7905 regulator?

[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]

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

Typo

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".

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.

[lilshawn]

you will find often amplifiers using  - voltages. to drive the speakers better they use a - voltage and a + voltage so if you use -5 and + 5 you can effectively get 10 volts from peak to peak. not only does this simplfy the circuit, you more efficiently use your power this way. (not throwing away power trying to invert the power later on)

this is called potential difference. The voltage between A and C is the sum of the voltage between A and B and the voltage between B and C... it's where you place your load that determines how much that is. if A and B is -5 volts and B and C is 5 volts then -5+5=10 volts... i know the math is wrong right??

-5 volts is 5 volts away from 0 volts ( what is that arbitrary point we called "Ground") and +5 volts is 5 volts away from 0 volts. so 5+5=10

the voltages don't even have to be the same. if you had -12 and +5 you get 17 volts. -62 and +90 volts would be 152 volts.

[SavannahLion]

I'd imagine the math would be 5-(-5)=10.

I know what your point is, eeking 7v is an old trick to downgrade 12v fans in PCs.

[lilshawn]

I'd imagine the math would be 5-(-5)=10.

I know what your point is, eeking 7v is an old trick to downgrade 12v fans in PCs.

I know what I was after... I maths fail. in all manners of maths. 

[SavannahLion]

I'd imagine the math would be 5-(-5)=10.

I know what your point is, eeking 7v is an old trick to downgrade 12v fans in PCs.

I know what I was after... I maths fail. in all manners of maths. 

I know what you know what you was after. 

[Ed_McCarron]

I know what your point is, eeking 7v is an old trick to downgrade 12v fans in PCs.

On an unrelated (well, kinda related) topic, using cheap 1N4148 diodes for their .7v drop (stack a bunch in series) is a cheap hack here.

[Blanka]

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

This would mean a 7905 is actually exactly the same as a 7805, but only different marked pins.

[lilshawn]

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

This would mean a 7905 is actually exactly the same as a 7805, but only different marked pins.

actually it's still marked "input" "ground" "output" it's the input that is different. you still have to feed a negative voltage to the input to get a negative voltage on the output.

[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

This would mean a 7905 is actually exactly the same as a 7805, but only different marked pins.

Almost.  The feedback network also gets moved so that it is between the "output" relative to "ground" in all cases.  The block diagram/representative schematics between the two parts (check the Fairchild datasheets) illustrate this.  There's an app note at the bottom of the Fairchild 78xx datasheet showing how to use it in a negative voltage application.

Note that in either "opposite" application, the regulator is basically regulating "ground", not the "output".  That is, the "output" is fixed at whatever rail you apply, and the regulator will adjust the level of "ground" to make sure that there's always 5V (or whatever) between the two nodes.  This means that you can't rely on "ground" to interconnect with anything else expecting the same notion of "ground".  Moving the feedback network is what changes this behavior and appears to be the only difference between the two parts other than the labeling of the pins.

[SavannahLion]

Note that in either "opposite" application, the regulator is basically regulating "ground", not the "output".  That is, the "output" is fixed at whatever rail you apply, and the regulator will adjust the level of "ground" to make sure that there's always 5V (or whatever) between the two nodes.  This means that you can't rely on "ground" to interconnect with anything else expecting the same notion of "ground".  Moving the feedback network is what changes this behavior and appears to be the only difference between the two parts other than the labeling of the pins.

So one can't really tie the ground of a circuit with a 7905 back into the ground of say.... your USB peripherals? I don't get that. Figure 7 of the Fairchild LM7905 specs show a circuit where the ground is tied into a 78** series.

Brutal, I just noticed the 7905's "tab" is tied to input whereas the 7805's tab is tied to ground. I'd imagine that's just asking to pop the 7905 or whatever else is tied into the heatsink if you're not careful.

[MonMotha]

So one can't really tie the ground of a circuit with a 7905 back into the ground of say.... your USB peripherals? I don't get that. Figure 7 of the Fairchild LM7905 specs show a circuit where the ground is tied into a 78** series.

You can, as long as that's the ONLY connection (remember, it's all relative, and "ground" is arbitrary).  So, if you're using a floating (i.e. isolated from everything, which is a common design) wall wart to power the 7905 circuit, you can happily hook the resulting "ground" up to USB/PC "ground" (which, remember, is also tied to Earth via the 3rd prong on your AC cable as well as to the metal components of the case, and some wall warts do the same).

However, if you're using 12V from your PC supply, you have a problem as there's already a connection, and you've redefined the meaning of "ground" when building your 79xx positive regulator circuit.  The same goes if you've already established any other connection between your circuit and the PC prior to defining the "ground" node.

Notice that the application circuit in Fairchild's datasheet for using a 78xx in a negative voltage application shows a transformer on the input.  I would hazard a guess that's probably to illustrate this fact.

You could in fact use 2x 7905 (or 7805) to create a +/- 5V dual supply with a common ground node, but you'd need two supplies that are isolated from each other to start with if you want to continue the notion of common ground throughout the circuit.  If you use a 7805/7905 pair, then you can get by with a single split supply (e.g. +/- 15V) on the input while keeping the same ground reference.

The 7905/7805 pair gets around this by flipping part of the topology around so that the ground reference is continuous throughout the circuit.  Rather than regulating "ground", it regulates the "output", which is more along the lines of what you expect and can probably wrap your head around.

In a nutshell, to save your sanity, use 79xx for negative voltages and 78xx for positive voltages

[lilshawn]

Do not try and regulate "Ground"... That's impossible. Instead... only try to realize the truth.

What truth?

There is no "Ground".

There is no "Ground"?

Then you'll see, that it is not the Ground that gets regulated, it is only yourself.