[SOLVED] Why does this circuit not turn on?

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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.
 

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.
 

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.

 

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

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