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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.

TIA,
basile

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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OddBot's picture

@ 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.

 

basile's picture

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

Take a look at the forum's update.

basile