Odd power supply circuit?

[FlippyBits]

I came across a schematic at work today with a really odd circuit to create +/- 5V from a 12V source.  The schematic is in the attached image, but I'll try to describe it as well as I can.

The 12V is regulated down to 10V with a LM2940T-10.  The output of the 10V regulator is used as the +5V.  The shared "ground" from the 12Vin and the regulated 10V is used as the -5V.  This "ground" net is also used as the input to a 7805. The 7805's ground pin is connected to the +5V (output from the 10V regulator), and the output voltage from the 7805 is used as the ground.

This circuit is in a product that we've sold hundreds of, so it works, but it gets really really hot.  My question is: Is/was this normal, or is it as wacky as I think it is?

[sleemanj]

That is a 7 NINE 05, not a 7805. 

7905 is a negative regulator.

[tszaboo]

There is a fundamental problem with power supplies like this, and drawing this circuit only proves, that the person drawing it has no real experience with power supplies.

The main problem is ignoring the fact that the 7805 and the 7905 is a one quadrant power supply. They can only source or sink current, not both. Now, if say to a potential, that it is GND, while it is the output of a linear regulator, is wrong.
Saying that a voltage rail is +5V it is 5V from GND and normally it can source current. -5V means it is 5V (the opposite direction) and it can sink current. GND can source or sink current, depending on the situation. Now if you only load the positive rail, that means there will be current flowing from the +5V to the GND, so the GND will have negative current. And since you have a voltage regulator there, which only can source current.
The problem with this is that sometimes it works, and we like copying circuits, even if we don't understand it fully.

[SeanB]

It works, but will dissipate the same power as the load on the 5V rail in the 7905. If say your device uses 500mA on the 5V rail and 100ma on the -5V rail the input regulator has to handle 500mA, and will dissipate 1W. The 7905 has to handle 400mA ( 100ma to the negative rail is current the 7905 does not have to carry) and thus will dissipate 2W. You run into problems if your negative rail draws more current than the positive rail, where you will have a negative voltage on the positive rail. If it is a low power rail it works fine so long as most power is on the 5V rail. If you want more than 500mA then you will be better off using a switching regulator to deliver 5v direct from the 12V input, and either having a separate -5V switching supply or using an overwinding on the 5V rail inductor to generate the -5V or using a capacitive voltage generator if the current is very low.