PIC and Transistor

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Basically what I want to do is use the output pins of my PIC to
activate some transistors. If i understand correctly, activating a PIC
pin essentially grounds it, so an NPN won't work. I am assuming then I
need to use a PNP, so when it is grounded, it activates the "switch".

Now, does it matter what is going through the collector and emitter? I
would like to have a Positive voltage sitting on one side of the
transistor, and the other side of the transistor would go to an ECU in
a vehicle. Under most circumstances, the transistor will be ungrounded,
which should mean that there is no flow through the transistor, and no
signal to the ECU (that is what I want). When instructed, the PIC will
ground the base, allowing current to flow through the transistor and
sending the positive signal from the other side of the transistor to
travel to the ECU. Basically it is -
http://wotid.com/tls/images/ssatre/transistor_tre.jpg- but the
transistor would be activated by a PIC.
Maybe I am missing something, but I cannot get this work no matter
which I use PNP, or NPN.
Is anyone able to shed some light on this for me?


Re: PIC and Transistor


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Which PIC are you using? In most cases making a PIC pin an output pin by
clearing a corresponding bit in TRIS register and then setting that pin
to 1 makes voltage appear on the that pin. On some PICs though, like the
very common 16F84 and I am guessing other 16 series chips, pin RA4 is
more like you understand it, an open drain output so if you make it an
output by clearing TRISA:4 then set RA4 to zero, it will ground whatever
is connected to it.

Quoted text here. Click to load it

It does. Collector goes to the positive side of the circuit.

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Notice that in the diagram you have given the link to, the transistor
sits between the ECU and the ground, not between the positive rail and
the ECU like you are saying which is quite different. You should
probably stick to that arrangement rather than having the transistor
between the positive rail and the ECU because if ECU is a high impedance
load, the emitter of the transistor will be close to the positive rail
voltage and that might create some unwanted side effects because a
transistor needs voltage difference of 0.6V between base and emitter to
switch. If you switch the transistor on with 5V on a PIC pin and the
emitter is sitting on 4.9V because voltage drop across the ECU is 4.9V,
then the voltage across base and emitter is around 0.1, maybe the
transistor will switch right off again?

Eugene.

Re: PIC and Transistor



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Most PIC output pins are "totem pole", meaning that they can source and
sink current.  An NPN will work fine with a PIC.

Quoted text here. Click to load it
ungrounded,

If you want the transistor to "be on top" (meaning between the battery
and the device being switched) and supply the positive voltage to a
device, then you usually want a PNP type.  The problem is that you need
to pull the base higher than the collector to switch it off.  If you are
using a PIC with 5V outputs and trying to switch 12V, you have a real
problem.  Never fear, the manufacturers have already solved it for you
though.  What you want is a "high side driver".  This is something like
a P-channel MOSFET with a built in charge pump to be able to pull the
gate (base equivalent) up to a high voltage, but using a logic level
input.  Take a look at the VN820.

OTOH, your schematic clearly shows a low side switch.  Many NPN type
transistors would be able to do this with relative ease.


Re: PIC and Transistor



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Put a resistor (10k) between the transistor base and the PIC output.
Then put another resistor (22k) between the PIC output and the 12v
rail. For added safety put a 5.1v zener diode between the PIC output
and ground.

Use a NPN transistor.  Ground the emitter. Collector to ECU.

HTH

Alan

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http://ozcomponents.com
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Re: PIC and Transistor


look up "tristate"
maybe you dont need a transistor, what is the load?

: Basically what I want to do is use the output pins of my PIC to
: activate some transistors. If i understand correctly, activating a PIC
: pin essentially grounds it, so an NPN won't work. I am assuming then I
: need to use a PNP, so when it is grounded, it activates the "switch".
:
: Now, does it matter what is going through the collector and emitter? I
: would like to have a Positive voltage sitting on one side of the
: transistor, and the other side of the transistor would go to an ECU in
: a vehicle. Under most circumstances, the transistor will be
ungrounded,
: which should mean that there is no flow through the transistor, and no
: signal to the ECU (that is what I want). When instructed, the PIC will
: ground the base, allowing current to flow through the transistor and
: sending the positive signal from the other side of the transistor to
: travel to the ECU. Basically it is -
:
http://wotid.com/tls/images/ssatre/transistor_tre.jpg- but the
: transistor would be activated by a PIC.
: Maybe I am missing something, but I cannot get this work no matter
: which I use PNP, or NPN.
: Is anyone able to shed some light on this for me?
:


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