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I realize that when you short the terminals of a motor, it stops the motor faster than just turning off the voltage.

I was wondering if putting a large diode across the leads so that when the motor is running current can't flow through the diode, but when the current stops, the diode would short the pins. In other words, connect the positive (anode) side of the diode to the negative pin of the motor and the negative (cathode) side of the diode to the positive side.

Would this have the same effect as shorting the leads of the motor?

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  • \$\begingroup\$ It doesn't seem like any of these comments directly answer the question. Will a diode connected in parallel with a brushed DC motor work as an effective brake if the diode is placed so that it is reverse-biased when power is applied to the motor? \$\endgroup\$ – user31348 Nov 6 '13 at 14:43
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    \$\begingroup\$ To answer the question, it won't work. The motor back emf opposes the supply voltage; the diode is in the wrong direction for braking. \$\endgroup\$ – user28910 Nov 6 '13 at 20:56
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A typical DC motor may be reasonably well modeled as an ideal motor in series with a certain amount of inductance and resistance. An ideal motor will always have voltage across its leads proportional to rotational speed, and current flowing through it proportional to torque. If a motor with frictionless bearings and 2.4-ohm internal resistance would have a no-load speed of 1200rpm at 12 volts, then such a motor rotated at 1200rpm would generate 12 volts. If the leads are shorted, such a motor would have 5 amps flowing through it, and would generate 5 amps' worth of torque.

As a pretty good approximation, shorting a motor which is spinning at a certain speed will generate braking torque pretty close to what would be obtained by driving a stalled motor with the same voltage as the spinning motor would generate open-circuit.

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    \$\begingroup\$ And, notably for this question, this generated voltage is related to the resistance. I expect that putting a diode in series with your generator would cause the output voltage to spike to dangerous levels. Perhaps a big zener would be better at dissipating this power than a standard rectifier in breakdown mode? \$\endgroup\$ – Kevin Vermeer Feb 5 '12 at 5:29
  • \$\begingroup\$ The generated voltage is not related to the resistance. It is determined by the number of turns in the winding, the strength of the magnets and the speed. See: Faraday's Law \$\endgroup\$ – user28910 Nov 6 '13 at 21:08
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This is a more popular method of braking a motor than many people seem to realize. The physics behind it boil down to the fact that an electric motor is a generator (the input power is provided as mechanical power from the shaft, e.g. when braking due to inertia) as well and can 'flip' from one operation mode to the other without any further extras. In practical applications this is done either by feeding back the energy stored in the motor into the supply (called regenerative braking), if the supply allows it and the amount of energy saved that way makes it economically worthwhile (the power electronics are more complicated - and expensive - in that case) or you can simply connect a resistor (imaginatively called braking resistor) across the motor during braking. I strongly suggest using a resistor and not simply shorting the motor to avoid overheating or other unwanted side-effects from the big reverse currents that are produced. You must keep in mind that the energy released through braking (accumulated as kinetic energy in the motor plus the mechanical inertia of the load e.g. flywheel) will be transformed into heat and you don't really want it to be dissipated in the motor, but rather in the braking resistor. Otherwise, happy braking! :)

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In some H-bridge drives you sometimes see modes where both low-side drives (with both high side drives OFF!) are turned on simultaneously to provide braking. This lets current generated in the motor loop around through the lower rail node, essentially shorting the motor; the current thus generated provides braking torque in the motor, and mechanical energy absorbed by the motor turns into heat in the resistive elements along the path of that current. (Of course, you could turn on both high side drives and leave the lower ones OFF, and get the same effect, with the supply rail node allowing the motor current to loop back.)

I believe the H-bridge has to be constructed with FETs as the active elements to accomplish this, b/c during braking, one of the sides has to conduct in the opposite direction than normal.

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