H Bridge

Hi:

When using an H bridge like L298 or L293, does the "fast motor stop" function really stop the motor instantly?, what is the difference between using this feature and disconnect from Vcc?

Thanks.

stop the motor instantly?

No, but the motor will stop more quickly than if you let it coast to a stop (as in an open circuit).

the fast motor stop is throwing the unit into both channels high. this locks the motor into position with all the power available.

so, motor energy in power, plus driven device power against driver output power.

think if it another way, you are driving at 5 MPH and slam on the brakes.

or you are driving at 90mph with a trailer, in the rain, on ice, downhill......

Did you miss the part about removing power and coasting?

123Splat:
Did you miss the part about removing power and coasting?

???

Quote from: 123Splat on Today at 11:24:25 am
Did you miss the part about removing power and coasting?
???

I think 123Splat was trying to figure out what dave-in-nj was trying to say. So am I!

In "brake" mode the H-bridge acts as a low resistance across the motor terminals. The motor continues to spin and generate power, but the power is dissipated as heat in the H-bridge, thus slowing the motor rotation down fairly rapidly. In "coast" mode the H-bridge acts as an open circuit so the motor continues to spin for some time longer. You will have to experiment with the different modes to see the actual effects.

With a large motor "fast stop mode" might burn out the H-bridge - the current that
flows is akin to stall current (except that the back-EMF is the source of voltage, not the
supply).

A more intelligent braking strategy is to monitor the current and dump it
back to the supply (if a battery) or a dummy load, using PWM to constrain the
max current. For tiny motors this is complete overkill, just use fast stop.

L298's and L293D's are only up to controlling small motors anyway so fast stop
will work.

If the bridge will withstand stall current, it should withstand braking current/power. Generally.

the data sheet for the L298 shows that two coils are powered full. that is using all the power to lock the motor in place.

if you have a stepper, wire all the wires together as a common. try to turn it. there is no coast. there is only brake. the power the motor generates as the field is altered goes into a reverse power to prevent movement.

try it and let us know what you found out.

I assumed that the OP was referring to a brushed DC motor, not a stepping motor. In that light the modes we are discussing are operable when 1/2 of the L298 is used for bidirectional control of the motor. I’ve attached a diagram from the data sheet showing the various modes and how they are activated by the control lines.

Ven = high, C=Ven=high, D=Ven=high

active power stop. nothing coasting about it.

the power of the motor has to overcome the inertia present in the drive train. if the motor has more power than the inertia, then a screeching of the brakes occurs and the thing stops dead.

as I posted earlier, if the power train is a CNC carriage with a 500 pound weight moving at high speed. the potential energy will be similar to a car, towing a trailer, in the rain, downhill. it will come to a stop. just not as quickly.

active power stop

No such thing with the L298, of any flavor.

For those who may be confused by this turn of the discussion, here is a simple explanation showing the current flow in an H-bridge, in terms of switches. H-Bridge Theory & Practice -- Chuck's Robotics Notebook

In this implementation, there are two brake modes (when either the top two switches are closed and the bottom two are open, or vice versa) which short-circuit the motor, and one coast mode when all switches are open. In neither brake mode, nor in coast mode, is any power consumed from the power supply.