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LM7805 unstable

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StillTrying:

--- Quote from: bogdant on February 19, 2019, 09:20:08 am ---The Vin=35V and the target is to have Vout=5V. As consummers I have a micro + LCD, should be no more then 50mA.

--- End quote ---

Max. 50mA @ 30V is 1.5W, you could just about get away with a 15V 2W zenner in series with the 35V, to share the max. dissipation to about 750mW each, in practice a little heat sinking for the reg.


--- Quote from: Jwillis on February 19, 2019, 05:21:18 am ---Sometimes data sheets can be a little unclear
--- End quote ---

It's clear they go to great lengths not to mention what happens below 5mA or above 25V. :)

bd139:

--- Quote from: StillTrying on February 19, 2019, 11:39:57 am ---It's clear they go to great lengths not to mention what happens below 5mA or above 25V. :)

--- End quote ---

That's always something my first EE mentor said to me. Look out for what the datasheet doesn't say!

One reason LT got a good reputation is they were honest where stuff didn't work.

Cliff Matthews:
Aren't some brands of 7805 rated for Vin 40V? I up'd the 7805's heatsink on my ADS200 (fighting back 33v at ~200ma)
AFAIK, it was running close to 80c (20c below its thermal protect) with the standard heat sink shown on the right..

ebastler:

--- Quote from: Cliff Matthews on February 19, 2019, 12:41:32 pm ---Aren't some brands of 7805 rated for Vin 40V?

--- End quote ---

Only if you interpret the term "7805" broadly, to my knowledge. ;)

Murata has (had?) switching regulators with 78xx pinout which can work up to 40V input voltage, with reduced current capability:
https://power.murata.com/data/power/dms-78xxsr.pdf

Zero999:
The LM78xx or LM317 can be cascoded with an external transistor, to work at higher voltages. At higher currents, Tr1 should be a Darlington pair and R1 can be higher.

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