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PNP transistor as a high side switch (2N3906) problem

Hi there,

I have a problem using a PNP 2N3906 transistor as a high side switch. I'm plugging a 18V line to the emitter, with a 3.3k pull up resistor from the

18V line from 2 9V batteries to the base of the transistor. The collector of the transistor is going to a 12V power regulator (so the transistor is acting as a high side switch). The base is eventually going to be pulled towards ground through a 1k resistor using a switch (later it's going to be connected to the serial port of a computer using an extra transistor to pull the base down).

The problem I'm facing is that the base is some how being pulled down (I'm seeing a Vbe of about -0.5V) and there is about a 9V signal on the collector. When I short the base and the emitter together, I still see a 9V signal on the collector. When I pull the base down for a Vbe of about 0.8V I get the full 18 V on the collector. Has anyone experienced this problem before? Am I using bad transistor model (I tested 2 different transistors and got the same problem)? Is my circuit flawed?

I've modeled the circuit in circuit maker and circuit maker is telling me that the theory is correct, that the transistor should work in this situation. Any advice would be appreciated.

Thanks

Paladiamors.

Reply to
Paladiamors
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First, check your wiring. Looking at the 2N3906 from the front (flat) part, it looks like this:

.-----------. | | | | | | | 2N3906 | | | | | | | '-o---o---o-' | | | | | | | | | | | | | | | E B C created by Andy´s ASCII-Circuit v1.24.140803 Beta

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You've probably either got your wiring mixed up, or hooked up something incorrectly before and fried the transistor. Make sure you've got a ground-referenced load on the collector of the PNP when you're testing. It should look something like this:

2N3906 .--------o----- ------------. | | V / | | | --- | | | | | | | o | | | | | --- .-. | | - 3.3K| | | | | | | | | --- '-' | .-. - | | R | | | '------o | | | | '-' | .-. | | | | | | 1K| | | | '-' | | | | | | | | o | | '\ | | \ | | o \ | | | | | | | '---------------o-------------' created by Andy´s ASCII-Circuit v1.24.140803 Beta
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Good luck Chris

Reply to
CFoley1064

Hey, thanks for the suggestion, turns out that I had the transistor backwards :) How embarassing :P

Paladiamors

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Reply to
Paladiamors

well it only takes aprox -.6 V to bias the pnp try leaving the Base floating and check that. it could be your getting so current some where if that is the case maybe a diode to the base will help since it will take a min of .6 before it starts to conduct. you may want to consider a board leakage problem?

Paladiamors wrote:

Reply to
Jamie 

--
And _you_ may want to consider reading a thread before responding to
it.  Had you done that you would have found that his problem was due
to the fact that he had the transistor connected backwards.
Reply to
John Fields

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