Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Physics behind voltage regulators

  1. Oct 26, 2011 #1
    Hello,
    I was given a voltage regulator today (to provide -5V to an op amp). I was told that I could create a -5V signal from a +5V and a ground. I was told that the ground was converted to a neutral level. I understand what that means, but not why or how it works. Could someone explain that to me how this works in detail? I am particularly interested in the physics behind it. It is an LM7905 if anyone is curious.

    Thanks,
    Chet
     
  2. jcsd
  3. Oct 26, 2011 #2
    You cannot create -5V from +5V and ground using a 7905. 7905 will take a raw negative voltage more negative than -7V together with the 0V(ground) to regulate to -5V output. But no way can a 7905 take only +5V and 0V(ground) and get -5V.
     
  4. Oct 26, 2011 #3
    Sorry, I listed the wrong one ( was given 2). LM7805 does the trick, yeah?
     
  5. Oct 26, 2011 #4

    f95toli

    User Avatar
    Science Advisor
    Gold Member

    No, a 7805 will give you +5V out.

    Converting a positive voltage to something negative is a bit tricky. One way of doing it is to create a "virtual ground", this will allow you to e.g. get +5 and -5V from a +12V supply.

    AFAIK there is no single-chip solution for creating -5V from a +5V supply.
     
  6. Oct 26, 2011 #5
    There must be a DC to DC converter that can convert +5V to -5V. Just that OP name 7905 on the first post. BUT not a single IC without monkeying with inductors and other components.

    Actually, if you are given +5V and 0V, you can change reference and use +5V as common and 0V(ground ) as -5V. You have to be very careful to call anything ground. In a straight sense, it is all relative.

    Back to the OP, there is no way to get -5V from either 7805 or 7905 if you have to keep 0V as ground from +5V.
     
  7. Oct 26, 2011 #6

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    there are switched capacitor converters that'll do it
    if you don't need much current.

    check Linear Technology's LTC 1983-3/LTC1983-5
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Physics behind voltage regulators
  1. Voltage regulators (Replies: 3)

  2. Voltage regulator (Replies: 3)

  3. Voltage Regulation (Replies: 3)

  4. Voltage regulation (Replies: 6)

  5. Voltage Regulator, (Replies: 2)

Loading...