Trying to use PIC to switch a darlington transistor

Thread Starter

dremelts

Joined Jun 8, 2007
2
I am trying to use a PIC microcontroller to switch a darlington transister to operate some 12volt, 3amp lights.

Here is a crude diagram of the circuit.


Here is a link to the spec sheet of the transistor
www.fairchildsemi.com/ds/TI/TIP102.pdf

When I measure the voltage at the emitter all I'm getting is 5v.

I could use some help figuring out what I'm doing wrong.

Thanks,
Dave
 

hgmjr

Joined Jan 28, 2005
9,029
You are using the TIP-102 in the common collector mode when you would be better off if you used it as a common emitter with the lamp as the collector load.

This new arrangement will mean that to turn the lamp on you will need to apply your logic high voltage to the base through a base resistor of around 10K ohm. A logic low should be able to the base to turn the lamp off reliably.

Are you familiar enough with transistors to know what I mean by a common emitter configured stage?

hgmjr
 

Ron H

Joined Apr 14, 2005
7,014
You are using the TIP-102 in the common collector mode when you would be better off if you used it as a common emitter with the lamp as the collector load.

This new arrangement will mean that to turn the lamp on you will need to apply your logic high voltage to the base through a base resistor of around 10K ohm. A logic low should be able to the base to turn the lamp off reliably.

Are you familiar enough with transistors to know what I mean by a common emitter configured stage?

hgmjr
Minimum beta is 1000 at Ic=3A (and that's at Vce=4V). I think the base current should be at least 4mA. This means the series base resistor should be around 875 ohms. 1k would probably be safe. 10k might work with some units, but I wouldn't count on it.
 

hgmjr

Joined Jan 28, 2005
9,029
That's a good point Ronh. I am used to darlingtons with a higher beta.

Greetings dremelts,

I would go with RonH's recommended base resistor value.


hgmjr
 

Thread Starter

dremelts

Joined Jun 8, 2007
2
You guys are awesome. I posted this just before going out to lunch and when I got back there were 4 replies!!!! Thanks for your help. I'll set it up as a common emitter and use the base resistance that you suggested and see if I can get this thing to work.

This project has been quite a learning experience. Never having programmed a PIC microcontroller before (or ANY controller for that matter), I had a lot to learn: interrupt service routines, registers, etc. But I muddled thru it and managed to get it to work in a simulator. Then I breadboarded it using switches for inputs and LEDs for outputs and found things that the simulator didn't show. Once the breadboard version was working the way I wanted then I built the working model with the power transistors to switch the heavy load and ran into this problem.

One step at a time... but I'll get there eventually.

Thanks again,
Dave
 
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