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Power Supply - don't want to fry anything — Parallax Forums

Power Supply - don't want to fry anything

DeskguyDeskguy Posts: 32
edited 2008-07-18 13:05 in Robotics
Hi folks,

I have a BOE-BOT (Rev C), and want to add on a bunch of accessories. I have a nice little table (4' x 8') which I will be using as an environment to be explored and hopefully mapped at some point. Given the fact that it is in a nice contained area (with 8" walls all around the table), I am hoping to run my BOE-BOT off of electricity rather than batteries - at least while doing all of the calibration and testing.

I have already purchased from Parallax the "7.5 VDC 1 amp Power Supply - Item 750-00009".

If I set the pin to Vdd, is it then OK to run the following all at the same time:

- 2 wheels
- Ping
- Ping servo
- IR remote
- Digital Encoder Kit
- Compass
- LCD

- and possibly the accelerometer

I'm guessing it would be OK, but having reassurance from a more experienced user would be greatly appreciated.

Thanks very much in advance.

Best regards,

David

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-13 21:45
    You basically add up the current requirements. Really the only significant current drain is the servo motors. These can draw up to 1 Amp peak each (basically just at stall from mechanical load). This is only going to happen when the BoeBot bumps up against the sides of your table and stays there trying to push the wall down. The supply would protect itself by providing less voltage at current drains above 1 Amp. It would eventually overheat, but not quickly.

    The regulator on the BOE is rated roughly at 1 Amp. It shouldn't overheat at 7.5V input although it might overheat with a 12V input at that current.
  • DeskguyDeskguy Posts: 32
    edited 2008-07-14 13:31
    Thanks for the tips. I'll have to go and look everything up and see what it totals up to.

    That is a different approach then what I thought - I was wondering more if I would fry anything, but I'm getting the impression that you are wondering if that power adapter could handle the load. Did I understand that correctly? BTW, if it hits the wall, it won't stay there for more than a second or so as I'll just move it out of the way (I'll babysit it).

    Thanks for help.

    David
  • allanlane5allanlane5 Posts: 3,815
    edited 2008-07-14 15:00
    The BOE-Bot will only "sip" as much current as it needs to do its work, as long as the voltage is "high enough". If the voltage goes too low, the BOE-Bot will "brown-out" and reset itself.

    So, if you use a wall-wart (the 7.5 volt DC, 1 Amp thing you plug into the wall) rated at 1 amp, or 2 amps, the BOE-Bot will STILL only "sip" the amount of current it needs -- having available a "larger current" is not the same as "forcing" that larger current down the BOE-Bot's throat. And the current situation is that the current is NOT forced down its throat.

    Now, a much much larger voltage WOULD "force" current down it's throat, so that would be bad. Thus we don't use DC supplies larger than 20 volts or so.

    The governing equation for this is "Ohm's Law" -- V = I * R, or Voltage is equal to Current Times Resistance. Although it's easier to understand this as I == V / R, or the Current in a branch is equal to the Voltage across that branch, divided by the Resistance of that branch. Because, the "Voltage" across a branch is the "driving function" here -- the current is a 'result' of the amount of voltage across the branch.
  • DeskguyDeskguy Posts: 32
    edited 2008-07-15 13:08
    Thanks to both of you for your help. I'm still trying to track down the draw from each component, but at least it sounds like I won't fry anything which was my main concern.

    Maybe I'll even stick with rechargeable batteries (with the Boe-boost) so I don't spend a fortune on regular batteries.

    Best regards,

    David
  • Edward HuertaEdward Huerta Posts: 7
    edited 2008-07-18 02:53
    let the batteries run the main BOT and power all of the accys up seperately.

    If you get a regulated supply for the accys you could make it safe to your needs in two ways. Insert an in-line fuse(the are available in mA versions if so desired. And/or install a current limiting resistor of the proper resistance and power rating.

    This is a pretty good way to keep it simple and safe for your testing phase.

    Edward
  • DeskguyDeskguy Posts: 32
    edited 2008-07-18 13:05
    Thanks for the suggestion - good food for thought.

    David
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