More about H-bridges

First, a definition of the term MOSFET:

It's an acronym for

Monolithic Oxide Semiconductor Field Effect Transistor

Now you understand why we call them MOSFETs...it's a whole lot easier to say!

For all practical purposes, you can think of a MOSFET as a solid-state On/Off
switch. We control this "switch" by applying voltages to the Gate terminal.

Current flow through the MOSFET will be between the Drain and the Source terminals.


Why is it called an H-bridge?

This arrangement of wires, MOSFETs and a motor can be thought of as looking like an H



The motor you want to control is located in the horizontal leg of the H.

There are four MOSFETS, two in each vertical leg of the H, one above the horizontal leg and one below.

The two upper ends of the vertical legs are connected to the Positive side of your DC supply. The two lower ends are connected to the Negative side.


How Does It Work?

With this arrangement, you simply turn On on upper MOSFET in one vertical leg of the H, and the lower MOSFET in the opposite leg of the H. In doing so, you send current from the DC voltage supply through the motor in one of two possible directions. And as a result, the motor turns in one of two possible directions.

To make the motor turn in the other direction, you turn Off those two MOSFETs and turn On the other two.

You must take great care to turn the correct MOSFETS On and Off in the correct order! If both MOSFETS in one leg are ON at the same time, it results in a DIRECT SHORT TO GROUND, which is a Bad Thing.

Here is an interconnect diagram of the H-bridge circuit I built.



I wish I could claim the credit for the pull-down resistors (thanks FunGus).

The pull-down resistors ensure that in the absence of any voltage applied to the gates of the MOSFETS, they will be OFF. I think of this as a safety measure, a "default condition" --everything is turned Off unless deliberately turned ON.

This also makes it easy to use an SPDT switch to control the direction of the motor. In the drawing you see a box that says "Polarity Reversal" .

Notice that one of the two wires leading to that box ties together the gates of two MOSFETS -- the "high" MOSFET in one vertical leg of the H and the "low" MOSFET in the opposite vertical leg.

And of course the other of the two wires ties together the gates of the other two MOSFETS in each vertical leg.

In this way we ensure that the correct MOSFETs will be turned On and Off in the correct order.

I'm saving the arcane details for later for the sake of simplicity. Things like "it's a bad idea to reverse the motor too fast and/or often", "DC braking" and "free spindown". Baby steps first.