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Thread: Question about L7805 Voltage regulator
www.piclist.com/techref/power/regulators.htm?key=7805
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face BY : Russell McMahon email (remove spam text)



> I am putting 24volts in to a 7905 to get a -5volts out.  The 7905 does get
> warm with a large heat sink...no it gets hot.  Is there something like the
> Black regulator that I could use to cool things down?  Keeping ripple to a
> minimum is very important, thats why I used a linear regulator.  I've
always
> had a difficult time reducing ripple.  Any help is appreciated.

You could use something EXACTLY lime the Black Regulator as a pre-regulator
provided it meets your power requirements.

For a negative input and output voltage you need to "invert" every active
component.
Replace any NPN with a PNP, and PNP with an NPN, reverse the "direction of
all diodes and zeners, feed input negative below ground and expect negative
output below ground (ground connection stays the same in old and new
versions)  and voila - a negative input & output black regulator. Using this
to feed you existing 7905 with about 8 volts will allow it to operate at
much lower temperature while still providing the good regulation of a linear
regulator.

As you are dissipating highish power now you are probably working at the
better part of 1 amp load. If so you will need to design the black
pre-regulator accordingly. Suitable output transistor and an inductor that
doesn't saturate at the required current levels are probably the main
requirements.

Roman may be prepared to suggest component values to get you started.



       Russell McMahon

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