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

P

Paladiamors

Jan 1, 1970
0
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.
 
C

CFoley1064

Jan 1, 1970
0
Subject: PNP transistor as a high side switch (2N3906) problem
From: "Paladiamors" [email protected]
Date: 8/27/2004 1:20 AM Central Daylight Time
Message-id: <[email protected]>

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.

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 www.tech-chat.de

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 www.tech-chat.de

Good luck
Chris
 
P

Paladiamors

Jan 1, 1970
0
Hey, thanks for the suggestion, turns out that I had the transistor
backwards :) How embarassing :p

Paladiamors
 
J

Jamie

Jan 1, 1970
0
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?
 
J

John Fields

Jan 1, 1970
0
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?
 
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