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[Newbie question] Vdd vs Vin for servos while using a 9v battery — Parallax Forums

[Newbie question] Vdd vs Vin for servos while using a 9v battery

anortonanorton Posts: 2
edited 2012-09-12 11:15 in Learn with BlocklyProp
My Intro to Engineering class has been tasked with a robotics project using the Board of Education. I've built robots before, but not with Parallax products.

I've just gotten to point of using servos in the "Robotics with the BOE Bot" book that comes with the BOE. The book says to put a jumper (between the servo ports) over Vdd if using a 7.5V DC power supply or over Vin if using a 6V battery pack. I'm using a 9V battery, which obviously isn't either. ;) So, where do I put the jumper? If someone could explain why I put the jumper there (e.g. voltage is greater than 6V, or something), that would be helpful, too.

I'm using a BOE Revision D with BS2 Rev. J.

//Andrew

EDIT: By the way... our instructor pointed us to the forums if we needed any help. He's fine with us asking questions on here--it's not cheating/unwanted in any way.

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2012-09-05 19:08
    Servos like 5-6V.
    Selecting "Vdd" with the jumper will send the 5V from the on-board regulator to the servo supply pin/s.
    Your 9V dictates selecting "Vdd".
    If "Vin" got selected then the 9V (too much) would be available for the servo (and wreck them.)


    They don't last long anyway, but little 9V batteries are short-lived running servos on a BoE.
  • anortonanorton Posts: 2
    edited 2012-09-05 19:21
    PJ Allen wrote: »
    Servos like 5-6V.
    Selecting "Vdd" with the jumper will send the 5V from the on-board regulator to the servo supply pin/s.
    Your 9V dictates selecting "Vdd".
    If "Vin" got selected then the 9V (too much) would be available for the servo (and wreck them.)


    They don't last long anyway, but little 9V batteries are short-lived running servos on a BoE.

    Thanks for the help!

    //Andrew
  • ercoerco Posts: 20,257
    edited 2012-09-07 13:20
    In some early Stamp experiments, servos were powered directly by small 9V batteries. It works briefly for simple experiments, but as PJ notes, the 9V batteries will die quickly. They have very high internal resistance, so their voltage drops way down (probably 6-7 V) when the servo motors start pulling lots of current. Otherwise, a more capable 9V supply would damage the servos if you use Vin, which are typically rated at 4.8-6.0V. And if you use Vdd, you may damage the BoE regulator or reset the Stamp when the servos need lots of current (can be over an amp each).

    With a BoE, you're better off using 4x AA batteries (like a BoeBot) to power Stamp and servos, using Vin.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-09-12 11:15
    Frankly, I suspect older servos were much more tolerant of high voltages. It isn't about the motor, it is the silicon that handles the demodulation of the PWM signal and the H-bridge.

    As chips have evolved, it seemed that 7 volts was the highest a servo would tolerate - never 9 volts DC.

    But now, even the brushless ESC for controlling r/c airplanes prove an extra +5 or +6 regulated DC output (often a choice to select one or the other) and this is just for the servos.

    In sum, newer servos will fry if you go too high. You can salvage the motor and use an outboard h-bridge, but the electronics are NOT going to tolerate 9VDC for a New York minute.

    Believe me as I have a nice new and very dead servo from momentary high voltage.
  • djnicedreamsdjnicedreams Posts: 1
    Followup question from a newbie: how am I supposed to move the jumper from Vdd to Vin? It's wedged between the X4 and X5 so one cannot pinch it with one's fingers. The Boe-Bot guidebook makes no mention of how to get at it.
  • msrobotsmsrobots Posts: 3,709
    There are tools called pliers. You will need a small one anyways.

    Enjoy!

    Mike
  • kwinnkwinn Posts: 8,697
    erco wrote: »
    In some early Stamp experiments, servos were powered directly by small 9V batteries. It works briefly for simple experiments, but as PJ notes, the 9V batteries will die quickly. ...........

    With a BoE, you're better off using 4x AA batteries (like a BoeBot) to power Stamp and servos, using Vin.

    Either the 4 AA's as erco suggests or a pair of Lithium Ion batteries would work better.
  • kwinnkwinn Posts: 8,697
    djnicedreams wrote: »
    Followup question from a newbie: how am I supposed to move the jumper from Vdd to Vin? It's wedged between the X4 and X5 so one cannot pinch it with one's fingers. The Boe-Bot guidebook makes no mention of how to get at it.

    Fine needle nosed pliers and tweezers are a necessity for working with small components, particularly surface mount parts.
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