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7905 regulator?

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Blanka:

--- Quote from: MonMotha on October 18, 2011, 02:51:40 pm ---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
--- End quote ---

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

lilshawn:

--- Quote from: Blanka on October 19, 2011, 03:46:25 pm ---
--- Quote from: MonMotha on October 18, 2011, 02:51:40 pm ---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
--- End quote ---

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

--- End quote ---

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:

--- Quote from: Blanka on October 19, 2011, 03:46:25 pm ---
--- Quote from: MonMotha on October 18, 2011, 02:51:40 pm ---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
--- End quote ---

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

--- End quote ---

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:

--- Quote from: MonMotha on October 20, 2011, 12:13:14 am ---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.

--- End quote ---

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:

--- Quote from: SavannahLion on October 20, 2011, 02:08:14 am ---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.
--- End quote ---

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 ;)

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