A third installement about transistors in this Easy Electrons series.
So far, I’ve shown how to get more current out of an I/O pin from an ATmega, since this will probably be the most common reason to use transistors in combination with a micro-controller. But these circuits all act as switches, i.e. they turn current on and off (or in the case of the voltage regulator: adjusting current flow to a certain value).
What if we wanted to control one or two DC motors for a little robot? Lots of fun stuff to do in that area, especially with wireless communication. To do this, we also need to be able to reverse the voltage placed on the motor, so we can make it turn forward or backward under software control. And if we want to make it a bit fancier, it would be nice if we could control the speed of the motor as well.
First things first. Reversing the direction of a motor can be done with a double-pole double-throw (DPDT) relay:
This low-tech solution will switch the +12V and the -12V poles to make the motor run clockwise or counter-clockwise. And if we were to use a transistor for the -12V (i.e. GND) side, we could also turn it on and off.
But that’s clunky! – let’s see if we can do differently. What we need is a way to place either a high or a low voltage on either side of the motor. Here’s a first (flawed!) attempt:
Look what happens when we put the proper voltages on A, B, C, and D:
- with A high and B low, the left side of the motor is tied to “+”
- with D low and C high, the right side of the motor is tied to “-”
- it will start running
And now the other case:
- with A low, B high, the left side of the motor is tied to “-”
- with D high, C low, the right side of the motor is tied to “+”
- it will start running in the opposite direction
And of course, when A = B = C = D = low, the motor will stop.
What the two transistors “on top” of each other do, is create sort of a push-pull circuit, since you can tie the central connection to either the “+” or the “-” voltage rail. This type of circuit is called an H bridge, due to it’s shape.
(note that I’ve left out 4 protection diodes, i.e. one across each C-E junction – they do need to be added in a real-world setup with DC motors)
There are several serious problems with this particular design, though:
- to pull A or D high, we have to apply 12V, since 3.3V won’t be high enough to raise the base 0.7V above the emitter voltage level
- if we pull A and B high, then we’ve got ourselves a short-circuit, with huge currents through both transistors on the left!
- same for C and D…
- and lastly, this thing needs a whopping 4 I/O pins
Let’s tackle that last point first: we can halve the I/O pin count by tying A and C together, and by tying B and D together. Now three out of the possible combinations will get us just what we want: stop, turn clockwise, turn counter-clockwise. But with both signals high, we still get a short circuit. Not good – we don’t want a software error to be able to start a fire…
The bigger problem though, electrically speaking, is that the input voltages involved are no longer suitable for an ATmega. This can be solved by adding an extra NPN transistor on both sides, for a total of 6 transistors. Instead of explaining the whole setup in detail, let me point you to some articles I found on the web:
- this one describes the basic idea using relays
- this page uses 6 transistors (lots more interesting pages on that site)
As you can see, it takes quite a few components to drive one small motor. Fortunately there are lots of H-bridge driver IC’s with various voltage- and current ratings. Some of these are quite small – such as the TC4424A I used on the DC motor plug, which is why I was able to actually put 2 of them on a single plug.
The second task we’d like to be able to do is control the motor speed.
This turns out to be fairly easy. The trick is to use pulse-width modulation (PWM). This is just a fancy term for a simple concept: we generate a set of pulses, and we control the on-time vs. off-time ratio of these pulses. As it turns out, DC motors are far too slow to follow these pulse trains if you generate them at 100 Hz or more. Instead, they will tend to average out the 0/1 values sent to them. And sure enough, a pulse train which is 100% off will cause the motor to stop, and a pulse train which is 100% on will cause the motor to run at full speed. Everything in between will lead to a motor running at intermediate speeds – simple!
For completeness’ sake, let me mention that the on-off power control circuits I’ve been describing in these last posts often use MOSFETs nowadays, instead of the traditional BJT transistors. For simple experiments and small DC motors, BJT’s are fine though.
Now if you think transistors are so great… wait ’till you see what MOSFETs can do!
I’ll go into those next week. Enough electronics for now.