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Power Supply for BS2 Homework Board — Parallax Forums

Power Supply for BS2 Homework Board

BrockMBrockM Posts: 2
edited 2011-01-20 08:39 in BASIC Stamp
Hi, I would like to connect a 12VDC 200 mA power supply to the homework board. There are two stripped ends which have alligator clips to the respective +/- terminals. I would like to connect them to the homework board. Is this okay?

Thanks

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-01-19 21:45
    It is ok with two caveats:

    1) Make absolutely sure you hook them up properly. There are few faster ways to destroy any kind of digital logic (including the Homework Board) than hooking up the power supply backwards. You'd be safer getting a 9V battery clip and permanently hooking it up to your power supply leads. Remember that most battery clips are backwards for this sort of use. Test the polarity at the battery clip using a multimeter before you connect it up to the Homework Board.

    2) The Homework Board's voltage regulator has no heatsink other than the board itself. Any supply voltage in excess of 5V has to be dissipated as heat (times the current drawn). The Stamp itself doesn't take much current, but anything you connect to an I/O pin (like an LED) will increase the supply current needed (and the heat generated). Eventually, the regulator will overheat and turn itself off to protect itself and the Homework Board until it cools down. Keeping the supply voltage to 7.5V to 9V will significantly produce less heat than 12V.
  • BrockMBrockM Posts: 2
    edited 2011-01-19 22:35
    Thanks Mike, this will save me some money on batteries. I have a few more inquiries.

    2.) I can use a 9V that gives 400 mA. Would that be better?
  • TomatoTomato Posts: 23
    edited 2011-01-19 23:41
    I use a 9V 2.5A laptop powerbrick (er, it's probably not that, but who cares) i found in the recycling bin. The current rating is ridiculously overkill but it was within arms reach lol. Works fine. A psu rated for 400mA is also plenty considering that you shouldn't be pulling more than like 20mA or so out of any given IO pin. I cleaned up my psu's output wires by soldering on two headers and wrapping them in heatshrink. Then I checked the polarity with my voltmeter and used a little bit of blue acrylic paint to mark the positive lead. No confusion, no problems. What i don't really get though is why I need 9V through a 5V regulator when there's a 5V regulator already on the BS2 lol. w/ever, I guess a straight 5V could be a little noisy.

    -Tomato
  • Mike GreenMike Green Posts: 23,101
    edited 2011-01-20 08:39
    BrockM,
    Yes, 9V would be better for reasons that I mentioned. 400mA is more than enough current for most things. It's probably not enough for use with motors though. They can draw much more than that for short periods of time. A single servo motor for example can draw peak currents of 1A or more. That's one reason why rechargeable batteries are used for robots. They can supply very high currents for short periods of time. 400mA should be fine for simple experiments with small motors since the motor is rarely under significant mechanical load.
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