I have a 12VDC motor controller with a brake feature. I am trying to use a normally energized 12 volt relay to act as a default brake when power is cut to the circuit.

But the relay won't release as long as the motor is spinning down. How can I get the relay to drop sooner?. The data sheet shows 1.2 volts as the release point. Is there any way to get much higher release point with just passive circuitry?

The controller circuit is posted without the relay just for reference. I have tried the relay contacts across the output to short the motor. I have tried it toggling the brake gate up to a holding capacitor. Both work fine until you try it with the motor in place. As long as the motor is spinning down, the relay holds! exactly what I don't want.

 

Why not scrap the relay and build a motor controller circuit?
With current sensing and feedback, you can have a speed controller that controls the speed of the motor as a function of control voltage. When the control voltage is zero the motor stops immediately by applying a reverse current. The circuit implements a negative resistance function.

 

Don't see offhand how to do it with passive circuitry but you could use a transistor to power the relay coil with a zener in series with the base with a voltage selected to turn off the relay at the desired voltage.
What cutoff voltage would you like?

 

The relay stays energized because the spinning motor's back-EMF powers the whole circuit back through the FET's intrinsic body diode.
When the relay armature is closed, it takes very little voltage to keep it that way.

If you put a high current diode in series with the entire circuit, then connect the relay BEFORE the diode, it will drop out almost instantly.

 

The thing is a slot racing controller. I want to deliver the full track power to the motor so a series diode is ruled out. Here is a short video demonstration of my Hall effect wiper based controller

Mr chips I would like to know more about a motor controller like you mention. Where can I find more information about it?

crutschow, I will experiment with your idea.

I just came across a you tube from Afrotechmods using a pfet in series at the front of a circuit as a reverse power protection. (the url would not paste in? Ill bread board it. it could be a way to isolate the circuit from the relay coil.