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Battery question — Parallax Forums

Battery question

nol1nol1 Posts: 19
edited 2005-05-30 01:27 in Robotics
This is a fairly simple but important question. Your help is appreciated. I know the BS2 has a +5v output pin for connecting other componets to, but is it safe to run 3 servos (hexapod application) off of this pin if Im using an 8.4v 600mah pack? To be more specific, the pack is a sanyo 7 cell 8.4v 600mah. Do I need any type of resistors etc-or can I connect my servos directly to the +5v pin? Thank you!

Comments

  • CJCJ Posts: 470
    edited 2005-05-28 03:10
    I'm pretty sure that would blow your stamp, might just as well be safe than sorry, get a separate regulator, like the 7805 from radioshack, its rated for a full amp, so I don't think it would be possible the kill it with the setup you are describing.
  • nol1nol1 Posts: 19
    edited 2005-05-28 03:16
    Thanks CJ! How would I go about hooking the 7805 up to my setup?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-05-28 03:46
    Hello,

    ·· CJ is correct in that what you propose to connect would draw too much current from the on-board regulator of the Stamp.· As for the 7805, if the chip is facing you with the tab up and the pins down, the pins are (From left to right) Vin, GND, Vout.· A heatsink is recommended.· Radio Shack may have one of these as well.· The 7805 is a TO-220 package.



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-05-28 14:48
    Get a copy of the BOE board's schematic. It is on the Parallax web site. It has a very nice schematic showing how to hook up a linear regulator -- which the 7805 is, and the 2840-5 (used on the BOE board) is.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-05-28 17:42
    Allan,

    ·· Close...The regulator is actually an LM2940-5.· This regulator is a better choice than the LM7805 when available.· Of course, you can get the LM7805 at Radio Shack, you'd have to order the LM2940 unless you have a local electronics store that carries this kind of thing.



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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-05-28 18:00
    Yup, the 2940-5 is a low-drop-out regulator, which means it will continue to regulate down to Vin == 5.5 volts. The 7805 is a 'normal' drop-out regulator, which means it stops regulating when the input voltage drops below 7 volts. (A 2-volt drop-out for the 7805 is higher than a 0.5 volt drop-out for the 2940-5).

    Now the BS2 does have an on-module regulator for its own use, but since a typical servo takes 150 mA or so by itself, you REALLY need a separate regulator for the Servos. Unless you are using a 6-volt battery (4 x 1.5), which can drive the servo +V directly.
  • nol1nol1 Posts: 19
    edited 2005-05-29 02:02
    Thanks for the info guys- As per CJ's suggestion, I looked at the BOE schematic, and still (unfortunately) am not sure how to hook the regulator up. Do I connect the +power leed from my battery pack to the vin on the regulator,then from the vout of the regulator to the bs2's vin or servo vin? A detailed description of this would be so appreciated-(think duplo blocks here[noparse]:)[/noparse])
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-05-29 13:39
    Oops, right.

    You hook your Battery + to the Vin on the regulator. Vout of the regulator goes to the Vdd pin on the BS2 -- which bypasses the on-chip regulator, but that's ok. If your battery is 7.5 volts or less, you can hook your battery + directly to the '+' line of the servo, otherwise you should hook the + line of the servo also to Vdd. Don't leave out the filter capacitors in all this, they are critical for proper operation of the regulator and preventing voltage drop-outs when you activate the Servo.

    Make sure your BS2 Vss (that's Ground) is hooked to the Servo ground, and both of them are hooked to the Battery -.

    The Servo 'signal' (I believe it is the middle wire) is hooked to a BS2 I/O pin, and is what the BS2 uses to control the beast.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-05-29 15:48
    Hello,

    ·· I would like to offer an alternative connection.· You're running an 8.4V battery pack, correct?· Connect the + (positive) side of the battery pack to the Vin on both the regulator and the BS2.· Leave the BS2 Vdd pin unconnected.· The ground (negative) side of the battery should tie to the BS2 Vss as well as the regulator ground and the Servo ground.· Definately use the proper filter capacitors for the 7805 (input and output).

    ·· On the servos, the connections are usually Black, Red, White.· Black is ground, Red is ~6V and white is the signal (Connect directly to stamp pin).· In this configuration there is less chance for the BS2 to reset if the servos stall suddenly.




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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • nol1nol1 Posts: 19
    edited 2005-05-29 20:15
    Thanks Chris and Allan-
    Would a 1000uf capacitor be ok to use as the filter capacitors? I picked some up at the shack, and they seem kinda large...?
  • nol1nol1 Posts: 19
    edited 2005-05-29 20:16
    I forgot to post that Ill be using the 7805 regulator
  • KenMKenM Posts: 657
    edited 2005-05-29 20:33
    Generally speaking I use two caps on the input, a large electrolytic and a small cap. They serve different purposes.

    The large cap is to temporarily provide current, or stablize voltage depending on how you want to view it. The small cap is to shunt high frequency noise.

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  • nol1nol1 Posts: 19
    edited 2005-05-30 01:27
    Thanks for the schematic Ken, I'll give it a go.
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