I still don't understand the problem of using PNP npn as a switch. 8050 is used as a switch and it keeps turning on and off.

tongshaoqiang

I still don't understand the problem of using PNP npn as a switch. 8050 is used as a switch and it keeps turning on and off. [Copy link]

 

Use the MCU IO output level (0-3.3) to control the on and off of 8050, thereby controlling the presence or absence of 12v and the power supply of the device. Now the problem is that it can't be turned off. I think the state of the transistor is not adjusted correctly (Rc resistance). Please give me some guidance.

dcexpert

The shutdown depends on the voltage. At 12V, VBE cannot reach the shutdown voltage regardless of whether PA12 is 0V or 3.3V.

vincentc543

This requires adding an NPN to perform level shift from 0~3.3 to 0~12V for normal control.

N MOS can also be used to perform level shift to 12V.

But the control will be reversed.

maychang

This 8550 has an emitter of 12V, and the base is connected to 3.3V via a 150k ohm resistor. The base current is more than 50uA. How can I shut it down?

maychang

"Remember that when VCC is 3.3V, control is possible."

Yes, if you raise the high level at PA12 to 12V, then you can control it when VCC is 12V.

tongshaoqiang

vincentc543 posted on 2022-5-25 18:34 This requires adding an NPN to do a level shift from 0~3.3 to 0~12V for normal control. Using N MOS can also do a level shift to 12V. ...

Thanks for your reply. I also thought about the problem yesterday and needed to convert it.

tongshaoqiang

maychang posted on 2022-5-25 18:53 This 8550, the emitter is 12V, the base is connected to 3.3V through a 150k ohm resistor, the base current is more than 50uA, how to shut it down?

Thanks to teacher maychang. I think no matter how to adjust the base current, it can't change the existence of Vbe conduction voltage difference, and the transistor is always in the on state.

tongshaoqiang

dcexpert posted on 2022-5-25 17:52 Shutdown depends on voltage. At 12V, VBE cannot reach the shutdown voltage regardless of whether PA12 is 0 or 3.3V.

Thanks!

Fred_1977

This type of control circuit is very common, R6 can be adjusted to a larger value, such as 10K or more.

maychang

tongshaoqiang posted on 2022-5-26 08:47 Thanks to teacher maychang. I think no matter how the base current is adjusted, the existence of the Vbe conduction voltage difference cannot be changed, and the transistor is always in the on state. ...

If you change the potential of the left end of R7 to 12V~8.7V, you can control the switching state of 8550.

When the left end of R7 is 12V, it is equivalent to 0~3.3V controlling the 0V of the NPN tube. At this time, 8550 is turned off. When the left end of R7 is 8.7V, it is equivalent to 0~3.3V controlling the 3.3V of the NPN tube. At this time, 8550 is turned on.

maychang

tongshaoqiang posted on 2022-5-26 08:47 Thanks to teacher maychang. I think no matter how the base current is adjusted, the existence of the Vbe conduction voltage difference cannot be changed, and the transistor is always in the on state. ...

NPN tubes and PNP tubes have opposite polarity. When used as switch tubes, in addition to swapping the positive and negative ends of the power supply, the input signal of the base also needs to be changed. The emitter of the PNP tube is connected to 12V, so it should be based on 12V. The zero point of the input signal is at 12V, so the previous reply said that the PNP tube is turned off at 12V. To turn on the PNP tube, the left end of R7 should be negative 3.3V, that is, 12V-3.3V=8.7V.

maychang

tongshaoqiang posted on 2022-5-26 08:47 Thanks to teacher maychang. I think no matter how the base current is adjusted, the existence of the Vbe conduction voltage difference cannot be changed, and the transistor is always in the on state. ...

In the early circuits using PNP tubes, for convenience, the positive end of the power supply is often marked as 0, and the negative end of the power supply is marked as -12V or -6V, etc. Because the PNP tube works as a switch, the input of the base through the resistor is 0V~(-3.3V).

maychang

tongshaoqiang posted on 2022-5-26 08:47 Thanks to teacher maychang. I think no matter how the base current is adjusted, the existence of the Vbe conduction voltage difference cannot be changed, and the transistor is always in the on state. ...

So the third poster is right: we need to add an NPN to perform a level shift from 0~3.3 to 0~12V for normal control.

The purpose of adding this NPN tube is not to amplify, but mainly to perform level conversion.

tongshaoqiang

maychang posted on 2022-5-26 09:31 So the 3rd floor is right: it is necessary to add an NPN to do level shift from 0~3.3 to 0~12V for normal control. The purpose of adding this NPN tube is not...

Thank you teacher for your guidance!

fjdeepblue

The above analysis is very correct. The 3.3V voltage is too low to be turned off. A transistor is needed to shift the level.

振动试验仪器

When using transistors to make switch circuits, NPN transistors are easier to control than PNP transistors, and most products use NPN transistors. Because the input base of NPN is connected to the power supply voltage when the potential is high, and the B pole is directly disconnected and grounded when the potential is low, this method is very convenient for general industrial control.