What's all this talk about H-Bridges? How do they work? Well let's see 

How do we make a motor turn?

You take a battery; hook the positive side to one side of your DC motor. Then you connect the negative side of the battery to the other motor lead. The motor spins forward. If you swap the battery leads the motor spins in reverse.

Ok, that's basic. Now lets say you want a Micro Controller Unit (MCU) to control the motor, how would you do it? Well, for starters you get a device that would act like a solid state switch, a transistor, and hook it up the motor.

NOTE: If you connect up these relay circuits, remember to put a diode across the coil of the relay. This will keep the spike voltage (back EMF), coming out of the coil of the relay, from getting into the MCU and damaging it. The anode, which is the arrow side of the diode, should connect to ground. The bar, which is the Cathode side of the diode, should connect to the coil where the MCU connects to the relay.

If you connect this circuit to a small hobby motor you can control the motor with a processor (MCU, etc.) Applying a logical one, (+12 Volts in our example) to point A causes the motor to turn forward. Applying a logical zero, (ground) causes the motor to stop turning (to coast and stop).

Hook the motor up in this fashion and the circuit turns the motor in reverse when you apply a logical one (+12Volts) to point B. Apply a logical zero, which is usually a ground, causes the motor to stop spinning.

If you hook up these circuits you can only get the motor to stop or turn in one direction, forward for the first circuit or reverse for the second circuit.

Motor Speed:

You can also pulse the motor control line, (A or B) on and off. This powers the motor in short burst and gets varying degrees of torque, which usually translates into variable motor speed.

But if you want to be able to control the motor in both forward and reverse with a processor, you will need more circuitry. You will need an H-Bridge. Notice the "H"-looking configuration in the next graphic. Relays configured in this fashion make an H-Bridge. The "high side drivers" are the relays that control the positive voltage to the motor. This is called sourcing current.

The "low side drivers" are the relays that control the negative voltage to sink current to the motor. "Sinking current" is the term for connecting the circuit to the negative side of the power supply, which is usually ground.

So, you turn on the upper left and lower right circuits, and power flows through the motor forward, i.e.: 1 to A, 0 to B, 0 to C, and 1 to D.

Then for reverse you turn on the upper right and lower left circuits and power flows through the motor in reverse, i.e.: 0 to A, 1 to B, 1 to C, and 0 to D.

CAUTION: You should be careful not to turn on both circuits on one side or the other, or you have a direct short which will destroy your circuit; Example: A and C or B and D both high (logical 1).

Semiconductor H-Bridges:

We can better control our motor by using transistors or Field Effect Transistors (FETs).

Most of what we have discussed about the relays H-Bridge is true of these circuits. You don't need diodes that were across the relay coils now. You should use diodes across your transistors though. See the following diagram showing how they are connected.

These solid state circuits provide power and ground connections to the motor, as did the relay circuits. The high side drivers need to be current "sources" which is what PNP transistors and P-channel FETs are good at. The low side drivers need to be current "sinks" which is what NPN transistors and N-channel FETs are good at.

If you turn on the two upper circuits, the motor resists turning, so you effectively have a breaking mechanism. The same is true if you turn on both of the lower circuits. This is because the motor is a generator and when it turns it generates a voltage. If the terminals of the motor are connected (shorted), then the voltage generated counteracts the motors freedom to turn. It is as if you are applying a similar but opposite voltage to the one generated by the motor being turned. Vis-ã-vis, it acts like a brake.

To be nice to your transistors, you should add diodes to catch the back voltage that is generated by the motor's coil when the power is switched on and off. This flyback voltage can be many times higher than the supply voltage! If you don't use diodes, you could burn out your transistors.

Transistors, being a semiconductor device, will have some resistance, which causes them to get hot when conducting much current. This is called not being able to sink or source very much power, i.e.: Not able to provide much current from ground or from plus voltage.

Mosfets are much more efficient, they can provide much more current and not get as hot. They usually have the flyback diodes built in so you don't need the diodes anymore. This helps guard against flyback voltage frying your MCU.

To use Mosfets in an H-Bridge, you need P-Channel Mosfets on top because they can "source" power, and N-Channel Mosfets on the bottom because then can "sink" power. N-Channel Mosfets are much cheaper than P-Channel Mosfets, but N-Channel Mosfets used to source power require about 7 volts more than the supply voltage, to turn on. As a result, some people manage to use N-Channel Mosfets, on top of the H-Bridge, by using cleaver circuits to overcome the breakdown voltage.

It is important that the four quadrants of the H-Bridgecircuits be turned on and off properly. When there is a path between the positive and ground side of the H-Bridge, other than through the motor, a condition exists called "shoot through". This is basically a direct short of the power supply and can cause semiconductors to become ballistic, in circuits with large currents flowing. There are H-bridge chips available that are much easier, and safer, to use than designing your own H-Bridge circuit.

H-Bridge Devices:

The L 293 has 2 H-Bridges, can provide about 1 amp to each and occasional peak loads to 2 amps. Motors typically controlled with this controller are near the size of a 35 mm film plastic canister.

The L298 has 2 h-bridges on board, can handle 1amp and peak current draws to about 3amps. You often see motors between the size a of 35 mm film plastic canister and a coke can, driven by this type H-Bridge. The LMD18200 has one h-bridge on board, can handle about 2 or 3 amps and can handle a peak of about 6 amps. This H-Bridge chip can usually handle an average motor about the size of a coke. There are several more commercially designed H-Bridge chips as well.

There! That's the basics about motors and H-Bridges! Hope it helps and be safe!

The H-bridge is a circuit used in electronic control of high current devices, particularly where the device polarity may be reversed, e.g. DC motors. The name comes from the fact that the circuit typically looks like a letter "H".

The circuit shown has four switches and a motor. To apply a voltage across the motor a pair of diagonally opposite switches need to be turned on. Depending on which pair of switches are turned on the motor will turn one way or another. If both the top or both the bottom switches are turned on the motor will have no voltage difference across it so it won't move at all. If the top and bottom switches on one side are turned on together by accident we will short out the supply, so you need to be careful if you're building your own H-bridge. Most pre-packaged ones will have internal logic to prevent this.

You can build an H-bridge like this out of relays instead of switches and control them with a lower current to drive big motors but most often the switches are replaced with transistors, a pair of PNP transistors (or p-type MOSFETS) at the top and a pair of NPN (or n-type MOSFETS) at the bottom.

PWM is a method of digitally controlling an output with a variable equivalent voltage. Essentially if you take the average of the signal over time then it has a varying analog level, however in the short term it is digital. This makes it easy to generate and efficient as transistors are most efficient when on or off rather than partially conducting. Most modern microcontrollers have the ability to generate PWM built in, including the Arduino and derivatives.

To control a plain H-bridge you need 4 signals to control the 4 transistors, to control it with PWM you need two PWM signals and two plain digital signals. You could theoretically use just the two PWM outputs, however you need to be careful about the polarity of the signal compared to the transistor you're driving. Some microcontrollers include a full bridge driver (Microchip call this Enhanced Capture/Compare/PWM or ECCP on their 8bit PIC chips) which drives 4 outputs and you can hook up to the H-bridge. However it still needs 4 control lines. If you can drive both your PNP and NPN transistors from the same logic line (i.e. your motor power and logic rails are the same) you can connect the PWM to both A and B inputs and a direction control to C and D. In this configuration however you have to invert the duty-cycle when you are running in reverse because it's the low part of the duty cycle that turns the motor on, shown as "Inverted PWM" above.

There are a couple of ways I can think of to make the H-bridge much simpler to control. Essentially what we want is to add an over-all enable line to the H-bridge so we can use one enable line controlled by PWM rather than using a complex H-bridge driver. One way is to add another transistor to the circuit that only connects the bottom rail of the H-bridge to the negative supply. Pulsing this will make the motor run only while the control is high so PWM on this would make an overall speed control without having to worry about which direction the motor is running. The cost of the extra transistor could be quite high if you're using high power transistors or MOSFETS and depending on your voltage rails this might be eating into your motor voltage as there'll be a 0.7-0.4V drop across each transistor or MOSFET. The other option then is to use logic to filter the control lines so that the PWM signal is combined with the direction signals.

In a lot of cases, especially with little toy motors, you don't need to build a whole H-bridge circuit from scratch. In fact using a chip can save you a lot of trouble with offset voltages; if you have a different motor supply voltage from your logic voltage you'll need drivers between the logic and the power transistors. There are a couple of chips I'd recommend and it depends really on how powerful your motors are which one to use, both are dual H-bridge chips (so you can control 2 motors) and both have 2 enable lines so you can connect a PWM line for each motor.

L298

The 298 is a high power dual-H-bridge driver which can handle 2A continuous current on each channel. It's in a big heat-sinkable package and if you're running it close to the limit I'd recommend using this heat sink. The 298 also has current sense lines on each of the two channels which you can add a low value resistor to ground and monitor the current being drawn by each motor. If you need to save some pins you can bridge the direction lines with a not gate and only use one line for direction control. Yu can also parallel up the inputs and outputs on this chip to make a 4A continuous drive bridge but you need to be careful to wire up the inputs in the right order. You need to provide your own protection diodes to suppress the inductive response of the motor and protect your circuit from negative voltages, these are wired "backwards" across your motor supply.

L293/SN754410

The L293 (and pin compatible SN754410) have a very similar block diagram to the L298 but a much lower current capacity. The continuous running current is 1A for the L293/SN754410 or 0.6A for the L293D. It is packaged as a normal DIP IC so if you need to heat sink it you're recommended to have a big ground plane connected rather than a heat sink block. This is fine if you have a PCB but it can be a bit difficult to achieve on strip board. The L293D can only handle about half the current of the L293 but has internal protection diodes so it really is a single component solution for driving low power motors. The SN754410 has the protection diodes for running inductive loads as well but can handle up to 1.1A continuous current, it has identical pinout and better absolute maximum ratings so is a safe drop in replacement for the L293D.

Datasheets:

st.com L298 Farnell: 403-295 ti.com L293/L293D Farnell: 147-0423 ti.com SN754410NE Farnell: 147-0411