[SOLVED] Why does this circuit not turn on?

[JohnJohn20]

Hi.

I want to use a PIC running on 3.7V to switch a 12V supply (where both power supplies have a common 0V).

I was using the PIC to control a TIP 120 but could only switch the negative line.

So I tried using the PIC to control a npn transistor (BC547) to drive a PNP power transistor (BD438) as shown in in the attached diagram.

But this doesn't work. Why?



- - - Updated - - -

Actually guys. It works fine. I had the BD438 connected up wrong. Sigh.

 

[betwixt]

The circuit will work but you need to add two more resistors, one in series with the base of the BD438 so that you don't get a direct current path through it and the BC547 across the supply lines, the other from the collector of the BC547 to 12V+ to allow it to pass collector current and to ensure leakage doesn't turn the BD438 on. I suggest 10K from the BC547 to 12V+ and 1K in series with the BD438 base as starting values.

Brian.

 

[BradtheRad]

Although the circuit can work as is, there is probably more current than necessary going through the NPN.

It is a good idea to install the resistors mentioned by Betwixt.

Here is a simulation, showing current through components.

 

[Jibby Benjamen]

BC 547 is burnt. Use a resistor of 1K between collector of 547 & 438 base

 

[amayilsamy]

Try do with the TIP122...

https://electronics.stackexchange.com/questions/21498/help-with-tip122-darlington-transistor