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separate power supply for motor(servos) and BS2... — Parallax Forums

separate power supply for motor(servos) and BS2...

celestino_galancelestino_galan Posts: 44
edited 2005-05-01 16:32 in Robotics
I have read in different places that is allowed to use one power supply for the motors (servos...) and other power supply for the BS2 and sensors...

¿What do you think?

Is not better to have the same battery pack for all?

I say that in order to "charge" the batteries... (Sould we charge the two batteries? isn't it more complicated?)


Thank you

Comments

  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-04-29 14:29
    One reason people use a seperate supply for the servos is the high current draw that they require. A battery is rated by mah (milliampere hours) these values are measured by draining them at a current draw that is close to the draw the battery chemistry is designed to support. If you drain the battery at a greater current than it was designed the battery will not supply the full mah, thereby reducing the effective amount of charge of the batteries. Higher drain also reduces the number of times the batteries will take a charge.

    An additional reason is batteries supplying current greater than they were designed for will experience a voltage sag, if this sag drops below the brownout voltage of the stamp, it will reset. This means even though the batteries have enough charge on them to continue operating the bot, it cant be utilized because the stamp keeps reseting due to overdrawing the batteries. Since the servos are the biggest culprit for this and they are less sensitive to sagging voltages, people choose to supply them with thier own power source.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-29 14:29
    Hello,

    ·· It is better to have more than one power supply if the supply voltage is close to the operating voltage of the Stamp Module and/or Motors/Servos are drawing large amounts of current in spikes (Sudden surges of current).· This can cause the Stamp Module to brownout due to voltage dropping.· In this case separate power supplies helps, although you must have a common ground (Unless the devices are electrically isolated).



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-04-29 14:32
    The reason different power sources are mentioned is that often people try to use the BS2's power (4 AA alkalines, or even a 9-volt 'transistor' battery) to power Servo's (which take WAY more current -- 150 mA per Servo is my current rule of thumb). When they do this, they 'brown-out' the voltage to the BS2, which makes it reset.

    The other problem is if you go through the BOE regulator, it is limited to 1 Amp total. And you can exceed this with enough Servo's, again needing another supply.

    However, for what you want, an RC battery pack with regulator driving both the Servo's and the BS2 should work.
  • celestino_galancelestino_galan Posts: 44
    edited 2005-04-30 11:17
    tHANK YOU ALL.

    Allanlane5: " ....RC battery pack with regulator... "

    What do you mean with regulator?

    for the servos?
    for the BS2?
    for both?

    Wich one do you recomend me?


    Thanks
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-04-30 12:23
    Well, a servo is supposed to be driven at 4.8 to 6 volts. You CAN drive them at 7.2 volts, with the risk of lowering their life a little.

    The BS2 'Vin' pin allows you to drive the BS2 from 7 to 16 volts, so that's not a problem. Definitely put a largish (20 uF to 100 uF) capacitor across the BS2 'Vin' to Vss pins to shield it from short brown-outs caused by turning on Servos.

    So, if you can get a 6-volt to 7.2 volt RC Car battery, you can drive your Servo voltage directly with that, and drive the BS2 'Vin' pin with that, and you won't need an additional regulator. I believe the BS2 on-module regulator is a low-drop-out type -- meaning as long as the input is 0.5 volts above the 5-volts output, the regulator will work. Parallax might confirm that.

    If you want to drive your Servo's with 5 volts, you'll need an external regulator to do that.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-04-30 16:16
    Hello,

    ·· I haven't actually gotten them and tried it yet to see how good it works, but on my Hex Crawler (Still waiting to be given life) I am planning on using 2 BARREL diodes to drop approximately 1.4V from the 7.2V R/C Pack.· I don't wish to take any life away from my servos.



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • celestino_galancelestino_galan Posts: 44
    edited 2005-05-01 11:10
    Chirs, could you tell us how those BARREL diodes work?

    Thanks
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-05-01 16:32
    Hello,

    ·· Essentially each diode (Depends on the Diode) will drop a voltage across it.· Barrel diodes are just an example of a higher current handling Diode.· But after looking at the a few I see some actually drop 1.2V across them, so only one would be required.

    ·· Most Silicon Diodes drop about .7V, while most Germanium Diodes· drop about .2V.· So there is some variance out there.· I guess you could say electronics is not an exact science!··tongue.gif·· You gotta do a little research.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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