Welcome to my h-bridge motor driver
tutorial. I’m going to explain what H bridges are and how to build one. So what is an h-bridge? An h-bridge is a circuit that you can
use to control the speed and direction of a brushed DC motor. There are many ways to
design an h-bridge but I’m going to show you a simple one that can work with input voltages
between 6 and 15 volts, for motor loads up to two amps. 30 watts
of power is plenty for small to medium sized motors. It’s not the most efficient H bridge,
but it’s easy to understand, and I think it’ll be fine for your first
motor controller. So here’s a 12 volt power supply, a 1000uF capacitor to help
filter it, four carefully arranged transistors, and a motor. I’ll put the exact part numbers
in the video description. Do you see the H? This is an H bridge!
If we turn these two transistors on, and these two off, here’s the equivalent
circuit. Now current can flow from your positive power supply rail, through this transistor, through the
motor, through this other transistor, down to your circuits ground. This will
make the motor spin in one direction. Now how do we reverse the direction the
motor is spinning? We do that by reversing the direction of
the current going through the motor. If we turn these two transistors on and
these two off, now current will flow from your power
supply, through this transistor, through the motor in this direction,
through the other transistor, down to ground. Since we changed the
direction of the motor current, we change the direction of the motor.
Hey… what happens if we turn all transistors on? The motor should go twice as fast!
(aww crap) Yeah so if you turn all the transistors on, you create two high current paths
straight from power to ground, effectively shorting out your power
supply and blowing up the transistors. Let’s make sure that never happens again.
Let’s make the circuit idiot proof so people like me can use it. First I’m going to set my current limiting
power supply to a 3 amp limit. That way things can get hot, but they
probably won’t catch fire. If you don’t have a current limiting
power supply, consider using 3 amp fuse and fuse holder like these. Just put it in series with the positive
side of your battery. Now let’s be a little more careful when
controlling the transistors. Lets tie the gates of these two
transistors together, and do the same on the other side. Then let’s add 1 a kilohm resistor between
the transistors gates and ground. Any charge that might be on the gates
will drain through the resistor. So by default the voltage here at the
gates will be 0 volts, and the n-channel FETs will stay off,
preventing any sustained short-circuits. But now the motor can’t do anything!
So let’s add a 47 ohm resistor here. These two resistors form a voltage
divider. So if we apply 12 volts here, the gate voltage will be 11.45 volts.
Now our H bridge will stay off by default, but we can easily apply enough voltage
to the gate to turn things on. The voltage between gate and source on
the n-channel MOSFET will be 11.45 volts, so the transistor turns on. On the top half of the bridge, the voltage between gate and source of
the p-channel MOSFET will be -0.55 volts so the P-FET turns off. On the other side of the bridge, the
opposite is happening because the gate voltage is still 0. On the left hand side, the N-FET is off,
and the voltage between gate and source of the P-FET -12 volts so the P-FET is on. So now motor current can flow again!
With this arrangement, the N and P FETs can never both stay on at
the same time. Now that’s not 100% true,
there is some shoot through that lasts a couple of microseconds, but for this simple low-power h-bridge
you don’t need to worry about that. If we want to reverse the direction of the current, we can start by applying 0 volts here,
or we can just leave it floating because we’ve got that 1k pull-down resistor to ground. Then we just apply 12 volts to the other side, and now the other side of the bridge is active. In the real world, how can we easily
switch from side to side? Well, if we add a single pole double
throw switch to the circuit now only half the bridge can be active
at any given time. I’m using a big switch for demonstration
purposes here but you don’t have to. Any switch that can handle 100 milliamps
will be fine. Alright, now instead of just forwards or
backwards, let’s control the speed of the motor. In a previous video I talked about a
technique called pulse width modulation and you can use it for speed control!
If we take the circuit from that video, we can use a potentiometer to get a
pulse width modulated signal. Here I am getting a square wave from
0 volts to 12 volts with a duty cycle ranging from nearly
zero to 100 percent. Now let’s just add that to the center
pole of the switch here and now we have a bi directional motor speed controller. Wait… what’s that noise? (Motor is making a whining noise)
(Arnold Schwarzenegger yells “Stop whining!”)
I can’t! The PWM frequency is 1kHz. That means power to the motor will be
pulsed in at 1kHz and that’s within hearing range. You’ll need a better H bridge circuit to
get rid of it. But as you can see, if you use PWM to change the percentage
time that current can flow through the motor, you change the average current of the
motor, which changes its speed. Finally, I want to highlight that it’s very
important to use the exact same components that I used in this circuit. The 555 timer should be an NE555 because it can source and sink a lot of
current on the output. A TLC555 will not work. And you should use the exact same MOSFETs
and the same resistor values. Alright, now you know what an H bridge is, and with
suitable components you can use this technique to build anything from an electric screwdriver to an
electric vehicle! Thank you for watching, subscribe, and check out the video
description section to see how you can support me if you want see more videos!