Transistors (by alaric)

Now, my knowledge of electronics really centers around digital stuff, so transistors have always remained a bit of a mystery to me. Sure, I know the gate or base input controls stuff flowing between the other two oddly-named pins, but I've never really known how this works out in practice - if a 12v battery is placed betwixt the collector and emitter of an NPN transistor, and the base is supplied from the +12v rail via a 10KΩ resistor, what would happen?

So I was actually quite happy when I found out that a project I'm working on would involve building a simple charger for NiMH batteries. A textbook reveals a circuit that generates a constant current source providing 6mA from a 12V supply, which is what is required, but I need to turn this charger on and off from a 3.6V CMOS logic output.

Which will clearly require transistors - you won't drive a relay directly from that tiny microcontroller...

Now, previous readings of books about transistors have quickly left me confused between emitters, collectors, NPNs, and PNPs, so I decided the only way I was really going to get to grips with these things was to produce a diagram showing the difference, and learning a few basic rules - the way my memory works, I remember better if I learn basic principles and work from them than if I try to memorise the entire behaviour of a system.

So I can now tell the difference between NPN and PNP transistor symbols, simply by knowing that the arrow always points towards an N layer in the device. An NPN has an arrow pointing away from the base, since the base is P, while a PNP has an arrow pointing towards the N base.

And the emitter is always the lead which has the actual arrow on it.

And the crucial thing is that the arrow shows which way the base current has to flow to activate the transistor; in an NPN, current flows from base to emitter to switch the transistor on and allow a larger current to flow from collector to emitter; in a PNP, current flows from emitter to base to switch the transistor on and allow the larger current to flow from emitter to collector.

And that, coupled with the knowledge that the emitter/base junction works just like a diode (no current flows until the voltage reaches 0.7V for silicon or 0.3V for germanium, then resistance rapidly drops towards zero) and that the collector current (as long as the emitter/collector voltage difference is high enough) is very roughly a few hundred times the base current, although somewhat nonlinear, describes just about all you need to know about bipolar transistors.

In my case, it was enough to tell me that I could control the +12V supply to my charger with a PNP transistor; the emitter connected to +12V, the collector connected to the power input to the charger circuit, and the base (via a current limiting resistor; bear in mind that just shy of 12v will be present there) pulled down to 0 to enable charging.

Sadly, the CMOS logic output that drives this isn't open-drain - when it's set to 1 it will be held at 3.6v, and I suspect the 12v or so being provided from the base of the PNP transistor would drive a significant current down to a 3.6v CMOS output, meaning the charger wouldn't really turn off, and possibly frying my microcontroller.

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3 Comments

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  By Ben, Tue 13th Sep 2005 @ 8:14 pm

  Just two things...

  1) You might want to actually upload your cards.

  2) When's my board going to be ready? 🙂

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  By andyjpb, Wed 14th Sep 2005 @ 7:43 pm

  I wish someone had told it to me like this when I was studying 2nd year electronics...

• 

  By Alaric Snell-Pym, Thu 15th Sep 2005 @ 6:34 pm

  I DID upload the cards... yes, they're there! Works For Me.

  Boards will be ready as soon as I can build them all, now the circuits seem to work on breadboard 😉

  And yes... I also wish transistors were taught that way. I've learnt about them many times before, but found myself forgetting the beginning before I got to the end. Why should something so simple be handled so verbosely?

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