Using Transistors and Driving Motors - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 73

Can you figure out why it's called an H-bridge? Notice that the motor in com-
bination with the four switches forms an uppercase H . Although the diagram
shows them as switches, the switching components are actually transistors,
similar to the ones you used in the previous exercise. Some additional circuitry,
including protection diodes, is also built in to the H-bridge integrated circuit.
The H-bridge has four main states of operation: open, braking, forward, and
backward. In the open state, all the switches are open and the motor won't spin.
In the forward state, two diagonally opposing switches are engaged, causing
current to flow from 9V, through the motor, and down to ground. When the
opposing switches are flipped, current then runs through the motor in the
opposite direction, causing it to spin in the opposite direction. If the H-bridge
is put in the braking state, all residual motion caused by momentum is ceased,
and the motor stops.
CREATING SHORT CIRCUITS WITH H-BRIDGES
BeawareofoneextremelyimportantconsiderationwhenusingH-bridges.
Whatwouldhappenifbothswitchesontheleftorbothswitchesonthe
rightwereopened?Itwouldcauseadirectshortbetween9Vandground.
Ifyou'veevershorteda9Vbatterybefore,youknowthatthisisnotsome-
thingyouwanttodo.Ashortedbatteryheatsupveryquickly,and,inrare
circumstances,couldburstorleak.Furthermore,ashortcoulddestroy
theH-bridgeorotherpartsofthecircuit.AnH-bridgeisararescenario
whereyoucouldpotentiallydestroyapieceofhardwarebyprogramming
somethingwrong.Forthisexperiment,youuseSN754410QuadrupleHalf-H
Driver.Thischiphasabuilt-inthermalshutdownthatshouldkickinbeforea
shortcircuitdestroysanything,butit'sstillagoodideatobecautious.
Toensurethatyoudon'tblowanythingup, always disablethechipbefore
lippingthestatesofanyoftheswitches.Thisensuresthatashortcannotbe
createdevenwhenyouquicklyswitchbetweenmotordirections.You'lluse
threecontrolpins:oneforcontrollingthetoptwogates,oneforcontrolling
thebottomtwogates,andoneforenablingthecircuit.
Building an H-bridge Circuit
With the preceding considerations in mind, it's time to build the circuit. The
H-bridge chip you use is the SN754410 Quadruple Half-H driver. Two Half-H
drivers are combined into one Full-H driver, such as the one shown in Figure 4-5.
For this exercise, you just use two of the four Half-H drivers to drive one DC
motor. If you want to make an RC car, for example, you could use this chip to
control two DC motors (one for the left wheels and one for the right wheels).
Before you actually get it wired up, take a look at the pin-out and logic table
from the part's datasheet (see Figure 4-6).