Drive TIP122 directly from a prop pin?

Hugh

Hi,

I've been trying to drive a TIP122 Darlington array directly from a prop pin. Does that make me insane, naive, overly optimistic or a combination of the three, or should this be possible.

With 5V at the collector and the prop pin providing 3.3V through a 500 ohm resistor to the base I expected more than 3.3V to be available at the emitter.

The output exactly matches the voltage at the base and it is the same with different transistors. A brief dalliance without the resistor didn't show any change. There's something fundamentally wrong in my approach but for the life of me I cant guess at what it is. I'd put a smaller transistor in first, but that would seem to defeat the point of using a darlington array.

Any ideas gratefully received. It is well over twenty years since I could even pretend to know about these things.

jmg

Hugh wrote: »

I expected more than 3.3V to be available at the emitter.

Why did you expect that ? - It is an emitter follower, which will always be < Vb.
How much less, depends on the load current.

With very low loads current, (eg a meter) the b-e resistors in a darlington give a direct path, and so you see "The output exactly matches the voltage at the base" - as the current load increases, that will drop to 2 x Vbe below Vb,

If you want Positive logic and voltage gain, a single Dual-NPN darlington will not work.
You need a NPN + PNP Darlington. As an example (this one for linear gain)

http://forums.parallax.com/attachment.php?attachmentid=96775&stc=1&thumb=1&d=1352345339

pedward

Why are you using it open emitter? The Base voltage reference is on the emitter, which means you need to be so many volts above the emitter for it to turn on. I suspect what you're seeing is base emitter leakage and not the transistor turning on.

With NPN transistors it's far more common to run open collector, so you are switching ground. If you want to switch the high side, you should be using a TIP42C or the like, which is PNP and not NPN.

Hugh

You both are, of course, correct and I was being particularly obtuse. Thank you.

[Goes off and kicks himself]

whicker

a third option I have brought up over the years is to use a high side switch. With the addition of another pin to ground and some more specifics about what voltage was being switched, this device would have worked as in the first post.

Internally they obviously use mosfets or other transistors, but externally they are 4 or more pin black boxes that act much closer to an ideal component with a low voltage level enable input and higher power, low on resistance switching output (with options like short circuit protection should you need it).

I hate to see people struggling with Bipolar Junction Transistors in this day and age, except as an educational exercise.