H-bridge


Posted on Feb 5, 2014

I had always assumed that dynamic braking would be most effective at higher speeds. However, actual tests using a Faulhaber (micro mo) gear motor showed that the brake would hold a motor almost stationary. These gearmotors are so well made that I can turn the 4 mm diameter shaft with light finger pressure. For the tests I had mounted a 90 mm diameter wheel, which gave me tremendous


H-bridge
Click here to download the full size of the above Circuit.

mechanical advantage. I found with the wheel mounted that I could barely turn the motor against the brake. When I spoke to Wilf Rigter about this feature, he suggested that the brake would be very power hungry as originally designed. He guessed it would draw around 50 to 100 mA which I confirmed by actual measurement. At Wilf`s suggestion I added a resistor next to each diode in the brake. He recommended resistors in the range 1000 to 5000 ohms; I found 1000 ohm resistors give almost instant braking with only 5 mA current draw. When I designed my general purpose H-bridge circuit board, I provided a jumper for each bridge. With no pins connected, the brake is disabled. With a pair of adjacent pins connected, the brake is applied by either a (0, 0) input, or a (1, 1) input, depending which way the jumper is connected. Of course, for a specific application it isn`t necessary to provide an actual jumper block. For the bridge input resistors, I show a 47k fixed resistor (to prevent overstaturation in case a trimpot gets turned down too far), a 100 k trimpot for fine adjustment, and a 1. 0 M trimpot for coarse adjustment. The pots I like to use are 6 mm Panasonics, sold by Digikey (about $. 50 US each). I found trying to fine tune a motor with a ten turn pot was a pain. Note that the Panasonic trimpots have a life of 50 turns, so they should not be used in a test-rig circuit that you will be constantly adjusting. There are plenty...




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