motor controller needs brake at power loss

Thread Starter

dlatch

Joined May 15, 2016
84
The circuit below is a slot car racing controller with hall effect wiper and braking fet. I am trying to figure out a way to create braking at power off. I have tried two approaches with a normally energized relay. Both fail as the relay does not release while the motor is spinning!

All 3 relays I have tired are low coil current as I am trying to minimize the overall current draw. (a diode across the motor makes no difference!)

How would one wire a capacitor to hold power to the whole circuit for at least a second or two? Long enough to brake the motor? My efforts at that have failed as well. FYI the circuit draws at least 150 mA as it is. (30 hall switches at 3-4 mA each)

A mechanical switch would be a last resort. (more conductors to the handle and I'm trying to keep it all solid state)

all comments are appreciated
 

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WBahn

Joined Mar 31, 2012
24,691
You talk about relays but your schematic doesn't have any relays.

Why can't you use a relay across the motor that has normally closed contacts and your controller uses power to keep them open?
 

Thread Starter

dlatch

Joined May 15, 2016
84
Yes Bahn...that was my first attempt. See diagram A. Weird...it works EXCEPT when you connect the motor and try it. Throttle up the motor and disconnect power and the motor spins freely. When it spins to a stop, the relay releases, not before. A diode across the relay coil makes no difference and neither does a diode across the motor!

The diagrams are crude...the relay coil is normally energized. Shown in the energized mode default brake off.

Attempt B where the gate selects a fully charged cap does the same thing! Nothing happens at the relay until the motor stops on its own.

I tried a pull down sdefault brake attempt A.JPG default brake attempt B.JPG ince it is cut free from its normal pull down. no help.

SOOO....basically the motor hash is somehow seen by the coil as falling voltage. I don't want to use a low R coil if that would help I don't have one handy.

I would like to simply hold power to the control circuit long enough to allow brake action if the Hall trigger is released at power off.
 

Thread Starter

dlatch

Joined May 15, 2016
84
The relay has a 1000 ohm coil. I had it laying around...I cant find any data on it but is a typical DPDT relay
 
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