getting -12v from a 7812 regulator

[maximafly]

Hi guys I'm new to the forums but not exactly new to electronics.
I recently downloaded the schematics for an old crt oscilloscope i still have from many moons ago and noticed in the
power supply section that the -12v supply was coming from a 7812? I have never seen this before as it would usually be a 7912 regulator. (See attached diagram)
I was not aware this could be done.
Is there a reason it was done this way? and is there a reason it's not done this way more often?

[babysitter]

Well, think of voltage as a potential difference and it becomes more ovious. Also, nobody prevents you from simply "stacking" voltage regulators and call something in the middle "the ground".

[MikeW]

Can I just ask a totally unrelated question about why you would put two diodes in series? What is that accomplishing that one diode doesn't? (D502-D507)

[wraper]

There is nothing wrong with using 7812 for negative supply. But you need a separate transformer winding and rectifier for this. In this case this is not a problem as all output voltages are different and would require this anyway. However if you need the same positive and negative supply (like for an opamp power) with the same voltage and common GND, that would be a big downside. In that case it would be better to use single winding with center tap and single rectifier to save on part count and simplify a transformer. But then only 79xx+78xx can be used as 78xx + 78xx are not possible in that arrangement.

Can I just ask a totally unrelated question about why you would put two diodes in series? What is that accomplishing that one diode doesn't? (D502-D507)

Maximum reverse voltage. That is HV circuit.

[VK5RC]

Re the diodes,  shouldn't there also be some very high resistance balance resistors eg 10M?

[maximafly]

Thanks for the info. Seems logical but never seen this before. That's what I like about electronics,  you will never stop learning.

[pelule]

In the "old" times this usage (generating a negative volatge with a positive regulator) was given as a typical application examples in the data sheets, f.e. in old Fairchild (uA7xxx) or Texas data books.
See page 15 @ following link: http://www.princeton.edu/~mae412/TEXT/NTRAK2002/124-141.pdf
At that time the cost of that parts was such high, it could have commercial benefit to use 2 x uA78xx instead of a 79xx and a 78xx (also reduced the numbers of parts in your inventory).
BR
PeLuLe

[wraper]

Re the diodes,  shouldn't there also be some very high resistance balance resistors eg 10M?

No need for it. Diode will limit max voltage on itself by increasing a leakage current, therefore resulting in more voltage drop on another diode. In such a way they will self balance good enough. In the worst case diode will become a zener.