Physics behind voltage regulators

  • #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
 

Answers and Replies

  • #2
5,527
195
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.
 
  • #3
Sorry, I listed the wrong one ( was given 2). LM7805 does the trick, yeah?
 
  • #4
f95toli
Science Advisor
Gold Member
3,113
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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.
 
  • #5
5,527
195
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.
 
  • #6
jim hardy
Science Advisor
Gold Member
Dearly Missed
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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
 

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