Weird Science

H-bridge

Polish ver­sion is here

What Is It?

An H-bridge is an elec­tri­cal cir­cuit that allows con­trol of the direc­tion of cur­rent flow in sys­tems powe­red by a sin­gle vol­tage source.H-brid­ges are com­monly used in robo­tics to reverse the rota­tion of DC com­mu­ta­tor motors (inc­lu­ding typi­cal DC micro motors and higher-power variants). H-brid­ges are ava­i­la­ble as inte­gra­ted cir­cu­its, but they can also be built using discrete com­po­nents.

How Does It Work?

The term "H-bridge" comes from the sche­ma­tic repre­sen­ta­tion of the cir­cuit. It con­si­sts of four swit­ches arran­ged in an H-sha­ped cir­cuit, as shown below:

Ilustracja

The swit­ches can be elec­tro­me­cha­ni­cal, such as relays. Howe­ver, modern cir­cu­its typi­cally use elec­tro­nic swit­che­s—tran­si­stors.

Let’s ana­lyze how this cir­cuit ope­ra­tes. If we close swit­ches 1 and 4, the cur­rent will flow as shown in the dia­gram:

Ilustracja

Clo­sing swit­ches 2 and 3 rever­ses the cur­rent flow:

Ilustracja

Notice that in both cases, cur­rent flows thro­ugh the motor (deno­ted by the let­ter M) in oppo­site direc­tions.

Buil­ding Our Own H-Bridge

To build a sim­ple H-bridge sui­ta­ble for ama­teur robo­tics appli­ca­tions, you will need the fol­lo­wing com­po­nents:

These com­po­nents are rea­dily ava­i­la­ble at any elec­tro­nics sup­ply store.

Assem­ble the com­po­nents accor­ding to the sim­ple sche­ma­tic shown below:

Ilustracja

The mecha­ni­cal swit­ches have been repla­ced by tran­s­i­stors. The tran­s­i­stor bases are bia­sed and dri­ven in such a way that when a vol­tage signal is applied to one of the Dir inputs, two tran­s­i­stors loca­ted dia­go­nally on the sche­ma­tic begin to con­duct. As a result, the motor's rota­tion direc­tion can be con­trol­led via the Dir ter­mi­nals: if both ter­mi­nals are at gro­und poten­tial, the motor rema­ins sta­tio­nary; if appro­xi­ma­tely 5V is applied to either ter­mi­nal, the motor will rotate in the cor­re­spon­ding direc­tion. The resi­stors in the con­trol cir­cuit pro­vide pro­tec­tion. Avoid a state where both con­trol ter­mi­nals rece­ive the sup­ply vol­tage simul­ta­ne­o­u­sly. The tran­s­i­stors have suf­fi­cient power capa­city to drive low-power DC motors (a heat­sink may be requ­i­red).

The fully assem­bled H-bridge sho­uld look as fol­lows:

Ilustracja:

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A large-value elec­tro­ly­tic capa­ci­tor ensu­res power sup­ply fil­te­ring.

Enjoy expe­ri­men­ting with this edu­ca­tio­nal pro­ject! :)

Fur­ther rea­dings:

Marek Ples

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