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Power Question

studysessionstudysession Posts: 66
edited 2006-08-13 11:09 in Robotics
Hi -

I ordered the BOE BOT kit and should be here later this week.

The Homework board and the BOE board have a connection for 9v battery. Would it hurt if I plugged in my 7.2v sub-c cell or lipo battery pack from RC's there instead since they are rechargable? Thanks

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-10 14:13
    Which connector is used for power isn't an issue...What matters is keeping the voltage the servos see below what they can handle.· Of course, if you have a Rev B. (USB) or Rev C. (Serial) BOE you could always set your Servo Jumper to Vdd (Default) and that will limit the voltage to the servos to only 5V.· If you have it in the Vin position that's when you need to be concious of the voltage going into the BOE.· In this case 7.2V is a bit above the rated voltages for a servo, but is still used in many situations.· It simply reduces the life of the servo a little.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • studysessionstudysession Posts: 66
    edited 2006-08-10 14:18
    So it won't ruin the board? I have a lot of 7.2v sub-c & LiPo batteries. And since they are rechargable, I want to use them.

    Thanks
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-10 14:26
    No, 7.2V won't ruin the board.· As you noted there is a 9V terminal.· Of course, 9V is too high for most servos, but 7.2V is a little more tolerable.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-08-10 14:27
    No, it won't ruin the board.

    The BOE has a low-dropout regulator on it, which acts as a 'variable resistor' to drop whatever power voltage you give it to 5 volts on its output. You can use up to 24 volts DC with no damage to the board. The BOE and BS2 are very good about this.

    Chris was referring to a jumper on the BOE, which lets you pass the unregulated battery voltage directly to the servo's, which are used as the wheel motors. This jumper lets you select whether to pass the regulated 5 volt 'Vdd' to the servos, or if using a 6 volt battery bypass the regulator (for the servo's only) and get a little more power to the servos.

    IF you use your 7.2 volt batteries, and IF you install the jumper so that 7.2 volts goes directly to the servos -- well, that's a little 'hot' for most servos. It may result in more 'sparking' inside the servo motor, and thus reduce its life. But it won't burn out the servo electronics (like driving it with 9-volts probably would). Nor will it have ANY effect on the other BOE circuitry.

    In any case, you can always choose NOT to bypass the regulator, and drive your servos with a nice regulated 5 volts.
  • studysessionstudysession Posts: 66
    edited 2006-08-10 14:32
    Cool thanks!@ It is nice to know it can handle up to 24v too. I might setup 2 Li-Po packs in parallel. Been reading some posts where people say batteries drain quick. Li-Po's are light and in parallel they will last longer. [noparse]:)[/noparse]
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-10 14:34
    While the regulator is rated for the 24V input the problem with doing this on the BOE is that the BASIC Stamp Module will run off the same input voltage.· Therefore the rated input voltage for these boards is 6-9VDC.· Running 24V into it would likely damage any BASIC Stamp Module connected.· =(

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • studysessionstudysession Posts: 66
    edited 2006-08-10 14:40
    OK - Miss-understood.
    May I ask what's the difference?

    I ordered two of the BOE-BOT's. Looks like older model than what is on the site. Looks like on the Parallax site now comes with bluetooth if I understand it correctly. The ones I ordered are serial and no bluetooth. I have been going through the "What is a Microcontroller?" from the kit I bought from Radio Shack. Wish I got into this sooner. I also bought some robot building "How To" books from Amazon. I want to make some of my own designs. I have a CNC Mill and a Lathe that I want to convert to CNC.

    After I go through the BOE-BOT book, I want to make it radio control after I am done. [noparse]:)[/noparse]
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-10 15:11
    What is the difference between what?· And no, our boards don't come with Bluetooth built-in now.· We added a package deal bundled with a Bluetooth module separately.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • studysessionstudysession Posts: 66
    edited 2006-08-10 15:14
    Sorry - Trying to figure out how it is rated for 24V but can only run 6-9VDC.

    I would think if it is rated for 24V then can use 24V???
  • allanlane5allanlane5 Posts: 3,815
    edited 2006-08-10 15:49
    Oops, sorry about that.

    Well, what we're talking about is "Vin" (which is the 'raw' voltage applied to the BOE, coming directly from your batteries or wall-wart) and "Vdd" (which is the regulated output of the voltage regulator). I'll mention "Vss" (which is 'ground') just to get it out of the way.

    So, on the BOE, it turns out (if I have this right) that the BS2 IS in fact powered from "Vin", NOT from "Vdd". Now, the BS2 module has its own linear regulator on-module, good for about 50 mA current, which drops "Vin" to 5 volts. BUT, this linear regulator (apparently) can only handle up to 9 volts DC on its input. Seems way low to me, but that's what Chris is saying.

    So that's where the 9-volts limitation is coming from. Personally, I've run my BOE with a 12-volt wall-wart without problems, so some more clarification may be needed on this issue.
  • studysessionstudysession Posts: 66
    edited 2006-08-10 16:19
    Do you have to do anything special to run on 12V? Or just connect it up the same as anything else? Are you using the servo's that came with it as well on that 12V?

    Thanks
  • Matt BattleMatt Battle Posts: 172
    edited 2006-08-10 16:28
    Hey everybody,

    I attached the schematics for the BOE REV C. As you can see that the BS2 is in fact powered from Vin. I think the reason for this is because the BS2 has its own voltage regulator, the LM2940-5.0. So what I gather from this if you ran the BS2 from the Vdd which is already at 4.75 to 5.25v being delivered by the LM7805, you run the risk that the voltage would drop below the minimum voltage, by the LM2940-5.0 on the BS2 and cause it to keep resetting it self. So what it looks like is the LM7805 main purpose is to deliver 4.75 to 5.25v to the servos.

    This is my two cents.
    -Matt

    I updated it so it sounded more clear.

    Post Edited (Matt Battle) : 8/10/2006 4:42:35 PM GMT
    pdf
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    BOErevc.pdf
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  • Mike GreenMike Green Posts: 23,101
    edited 2006-08-10 16:37
    Although the regulator on the BOE is rated at 40V, it has no heatsink. Any voltage over 5V has to be dissipated as heat. The Stamp itself doesn't draw much current, but anything you attach to it (and run off the 5V Vdd supply) will add up. At 9V and 1A maximum draw, the regulator is dissipating 4W which is a lot without a heatsink. If you attach a large heatsink to the regulator, it will be very happy at higher voltages. I would recommend using a commercial switching regulator if you need to use this a lot with 24V, something that will reduce this to 6V ideally. Switching regulators are much more efficient (and more expensive).
  • Matt BattleMatt Battle Posts: 172
    edited 2006-08-10 16:44
    sad.gif OPPS. I guess I will withdraw my two cents and go back in my corner.

    -Matt
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-10 16:47
    Matt has the right idea...The BASIC Stamp Module has it's own on-board regulator.· It is limited in maximum input voltage and so we rate the board at 6-9V to prevent damage to the BASIC Stamp Module.· Using Vdd from the regulator opens more potential for devices (especially servos) running from that regulator to cause a voltage drop (sagging) which could reset the BASIC Stamp.· Basically, when in doubt, follow the ratings on the board.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • MikeSMikeS Posts: 131
    edited 2006-08-11 15:47
    This is taken from the Basic Stamp Manual v2.2 for BS2



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    MikeS
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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-08-11 16:31
    That is the literal maximum voltage range allowed.· The BOE lists 6-9V due to the number of unregulated power supplies that are more comonly used and the fact that they run at a higher voltage than rated when not at their rated current load.· If we listed 15V and someone connected an unregulated 15V supply the BASIC Stamp would be damaged.

    By specifying 9V max it keeps the 9V unregulated supplies (typically at 14-15V) within the spec.· So if you have a regulated supply at say 15V on a BS2 you could use it.· Current output from the on-board regulated of the BASIC Stamp would be limited due to higher thermal dissipation of excess voltage.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Tricky NekroTricky Nekro Posts: 218
    edited 2006-08-13 10:33
    Doesn't the BS2xx modules have onboard regulators?!?! What if the were changed and a very small heatsink added?

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  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-08-13 11:09
    Provas -

    I'm sure it does, but what would be the point of doing that? Here's why I ask:

    The CPU takes a finite about of current (which is published).

    The individual I/O pin ports and sets of pin ports have a finite and limited amount of current that they can sink/source.

    The ancillary chips used with a BS-2xx also use only a finite amount of current.

    Above and beyond that, on most if not all BS-2xx modules there is some overhead built in.

    Parallax has done a careful study of all of these current loads, and the overhead, and has chosen an appropriate voltage regulator for each of the BS-2xx modules. In fact, if for some strange reason you want the Stamp to NOT use the onboard regulator, you can do that as well. Check the PBASIC Reference Manual for the difference between powering the PBASIC Stamp from the Vin pin vs. the Vdd pin.

    The implication of adding heat sinks is that you want to drive the onboard voltage regulators "harder" which is quite contrary to the way Parallax has things planned. One of their chief concerns with ANYTHING they produce is product longevity, and reliability. Why would one want to cast that to the wind, when one can add an external voltage regulator, or use a carrier board which has one installed, and such a method is so much easier, safer, causes no excess heat, and has no downside?


    Regards,

    Bruce Bates

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