Hello everyone,

Can you recommend me the most common fast switching transistors? it is for inverting the Tx signal of a PIC (serial communication), it has to be PNP. This is the schematic

Thank you in advance!!

 

What is the maximum speed for the Tx signal?

 

What is the maximum speed for the Tx signal?

I've done 1MHz using SPI. Usually slower in DIY applications.

A 2N7000 works for low-side switching. I'm guessing any old 2n3906 should work.

 

What is the maximum speed for the Tx signal?

It would be 9600 bauds

 

NPN: 2N2222
PNP: 2N2907

Or almost any other small signal transistors you can think of.

 

I've done 1MHz using SPI. Usually slower in DIY applications.

A 2N7000 works for low-side switching. I'm guessing any old 2n3906 should work.

I tried with 2n2907, but it didn't work. Do you think it isn't right for the application, or it's right and I should start looking for the fault somewhere else?

 

It would be 9600 bauds

You could almost switch by hand at those speeds

 

NPN: 2N2222
PNP: 2N2907

Or almost any other small signal transistors you can think of.

Ok, that answers my last reply. Thank you!

 

You could almost switch by hand at those speeds

Understood! xD

Thank you!

 

I tried with 2n2907, but it didn't work. Do you think it isn't right for the application, or it's right and I should start looking for the fault somewhere else?

The 2n2907 should work. People get e, b, c mixed up on pnp transistors - did you connect it wrong? What is your load? Do you even need this extra current? What is your application and cable length.

 

The 2n2907 should work. People get e, b, c mixed up on pnp transistors - did you connect it wrong? What is your load? Do you even need this extra current? What is your application and cable length.

I think I did connect correctly. I even made the test switching by hand and measuring the output. It seems to work fine, but the communication isn't working. The maximum current would be 15 mA. The application is for MDB protocol (a coin changer), and the cable length is about 60 cm.

 

I think I did connect correctly. I even made the test switching by hand and measuring the output. It seems to work fine, but the communication isn't working. The maximum current would be 15 mA. The application is for MDB protocol (a coin changer), and the cable length is about 60 cm.

The pic can easily drive 15 ma if I remember correctly. Check the specific pin used for transmit on your chip.

 

I don't understand how that circuit works.
What's the purpose of the top transistor?

I think it is some sort of current limiting in case of a short on the cable.

 

If this is for RS-232, the receiving end might need to see the signal go below ground, not just to ground.

By the way, what's the purpose of the top transistor?

 

The pic can easily drive 15 ma if I remember correctly. Check the specific pin used for transmit on your chip.

I think the PIC is capable to drive up to 20 mA. But the MDB protocol states that circuit for interfacing between master and slave, so I have to include it.

 

Regarding the circuit above, I made a question about it some time ago, and some people kindly answered:

http://forum.allaboutcircuits.com/t...on-pnp-transistors-array.126955/#post-1034524

 

If this is for RS-232, the receiving end might need to see the signal go below ground, not just to ground.

By the way, what's the purpose of the top transistor?

It isn't for RS232. It's MDB (multidrop bus)

 

BC547 would do fine.