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Author Topic: Different types of Off on a motor?  (Read 634 times)
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Hello all

Looking at the schematic of a SN754410 quad half bridge I was wondering about the varios types of off that a motor can have.

The motor could either have no power at all, putting it in 'Freewheel' mode or could have some other mode where both inputs are high or both low... not sure if there is a difference there.

Anyway, The function table on page one of the datasheet states that if the ENable pin is low the output is high-impedance (off).

Could anyone explain what this high-impedance means and the end result, as opposed to me setting the pins so that the two wires going to the motor are both Low for example.

The reason I am asking is I need to decide whether I will simply set the EN pins high with a pull up resistor or whether I will gain an advantage by actually having programatic controll over the EN pin.

Thanks for any advice/opinions.

 smiley-cool
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[The motor could either have no power at all, putting it in 'Freewheel' mode or could have some other mode where both inputs are high or both low... not sure if there is a difference there.

There is. If both enable inputs are off then the bridge transistors are all off and energy stored in the motor circulates current through the freewheel diodes and back to the power supply, acting as sort of a "regenerative braking" mode. Current in the motor decays quickly in this case and the motor feels "strong" braking.

If you set both sides of the motor coil to the same potential then the energy stored in the motor circulates coil through the bridge transistors only (or freewheeling diodes). Current in the motor decays more slowly and the motor feels weaker braking.

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Anyway, The function table on page one of the datasheet states that if the ENable pin is low the output is high-impedance (off).

Could anyone explain what this high-impedance means and the end result, as opposed to me setting the pins so that the two wires going to the motor are both Low for example.

The high-impedance means the bridge transistors are off and current can only go through the freewheeling diodes. The end result is as I described above.

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Great - Thanks RuggedC

So, to sumarise. If I want active breaking then I should have programatic control to the ENable pins.

If I am happy with the motors to freewheel to a stop then I can hard-wire the EN pins and simply set both motor wires to the same polarity.
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Frankly, I'd recommend programmatic control to the enable pins anyways, as then you can have speed control using pulse-width modulation (PWM). Otherwise you're correct. I would perhaps add my own freewheeling diodes to the outputs (even though there are ones built-in) if they're going to get a lot of usage as that will be less stress on the 754410.

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If you set both sides of the motor coil to the same potential then the energy stored in the motor circulates coil through the bridge transistors only (or freewheeling diodes). Current in the motor decays more slowly and the motor feels weaker braking.

Is that right?

If you connect both sides of the motor to the same potential they are effective shorted together and you get flywheel breaking. If you disconnect the power you get free wheeling. That is always what I have found in practice anyway.
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If you set both sides of the motor coil to the same potential then the energy stored in the motor circulates coil through the bridge transistors only (or freewheeling diodes). Current in the motor decays more slowly and the motor feels weaker braking.

Is that right?

If you connect both sides of the motor to the same potential they are effective shorted together and you get flywheel breaking. If you disconnect the power you get free wheeling. That is always what I have found in practice anyway.

Mike you are right to query this - shorting the terminals of a high power efficient DC motor will cause EXTREME braking - usually breaking...  A lesser motor will behave in a less extreme (but abrupt) manner.  Basically the current will rise to a value roughly = backEMF / copper resistance.

If you leave the motor open circuit then it can only push current through the freewheel diodes if it is spinning fast enough that the back EMF exceeds the supply.

Basically the advice RuggedCircuits gave is wrong for DC brushed motors (or Hall-commutated brushless) for which voltage = speed, current = torque.

I've witnessed the effect on a 1kW bicycle hub motor where even turning the wheel by hand at a few inches a second is very hard work with the terminals shorted!

If you want controlled regen braking you can PWM between open and shorted, being cautious - this is effectively acting as a boost converter from the back EMF to the power source...
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