7805linearpowerpower supplyvoltage-regulator
I was wondering whether I could use a negative voltage regulator, specifically a 7905, as a drop-in replacement for a positive one, 7805.
On closer inspection of the datasheet, I realized that pin 1 is the most positive, pin 2 is the most negative, and pin 3 is somewhere in between.
P.S. Worst-case scenario I can simply do a quick and dirty DIY regulator with a zener, op-amp, and a transistor.
You could do it, for just a few milliamps, with two zeners and a resistor.
If you did it this way you could probably get away with a 1206 SMD resistor (0.5W) and a dual series SOT23 zener, in total smaller than most linear regulator packages. Here is a possible circuit (output is 9.1V per rail):
____
48V -|10k_|-+-- 9V
|
|__|__
/ \ | 9.1V
/___\
|
|
+-- GND
|
|__|__
/ \ | 9.1V
/___\
|
Source COM -+-- -9V
Your first question makes no sense since the negative outputs will be taken from the anodes with the cathodes common, with the proviso that the outputs of both supplies are floating.
The answer to your second question is:
Best Answer
No, a negative regulator is not the same as a positive regulator with the pins swapped or something.
Think about it. If the same device could work in both roles, then why would there be separate positive and negative regulators? Do you really think it didn't occur to National Semiconductor and engineers who have been using these things for the last 40 years that a negative regulator could be used as a positive regulator if that were actually so?