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3.3v Regulator Problem — Parallax Forums

3.3v Regulator Problem

mhamen3mhamen3 Posts: 69
edited 2011-04-25 13:47 in General Discussion
I bought a couple 3.3v regs from parallax but am having trouble with both of them. I have them hooked up according to the datasheet, feeding them with an 8.5v 250mA power supply but instead of outputing 3.3v they are outputing 7.5v. Is it possible that they are defective? I really don't see how I could be using them wrong. Any suggestions?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2011-04-21 10:49
    What caps are you using on the input and output? Regulators require the proper caps (specified in the datasheet) for regulation and stability.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2011-04-21 10:58
    The output capacitor needs to be close to the regulator leads, with short connections.
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2011-04-21 11:08
    Since you are feeding 8.5 and getting 7.5 I think that your
    wiring is incorrect. Do a google image search on the regulator
    and you should turn up some pictures of the regulator wired
    correctly.
  • KJenneKJenne Posts: 2
    edited 2011-04-23 12:55
    Yes, me too I just bought a few small components and one 3.3V regulator.I wired just like in
    the datasheet ,same pins assignment as the common 7805 or 7812
    never could get it to function,always put out about 0.5 volts less than input.
    Just assumed I got a bad one. or I zapped it when I touched it.
    mine was labeled LM2937ET -3.3
    no big deal, just used a LM317 adj one I had
  • Mike GreenMike Green Posts: 23,101
    edited 2011-04-23 12:59
    Not all regulators have the same pinout as the 78xx series. Check the part number and its datasheet.
  • KJenneKJenne Posts: 2
    edited 2011-04-23 13:13
    This regulator does have the same pins config as a common
    7805 or 7812, according to the datasheet, I would assume the datasheet is correct
    Datasheet is from Parallax and is the correct data sheet for the number on the part itself
    I would not connect it without verifing that.
    Anyway it is only a $1.25 part. just info for others if they have a problem

    UPDATE: MY APPOLOGIES, I just tested again and apparently, just like datasheet
    says, a 10uF cap on at least the output is NECESSARY for this regulator to even function.
    Apparently even when immediatedy following another clean stable regulator, and no matter the output load.
    Now with the cap it jumps to 3.3v and is steady under different loads.
    I had never experienced this behavior before with 3 legged regulator, mostly
    the 78xx series. Of course this is NOT a 78xx series.
    Thanks Phil and Leon for the hints!
  • LeonLeon Posts: 7,620
    edited 2011-04-23 13:19
    Which regulator?
  • RDL2004RDL2004 Posts: 2,554
    edited 2011-04-23 14:04
    Like Phil asked, did you include the required, low ESR, output capacitor? Even though the pin-out is the same as the 78xx series, these LDO type regulators are not quite as simple to use.

    2937.gif
    2937.gif
    437 x 250 - 8K
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2011-04-23 23:08
    I had no idea the low drop out types were so much more
    finicky....this is good to know! :-)
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-04-25 00:01
    Yes indeed, just like all transistors are not the same even if they have the same physical configuration - you must verify what your particular voltage regulator requires in addition to itself. The LDOs do indeed have some odd capacitor requirements that MUST be met.

    In other words, you cannot merely swap an LDO for a 7805 in a circuit and expect it to behave. These are NOT generic replacements.
  • Invent-O-DocInvent-O-Doc Posts: 768
    edited 2011-04-25 13:47
    When I forget the caps and the same thing happens to me too.
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