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How do I determine how powerful an H-Bridge I need to control a DC-motor?

In particular, does an H-Bridge need to be able to supply the full amount of current a motor will use when stalled, or would it be okay to use an H-Bridge that supplies enough power for a motor under standard load (and then some, but not stall conditions), and if so, what happens if the motor wants to draw more power than is available? (Does the H-Bridge fry, or does the motor just stop?)

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How do I determine how powerful an H-Bridge I need to control a DC-motor?

If you want to provide full stall current, measure the DC resistance of the motor leads. (e.g. a 0.3 ohm motor connected to a 12V battery will draw 40A at stall) If you want to be able to drive the motor to full speed, then reverse it, it will draw twice that current.

If you have a current sensor, you can (with suitable control circuitry) limit the current to any value you choose, for a "soft-start". This is usually a good idea. A less complicated route is to have a current trip sensor which then turns off the H-bridge (do NOT short the motor out, it will overcurrent if it is spinning) if there is an overcurrent detected, then retry some time period later. This has inferior performance to current limiting but will generally be adequate.

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A current sensor, such as a resistor in series with the load, is often used to protect the system. A better solution may be found in Linear Technology Design Note 407 that is titled Dual Current-Sense Amplifiers Simplify H-Bridge Load Monitoring.

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I like the above, but if you want the best protection try using a PTC resettable fuse in line with the motor that will break before your H-bridge is damaged. Ideally you never want to stall your motor, so I'd design for ideal conditions, add 25-50% of overhead and then put a PTC in line with the motor that will trip when those conditions are exceeded.

I suggest the overhead because you never know what your robot is going to see. I have a roomba that had a failing motor - the shaft kept getting stuck with friction. The H-bridge couldn't drive it loose when it got like that, but if I applied the voltage directly to the motor with a 3A source then it would break free for a bit and continue running. It was likely operating under increased load conditions before that occurred - but it kept running. The overhead will ensure that if you've misjudged the power requirements your robot won't just stop dead if it hits a snag.

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