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7905 regulator?

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SavannahLion:
I bought some stuff and the guy threw in a grab bag of parts. Nothing critically interesting. Mostly resistors, a few basic diodes and what not. What sparked my interest is a single LM7905 regulator from Fairchild.

So looking around on the internet and pulling up the spec sheet this is an inverted power regulator. Errr.... I'm having a little trouble understanding what this does. Everyone seems to have a different comment about this. I have under -5v and GND, feed those into this and I get a nice steady -5v. OK, makes sense. But I see a few comments such as thread that seems to imply this is actually an inverter? So if I have +5v/GND, I can get GND/-5v?

Does this mean that one can use, say a PC power supply with extremely weak -5v (or non-existent) line, pop this sucker on (or one of the related 79** series) and get -5v and +5v lines for those PCB's that need it? Something tells me this isn't right. Since I don't see a whole lot of people actually doing this here, this also tells me that I have the wrong idea.

danny_galaga:

An inverter is the opposite of a rectifier. DC in, AC out.

MonMotha:
The 79XX series regulators are just linear regulators built to operate on the "other side" of ground.  That is, you put a larger (in magnitude, i.e. "more negative") voltage on the input, hook ground up, and the output will have a smaller magnitude ("less negative") negative voltage.  e.g. -12V in, -5V out.

This is not an "inverter", so you can't make AC from DC with it.  This is not an "inverting regulator" which would flip the magnitude of the voltage (e.g. 12V in, -12V out).  It's just a simple "negative regulator".

SavannahLion:

--- Quote from: danny_galaga on October 18, 2011, 03:50:57 am ---
An inverter is the opposite of a rectifier. DC in, AC out.

--- End quote ---

Typo



--- Quote from: MonMotha on October 18, 2011, 10:19:48 am ---The 79XX series regulators are just linear regulators built to operate on the "other side" of ground.  That is, you put a larger (in magnitude, i.e. "more negative") voltage on the input, hook ground up, and the output will have a smaller magnitude ("less negative") negative voltage.  e.g. -12V in, -5V out.

This is not an "inverter", so you can't make AC from DC with it.  This is not an "inverting regulator" which would flip the magnitude of the voltage (e.g. 12V in, -12V out).  It's just a simple "negative regulator".

--- End quote ---

That is exactly what I had figured.  I thought it was too easy a way to pulling -5 from the + line.

So in the link in my post all he is doing is changing the reference to "ground"?

MonMotha:
Something like that.  You have to remember that ground is not an absolute.  Ground is an arbitrary reference node in the circuit.  Voltage is always relative; we just tend to pick a convenient middle point, call it "ground" and write "absolute" measurements which are actually just relative to that arbitrarily (but conveniently) selected "ground".

In utility systems, "ground" literally means the earth, but in small electronics, it's just the node so designated.

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