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BoeBot 9v supply for servos — Parallax Forums

BoeBot 9v supply for servos

kshurtzkshurtz Posts: 2
edited 2007-09-28 23:48 in Robotics
The BoeBot manual says to use only 6-7.5V DC adapters for the servos, but the board itself allows 6-9V. If I switch select Vdd as the source for the servos, doesn't the power come through the 5V regulator? Seems like I could use a 9V adapter (which I have) and be OK. Any thoughts? Thanks

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2007-09-28 17:38
    Do not use 9V to power the servos. They're designed for no more than about 7.2-7.5V as a power source. More than that may destroy the controller inside the servo and will certainly shorten the life of the motor's brushes. It's best to run them off 6V or even as low as 5V.

    If you select Vdd as the servo power source, the power does indeed come through the 5V regulator and that's limited to about 1A. Servos can each draw more than that if they're stalled. It's not good for them to stay stalled for long, but that kind of surge can happen. The BoeBot board will work with a 9V supply, but the extra voltage has to be dissipated as heat in the regulator and that doesn't have a heatsink (other than the board itself). If you're just running the Stamp and a little logic and a few LEDs, that's not much power to have to dissipate, but, when you add the servos, that's a lot of heat. Figure it out: Power = Voltage x Current = (9-5) x (200ma+200ma+50ma+...) = maybe 2 watts when running 2 servos, the Stamp, plus a little more.
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-09-28 19:27
    Yes, you are correct, IF you select the 5 volt jumper so the voltage for the Servo's comes from Vdd, and NOT Vin, you can run the board/servo combination on 9 volts.

    As Mike says, your regulator could run hot in this circumstance, depending on what mechanical load your servo's face. If it gets too hot, it does incorporate a "thermal shutdown" circuit which will shut off your BOE until it cools back down.

    The reason most people don't do this is they want the combination to be battery powered. A 'stock' off the shelf alkaline 9-volt battery will only supply about 10 mA at a time, so it won't drive a servo for any time at all. Using an RC rechargeable 9-volt is possible -- but then you're burning almost half your power in the regulator, so people don't want to do that either.

    The best comprimise is a 6 or 7.2 volt RC battery pack. Then you can run the servo's off the full voltage and current of the battery, and let the BOE regulator only have to deal with the BS2's current needs.
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-09-28 19:29
    Oh, and the one thing you MUST NOT DO is try to run the Servo's off Vin, when you've put a 9-volt RC battery on Vin. I believe this will burn up the servo's pretty quickly.
  • kshurtzkshurtz Posts: 2
    edited 2007-09-28 23:48
    Thank you both. That was very helpful.
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