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Power Question — Parallax Forums

Power Question

Computer Geek 101Computer Geek 101 Posts: 179
edited 2011-12-07 13:25 in General Discussion
I am wanting to build some battery powered Propeller devices. I will need 5vdc and 3.3vdc. What would be the best way to get this from one single battery source? Thanks for the help.

Comments

  • Martin_HMartin_H Posts: 4,051
    edited 2011-12-07 06:36
    A pair of voltage regulators are what you need. You can either wire them up yourself, or buy something like this:

    http://www.powertwig.com/
  • Computer Geek 101Computer Geek 101 Posts: 179
    edited 2011-12-07 06:43
    Martin,
    What regulators would get me the most battery life? I have used voltage regulators, but they seem to go through batteries like crazy.
  • Martin_HMartin_H Posts: 4,051
    edited 2011-12-07 06:51
    I've found using six NiMH AA cells and running the motors and servos directly off the batteries works well. They use most of the current, so that usage bypasses the regulators entirely. I then use low drop out 5 and 3.3 volt regulators for the electronics. Note that this is my hobby and my EE training ended with C&E II.
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2011-12-07 08:03
    The Microchip MCP1702 series are a good option for relatively low currents (up to 250 mA) combined with low dropout (0.5 V). Their quiescent current is only 2 µA at no load and increases to only `~100 µA at 250 mA output. They use a pmosfet internally as the pass element.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-12-07 08:04
    The 'good old' standard linear regulators need some extra voltage to work properly. A 7805 regulator requires about 7 to 7.5 volts DC to operate. The similar 3.3 volt regulator would also need an addition 2 to 2.5volts to work properly.

    So most people go to LDO linear regulators or small switching regulators to make a good fit to the batteries. With NiCd, NiMH or Alkaline 1.5 cells, you need 5 to provide 7.5, but it is hard to get 5 cell holders (Parallax actually has made some). Power in a sixth cell is just burned up as heat in the linear regulator - it is better to create a dummy battery to fill the slot in the holder.

    LDO are "Low drop out" regulators and often operate well with as little as 0.5volt headroom - so over 5.5v or 4 NiCd/NiMH/Alkaline cells will work.

    Switching regulators are even better as some of them will go up in voltage as well as down AND they waste less energy than the linear regulators. You might take a look at what Pololu has in these items.

    Personally, I prefer going with the 7.2 Li-ion power pack. Alkaline cells are expensive and don't recharge; NiCd or NiMHtend to loose charge rather rapidly when not in use. And with the Li-ion, I can use any sort of regulation. Take a look at Parallax's Li-ion board.
  • Computer Geek 101Computer Geek 101 Posts: 179
    edited 2011-12-07 08:32
    @all Thanks for all the ideas!

    @tracy allen: Is this the 3.3v unit?
    http://search.digikey.com/us/en/products/MCP1702-3302E%2FTO/MCP1702-3302E%2FTO-ND/1098463

    I will try out a few different iudeas and see what ones work best for my application.
  • Martin_HMartin_H Posts: 4,051
    edited 2011-12-07 13:25
    Looking at the description it says "IC REG LDO 3.3V 250MA TO-92-3" So it certainly looks like a 3.3 V LDO regulator to me.
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