Using Motor Bridges

Bridges allow your robot to control high current motors. They literally `bridge` between the low-current logic circuitry of your robot`s brain to the high current demanded from the motors. Many, but not all, use the common `H` arrangement of control circuitry (usually power transistors) to provide on/off and directional controls to the motors - h

ence the oft-cited name H-bridge. Though often considered "old school, " motor bridges using relays solve a number of problems, and easily provide for handling motors demanding many amps of current. Especially when purchased surplus, relays offer a cost-effective means to control your robot`s motors. The following two circuits were contributed by robo-maestro Russell Cameron. Circuit 1 is a hybrid solution using a DPDT relay for controlling direction, and a power transistor for on/off control. The transistor can also be driven in low-frequency PWM (pulse width modulation) mode for controlling motor speed. Circuit 2 is a half-bridge using a SPST and SPDT relay and transistor-controlled logic to manage on/off and direction. The bridge requires a split supply (two sets of batteries), and is well-suited for use when driving those small toy motors that are meant for 3V operation. (Like all split-supply drivers, one set of batteries may deplete faster than the other, depending on the driving habits of your `bot. ) A very popular and reasonably priced all-in-one H-bridge motor control IC is the L298. It can control two motors, not just one. It can handle 2 amps per motor, though to get the maximum current be sure to add a heat sink. The L298 has a large cooling flange with a hole in it, making it easy to attach a homebrew metal heat sink to it. If there`s a downside to the L298 it`s that it comes in a special Multiwatt 15 ...

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