Let's Make Robots!

Parallax HB-25 Motor Controllers and Braking

In a robot my friend and I are building we are using 2 Parallax HB-25 Motor Controllers to control 2 motors.
The problem I am having is making the motor brake (as in freeze robot from moving).

A friend have found this thread in the parallax forum that has a solution for the not-braking or free spinning of the wheels and I intend to proceed with constructing the "option 1" or in short to short the 2 leads of the motor.

I intend to connect 2 relays (one for each motor) like this: http://letsmakerobots.com/node/528#comment-15665 and http://letsmakerobots.com/node/23424#comment-56001

My question is as to what specs must the relay have, the transistor that will drive it and the diode in parallel with the transistor must be.


P.S.: you can see a small preview of the robot here: http://www.youtube.com/watch?v=CrQDC0QnKMM
P.S.2: it's this robot after a change in chassis and an arm addition: http://letsmakerobots.com/node/33553

-- Update 2012-10-16 14:00 --
The motors are 4 and they are connected 2 per controller (in parallel).

-- Update 2012-10-20 10:37 --
Sorry for the long time it took me to respond.
I was thinking of two options:

  1. The one uses one relay for 2 motors and
  2. the other uses separate contacts per motor. (I prefer this one better - the motors could be even slightly different)

Image for option 1:

Image for option 2:

From OddBot I know that the relays must be rated to handle the stall current of the motor which is 1.5A so:
for option 1 I should get a relay rated at least for 4A and
for option 2 I should get a relay rated at least for 2A.

But what about the diode, transistor and resistor?

P.S.: this circuit is going to be replicated twice (the robot is using 2 HB25s) and the transistor will be used to trigger both relays.

-- Update 2012-10-29 12:49 --
Just a small update in the diagrams with part numbers I am thinking of using.

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@ basil, your motor stall current apears to be 1.4A per motor. I would recommend your relays be rated for 4A as you will have 2 motors in parallel.

Note: if you accidentally short the motor controller output then you might need new relays or a new motor controller.

You are much better off using a motor controller with brakes built in.


Thank you and sorry it took me this long to respond.
I was swamped at work.

Take a look at the forum's update.