With reference to Olin's answer to Connecting Voltage Regulators 7812 in Parallel for High Current, and likewise the examples from the Fairchild, TI and ST datasheets, a PNP transistor is used (i.e. TIP42, BD536, 2N6133, BD536, respectively) to provide the higher current, when using an LM7812.

Looking at the Fairchild example:

Fairchild LM78xx High current schematic

I would assume that (in a dual rail supply), for the negative rail, using an LM7912, a complementary NPN transistor equivalent would be required (i.e. TIP41, BD535, 2N6130), instead of the PNP for the 7812. Is that correct?

Likewise, for the short circuit protection schematic using the LM7812, an additional PNP transistor is used (i.e. 2N6124 (for the TI and ST datasheet examples), and TIP42 (for the Fairchild example)). Taking the Fairchild example again:

Fairchild LM78xx High current and short circuit protection

So, for the LM7912, is an NPN transistor equivalent used for the short circuit protection transistor (i.e. 2N6121 and TIP41)?

In addition, I presume that the orientation of the transistors is the same - that is to say, the Collectors and the Emitters of the NPN equivalents remain connected the same - and they are not reversed? To be exact, the Collectors of the two NPN transistors go to the input and output, respectively, of the LM7812?

I only ask as neither the NS nor the TI LM7912 datasheets show a high current (with or without short circuit protection) schematic, hence my doubt.

Please note I'm not looking for component recommendation, but rather I would like to confirm that I have got the BJT polarity configurations (PNP versus NPN) correct. I have merely included the component identifiers for completion.


1 Answer 1


Yes, when you reverse the polarities of all the voltages, you also swap BJT types. Your thinking is exactly right.

  • \$\begingroup\$ Thanks Dave. Just as a sanity check, do the NPN Collectors and the Emitters remain with the same connection orientation, as the PNP equivalents, they aren't swapped around, are they..? Whilst drawing the schematic in EagleCAD I had a doubt. I can update my question, if need be, just to add this query. \$\endgroup\$ Commented Oct 15, 2019 at 9:30
  • 1
    \$\begingroup\$ No, don't swap emitters and collectors. \$\endgroup\$
    – Dave Tweed
    Commented Oct 15, 2019 at 11:48

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