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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.

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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?

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No, you cannot replace a positive regulator with its negative equivalent.

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  • \$\begingroup\$ Could you elaborate? What exactly will happen? What will the output be? \$\endgroup\$ – fuzzyhair2 Aug 29 '13 at 4:36
  • \$\begingroup\$ What will happen is it simply won't work. Those regulators have internal feedback loops to main their output voltages. You cannot change those. If the datasheet says -5V out, that is firm. There are hacks to make it adjustable, but there will always be -5V between the output and ground pin. \$\endgroup\$ – Matt Young Aug 29 '13 at 16:27

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