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Power supply. — Parallax Forums

Power supply.

Special_KSpecial_K Posts: 162
edited 2006-05-16 14:43 in Robotics
I have a robot that I am building. It is for the time being controlled by a Parallax Servo controller hooked directly to the USB bus of my computer. 16 servos running at the same time (legs). I can only get the 4 double A battery pack to power the PSC and servos for about 10 minutes. After that the robot can no longer be reliably positioned since the servos are under powered and begin to stop holding the robot body up. Would the 7.5 volt 1amp power supply that parallax sells reliably power the PSC so it can give ample power to the servos. Also what should I do about the power supple since 10 minutes is not really enough time to do much of anything.
is there any documentation that tell you what the max and min power rating for the PSC is and what the polarity is???

Post Edited (Special_K) : 5/5/2006 9:41:42 PM GMT

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-05-05 22:32
    In most cases you will want a separate power supply for the servos than what you're powering the PSC from.· Even half that number of servos would quickly run those batteries down.· You'll need a NiMH battery pack like those used on R/C cars.· Get one rated for at least 3000mAh and run the PSC power from a solid 5V.· If you're running it from 6V you will quickly damage the PSC.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Special_KSpecial_K Posts: 162
    edited 2006-05-11 21:08
    I think I need a little more help here. I am unclear on the implementation
    of what you are suggesting. How do I add another power supply just for the
    servos? I know that of the three wires from the servos the red is power and
    the black is ground. So do I wire these to the additional power source and
    the white to the PSC?
    There are no directions with the PSC to help wire the servos to
    another power source. Can you possibly send me a link that might help?
    I looked online and I see that the Crust crawler
    looks like it is using the Parallax PSC and is running all the servos off the power supplied to the PSC. Are they using a different PSC? Also I do not know what battery combination will get me a straight 5V. Three AA are too little and 4 are too much. even the camera batteries are to little or to much. There is an additional piece of equipment that I could buy from crustcraler that looks like it will do what you are suggesting
    http://www.crustcrawler.com/products/powercom.php?prod=55
    But this is more money then I want to lay out. Also I have 2 PSCs. Will I have to get two additional power sources for the servos connected to the PSC plus 2 power sources for the PSC themselves and then the power source for the Board of education?

    Why can't the PSC power all 16 servos? If the PSC needs more power to handle the servos I risk burning out the PSC. I am having trouble wrapping my head around this mad.gif

    Post Edited (Special_K) : 5/12/2006 2:11:42 AM GMT
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-05-12 14:36
    I'm not sure where I am being unclear.· Every document I referred you to clearly defines the information.· For example page 3 of the PSC manual clearly shows the PSC connected to the BOE which powers the PSC but shows the servos being poewred from an R/C Battery Pack, which clearly is a secondary power source.· The PSC takes a regulated 5V, nothing more nothing less.· The servos can run from a range of voltages from 4.8 through 7.2 nominally and take a huge amount of current when you have more than one.·

    The CrustCrawler link was posted as an example of using more than one servo, although this is considered advanced robotics and they too are using an alternate power source for the servos.· There is no way to get enough current for the Hex Crawler through the BOE power, I have tried it.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-05-12 16:12
    "Power" is Current times Voltage (P = I * V). What Chris is telling you is that you don't currently have enough "Current" available.

    The PSC wants +5 volts, and enough current to drive all its servo's. I use about 100 mA per servo as a rule of thumb, so if you have 16 servo's that's 1.6 AMPS. 1.6 Amps is a lot of current, but it's do-able.

    Now, to use "Separate Power Supplies", what you need is one supply for the BS2 board, and a second supply for the PSC board.· I assume the PSC board has a 'linear regulator' on it (a three-legged device, part number like 7805 or 3840-5).· The 'linear regulator' drops the incoming 6, 7.5, 9, 12 volts down to the +5 needed by the BS2.· The servo's will also run off 5 volts, but pull much more current, thus the need for the second supply.

    The BS2 then only drives the Ground and Signal wire to the PSC -- the Power wire is disconnected between the two boards.· A separate ground wire should also be run between the BOE and the PSC boards -- you MUST have a 'common ground'.

    Post Edited (allanlane5) : 5/12/2006 4:29:12 PM GMT
  • Special_KSpecial_K Posts: 162
    edited 2006-05-16 03:06
    will this work

    Specifications:
    3500mah 11.1v Li-polymer Battery
    >Size: 137.9mm x 46.7mm x 19.1mm
    >Capacity: 3500mah
    >Voltage: 11.1v
    >Charger Time: about 3.5 hours (by input 1A)
    >Weight: 238g
    >For use the helicopter or plane: Hummingbird, Feda, E-Sky, Dragonfly, Aerohawk or other mini helicopter or Airplane
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-05-16 14:43
    Very nice battery. Yes, it should work, IF you use a linear regulator to drop the voltage from 11 volts to 5 volts -- 11 volts directly into the + voltage of a Servo will burn out its electronics.

    I don't know enough about the PSC board to know if there's a linear regulator on it.

    Oh, dear. I've just looked at the PSC page, and the PDF on it. http://www.parallax.com/detail.asp?product_id=28023

    It turns out there IS NOT a linear regulator on the PSC board. They expect you to wire in a 6 to 7.5 volt battery pack on their 'screw terminals'. So your 11.1v Li-Po battery would NOT be a good unit to wire directly to the PSC.
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