Today, a second example of a transistor circuit. As announced yesterday, this one is about boosting the output current further…

With a transistor which amplifies current say 100 times, there are still limits to what you can control from an ATmega I/O pin: 10 mA x 100 = 1A. What if you want to drive a DC motor with a stall current well above that?

Quite simple, really: use two transistors “in series”. But how?

Note that this is not the way to do it:

On first sight, it looks exactly like what we need: after all, when the inout voltage rises above 2x 0.7V, both transistors will start to conduct, right? And since each of them can amplify their input current by 100, that means we could control a current which is a whopping 10,000x as large as the input current! So with the proper high-power transistors, this ought to work, right?

There is a sneaky problem, though. Keep in mind that the base voltage is always capped to at most 0.7V above the emitter, since the B-E junction is like a diode. So when the first transistor starts conducting, it’ll drive the second base up. Up to 0.7V, in fact. And that’s where things go wrong: the first transistor will drive 100x its base current into the second base. With 10 mA, that might well end up being around 1A. But most transistors can’t handle that: they expect small base currents to drive large collector currents. Another way to explain the problem, is that the C-E junction of the first transistor will always be 12 – 0.7 = 11.3V, so with a large current, it’s bound to overheat.

What we need, is a way to limit the current into the second transistor. Here are two ways to do it:

Both should work. The former is like a double amplification stage, whereas the latter uses the voltage follower approach described yesterday. I don’t really know which one would be better, because I’ve never tried either one.

That’s because there is also another circuit called the Darlington transistor, named after the person who invented it, Sydney Darlington:

The difference is subtle but important: the collector of the first transistor is no longer tied to the positive power supply rail, but to the collector, i.e. the lower side of the load.

The practical advantage of this combination, is that it’s still a 3-pin device, so you can use it wherever a transistor is being used, and that’s indeed it’s main raison d’être. Roughly speaking, a Darlington transistor acts like a transistor with a much higher current gain than a single transistor.

Its main drawback is that the saturation voltage, i.e. the voltage over the (combined) C-E junction, is higher than with a regular transistor – i.e. more like 1.1V than 0.4V. This is the voltage you get when driving the transistor all out – and it affects the amount of power absorbed by the transistor (P = E * I). So although a Darlington can switch higher currents from an I/O pin, it also generates roughly three times as much heat.

The reason I’m going into Darlingtons, is that they are very convenient and widely used. There is a 18-pin chip with 8 Darlingtons, tied into a tiny package, the ULN2803:

This is what I used on the Output Plug, by the way. Not only does each pin drive 500 mA at up to 50V, you can actually tie them together to control larger currents. Each Darlington on this chip includes the necessary resistors, so it’s simply a matter of tying an I/O pin to its input, and it will “sink” up to 500 mA to ground.

An extra benefit of the ULN2803 is that it has a built-in reverse kickback protection diode, as needed when hooking up relays and motors. So that’s 8 more components saved, all by using this single chip.

Don’t get you’re hopes up too high, though. When driven from a 3.3V input, the output current of the ULN2803 might not go much higher than 300..350 mA on each pin. The components on the chip seem to have been designed for 5V. But it does work fine at 3.3V!

To summarize: the Darlington transistor pair is an easy way to get just a little bit more current (or more accurately: a higher gain) than from a single transistor.

So much for switching current on and off. Tomorrow, as last part of this mini-series, I’ll describe how to reverse the current as well, so we can also control the direction of a DC motor. And how we can control their speed. All with just a bunch of transistors.