motor (a generator at this time) will flow back into the microcontroller through the Q1 collector junction, but it still needs to pass through resistor R1.

sky999

tagetage posted on 2024-9-7 10:17 This circuit is so simple, you will know it if you experiment it yourself.

I have welded it and it works fine.

sky999

maychang published on 2024-9-7 10:17 If the motor is driven by an external force and the speed is higher than when it is directly connected to the battery, then the motor will become a generator, Q1 collector junction is forward biased, and the motor...

The conduction condition of a PNP transistor is E>B>C. If the IO port is high level, it is only 3.3V. Logically, the transistor should not be turned off. Why can it be turned on and off normally after I solder it? Is it because the battery voltage flows back to the base through the collector? The transistor is SS8550

maychang

sky999 posted on 2024-9-7 10:30 The conduction condition of the PNP transistor is E>B>C. If the IO port is at a high level, it is only 3.3V. Logically, the transistor should not be turned off. Why...

[The conduction condition of the PNP transistor is E>B>C. If the IO port is at a high level, it is only 3.3V. Logically, the transistor cannot be turned off... Is it because the battery voltage flows back to the base through the collector?]

Instead of [returning from the collector to the base], the battery voltage returns to the microcontroller I/O pin through resistor R1, causing the pin voltage to rise to higher than 3.3 V. As a result, the base of Q1 cannot get enough base current (not no base current!), and the motor does not rotate.

sky999

maychang posted on 2024-9-7 10:54 [The conduction condition of the PNP transistor is E>B>C. If the IO port is at a high level, it is only 3.3V. Logically, the transistor should not be turned off...

Um, but I have done the welding here, and it can work normally. The motor can rotate and stop normally, there is no problem, even with load. What is the reason?

maychang

sky999 posted on 2024-9-7 11:36 Um, but I have welded it and it works normally. The motor can rotate and stop normally. There is no problem. It can even carry load. This is...

I have explained it on the 10th floor. I did not say on the 10th floor that this circuit cannot stop the motor.

maychang

sky999 posted on 2024-9-7 11:36 Um, but I have welded it and it works normally. The motor can rotate and stop normally. There is no problem. It can even carry load. This is...

In fact, if Q1 is replaced with an NPN transistor and the motor is connected between the positive terminal of the battery and the collector of the transistor, the microcontroller can still control the transistor, and it doesn't matter how high the battery voltage is (as long as the transistor can withstand the voltage). It is much better than using a PNP tube.

The only difference is that the original microcontroller outputs a low level to rotate the motor, while after the change, the microcontroller outputs a high level to rotate the motor.

sky999

maychang posted on 2024-9-7 15:48 In fact, if Q1 is replaced with an NPN transistor and the motor is connected between the positive terminal of the battery and the collector of the transistor, the microcontroller can still control the transistor, and the battery power...

OK, I know. I'm just worried that the 4.2V voltage will flow back to the IO port. Now I'm curious why this circuit can switch normally. Shouldn't it be impossible to turn off the motor?

maychang

sky999 posted on 2024-9-7 18:28 Well, I know, I am just worried that the 4.2V voltage will flow back to the IO port, and now I am curious, why this circuit can switch normally, it shouldn't be...

The motor does not rotate because there is not enough current in Q1, that is, the microcontroller pin outputs a high level. If the microcontroller can withstand a voltage slightly higher than the microcontroller power supply voltage when outputting a high level, there will be no current in R1 and the motor will not rotate. This "slightly higher" varies with different microcontroller models. For example, a certain model of microcontroller is used for a 5V power supply voltage, but the power supply voltage is allowed to be as high as 6.6V, and the voltage allowed to be borne by the pin is as high as 7V, which is 2V higher than the rated power supply voltage.

maychang

sky999 posted on 2024-9-7 18:28 Well, I know, I am just worried that the 4.2V voltage will flow back to the IO port, and now I am curious, why this circuit can switch normally, it shouldn't be...

But if the voltage supplied to the motor in your circuit is 15V, then nothing will work. As mentioned on the 13th floor, no matter how high the voltage supplied to the motor is, it can be used as long as the transistor can withstand it.