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; do not connect the servo's Vss line directly to the Basic Stamp modules Vss pin? — Parallax Forums

; do not connect the servo's Vss line directly to the Basic Stamp modules Vss pin?

garyggaryg Posts: 420
edited 2012-05-07 14:31 in BASIC Stamp
I've just now started working on a circuit to drive my Parallax Standard Servo which I purchased from Radio Shack.
The plan is to use my BS1 module to control the servo as best as it can.
I want to find out if I can get enough control out of the BS1 module to control the servo in a manner that's
acceptable to me.
At the Parallax store section there is a note: that says not to connect Vss line directly to the Basic Stamp Vss pin.

What exactly is this statement telling me?

My plan is to connect a 9Volt battery to drive the Basic stamp and
to connect a heavy duty 6Volt battery to drive the Servo.
If I connect the (-) gnd wire from the 9Volt and 6Volt battery together, am I not connecting the Vss line
directly to the Basic Stamp?

Thanks for any comments, suggestions.

On another note:
I just finished building a timing circuit using a BS1 module (2 inputs, one output are used)
It's an automotive application that I needed a timer for.
I left the unused pins not connected to anything.
What harm would this possibly cause in my recently completed and installed circuit?
The timing control functions perfectly, but I would like to, in the future build the most reliable circuits.

and
Since this is my first post, please let me know if I need to change this format.

Thanks
garyg

Comments

  • ercoerco Posts: 20,257
    edited 2012-03-18 12:00
    The Vss (ground, -voltage) lines usually do connect together, providing a common ground to all batteries, servos & sensors ground lines.

    Please give the link to the page which says no on connecting Vss lines.
  • garyggaryg Posts: 420
    edited 2012-03-18 12:27
    Here's the link to the page
    http://www.parallax.com/Store/Accessories/MotorServos/tabid/163/CategoryID/57/List/0/SortField/0/Level/a/ProductID/101/Default.aspx

    It's part of the information in the store.
    Thanks for your response, I'll attempt to build the circuit according to the plan of connecting the (-)gnd wires from both batteries together.

    Garyg
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2012-03-18 12:35
    Egad!
    They have it there, under RoHS Compliant -
    "Note: Servo current draw can spike while under load. Be sure that your application's power supply and voltage regulator is prepared to supply adequate current for all servos used. Do not try to power this servo directly from a BASIC Stamp module's Vdd or Vin pins; do not connect the servo's Vss line directly to the BASIC Stamp module's Vss pin."

    What the heck?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-03-18 12:44
    That's actually good advice, but it needs to be more explicit. It ought to read:
    Do not connect the servo's Vss line directly to the BASIC Stamp module's Vss pin. Instead, connect both the Stamp's Vss pin and the servo's Vss pin to a common point right at the Vss power rail.

    The idea is that you don't want a current surge from the servo to cause voltage glitches on the trace leading to the Stamp's Vss pin. By connecting both to a common, low impedance point near the power return, this can be avoided. A schematic showing right vs. wrong would have been helpful, too.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2012-03-18 13:16
    It's often called a "star point ground".
  • ercoerco Posts: 20,257
    edited 2012-03-18 15:11
    Barely off-topic: IIRC, there's an offbeat system/convention which uses Vss as the positive battery connection.

    Kids, don't try this at home!
  • garyggaryg Posts: 420
    edited 2012-03-18 22:10
    Thanks Phil
    I'm thinking that I understand your comments.
    In the time of my life that I was an electromechanical technician, I saw a number of mysterious problems that I tracked down
    to be poor grounding of equipment. Your comments and Leon's comments helped me remember things that I had 1st learned in the 80's.
    Thanks. I will attempt to set up a low impedance point near the 6Volt battery.
    I don't know of any way of measuring surge current in the ground connections.
    Is there a way to measure Surge current in ground connections?
    Thank you and Leon too.
    garyg
  • LeonLeon Posts: 7,620
    edited 2012-03-19 04:54
    How about putting a low value resistor in series and looking at the voltage across it with a scope and a differential probe?
  • whizbangwhizbang Posts: 1
    edited 2012-04-22 13:39
    Can you connect a Vex motor module directly to Vss though?
  • garyggaryg Posts: 420
    edited 2012-05-07 14:31
    whizbang wrote: »
    Can you connect a Vex motor module directly to Vss though?

    Hi
    I don't know what a VEX Motor Module is.
    It appears that, from what I've gleened from all of the comments so far is:
    Motors including Servos draw lots of current when they are starting up.
    If one uses a ground that is in very close proximity to the motor and Basic Stamp module,
    The current flow that happens in the ground leads going back to the motor, could interfere with the
    Basic Stamp module.
    On my Servo circuit, what I did is, I made a large copper and solder pad that I believe created a low impedence
    path to ground for the servo motor.
    Since the electrons will take the easiest path, In my Servo case, the easiest path to ground is through the
    Copper/solder pad back to the Servo 6Volt high current supply.

    Is the VEX motor module, a motor controller board, or is it an actual motor?
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