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Parallel Volt Regs And Ballast Resistors

Discussion in 'Electronic Basics' started by Michael, Jun 23, 2007.

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  1. Michael

    Michael Guest

    Hi,

    I'd like to put some volt regs in parallel and the datasheet advises a
    0.015R resistor in the output. Is this a requirement or can I get away
    without it? If I need it, is there a 'trick of the trade' to getting the
    right value as the datasheet suggests 2ft of 18ga wire which I'd rather not
    use due to space constraints.

    Cheers,

    Michael
     
  2. Chuck

    Chuck Guest

    Hello Michael,

    Can't comment on the importance of that
    resistor, but fractional ohm resistors
    like that are available commercially if
    you want to avoid the space of a
    home-made resistor.

    Do a Google search on "current sensing
    resistor .015 ohm" and also try Ebay.
    I've seen them there from time to time
    fairly cheap. Obviously, not all current
    sensing resistors are fractional ohm values.

    Good luck.

    Chuck
     
  3. Michael

    Michael Guest

    Thanks Chuck, I looked at sites like Newark and they didn't turn up anything
    so figured they didn't exist.

    Cheers,

    Michael
     
  4. kell

    kell Guest

    Go to www.digikey.com click site map, click part search, scroll down
    to resistors and click current sense resistors. Check out the Ohmite
    brand resistors for the best prices.

    But you might not need to spend any money at all if you just want some
    ballast resistance. Try using paper clips. The cheap steel in paper
    clips has much higher resistance than copper does, so you can use
    short pieces. Dab the end of the paper clip in some flux to make it
    easier to solder (electronics flux, not plumber's flux).

    About the only difference between cheapo steel versus current sense
    resistors is that the steel has a big temperature coefficient, but
    that's actually a good thing for current-limiting.

    Another thing you could try is get a hold of some nichrome wire,
    perhaps by cannabalizing a power resistor, but nichrome doesn't solder
    well.
     
  5. Chuck

    Chuck Guest

    Great idea!

    Michael, if your ohmmeter doesn't let
    you measure down to 15 milliohms, you
    can pass a current of, say 1 amp,
    through a piece of clip and using a DMM,
    measure the voltage drop. Most DMMs
    ought to read 15 millivolts OK. This is
    a four-wire resistance measurement FWIW.

    Chuck
     
  6. Phil Allison

    Phil Allison Guest

    "Michael"

    ** Really ?

    Post the link.




    ........ Phil
     
  7. If you are going to try putting regulators in parallel, the resistors
    are generally necessary. Each regulator will be trying to control its
    output and doesn't expect other regulators to be doing the same thing
    to the same output, where some conflict can arise. The better value
    of the resistors will depend on the load and the regulators
    themselves, but the resistors provide a little bit of "working room"
    for each of the regulators to have some measure of control over their
    own outputs, as they were designed to have, while allowing their
    aggregate current handling to be summed into the load. Just wiring
    their outputs together isn't wise.

    What kind of current are you talking about, load-wise? What's the
    regulator? (And are they just trying to sell you regulators, when a
    bypass transistor around it might do a similar job for you?)

    Jon
     
  8. Chuck

    Chuck Guest

  9. Jasen

    Jasen Guest

    I've never had a problem soldering zinc plated paperclips using only
    electronics solder. I use pieces of them them between D connectors to
    make custom adaptors
    or a toaster etc...
    also 0.015 ohms would be a very short piece.
     
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