How to stop a DC motor quickly

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I have an application involving a small DC gear motor rotating slowly (~ 5 RPM). I'm using it kind of like a stepper in that it rotates until a sensor detects the right position and then it has to stop. I've been playing around with an L293 bridge chip which has a fast stop mode that stops the motor faster than just killing the power. The L293 data sheet doesn't say what the chip is doing to stop it so fast. It's not reversing the current because I don't have it hooked up that way. An app note says only that the motor is "shorted by one of the transistors." It's still not clear to me what's going on. I'd like to understand since, if I can duplicate it with one or two transistors, I'd rather do that than use the L293. Can anyone clue me in? Thanks.

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The motor's wires are shorted. That causes it to stop quickly, because it is then basically a shorted generator.

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Sounds like the chip is applying a "brake". This is achieved by shorting the motor's contacts together. You may be aware that a motor that is mechanically driven can act as a generator. By shorting the motor(generator) contacts the power effectively goes back in to the motor in "reverse polarity" if you like to force the motor the opposite way to the applied force, thus effectively acting as an electrial brake.

This principle can be easily demonstraited. Simply turn a motors shaft with your fingers with and without the shorted contacts. You should notice much greater resistance with them shorted.

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Aah, I see. Thanks. But if I have the motor hooked up as M1 in figure 5 of the attachment, how can the contacts be shorted?

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There are three output possibilities with the L293:
1. output volts
2. output to ground
3. output open

if #2 activates the motor, #1 will brake it and #3 will allow it to 'free wheel'

[edit]
Seems I just repeated what the datasheet says!

You can just put a transistor to switch across the motor. Obviously, if you have one transistor to apply power to the motor, the brake transistor can't be on at the same time otherwise you get smoke. The L293 has circuitry to do all this for you.

Note, that the braking energy gets disipated in the transistor and motor. Just be sure the circuit can cope with that energy.