Electronics > Beginners
LM7805 unstable
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Jwillis:
Regardless whether you use a 78xx or a lm317 you still need to remember that  Power in Watts  = (Volts in - Volts out) X I . The LM317 won't run any cooler than a 78XX so adequate heat sinking is still required.
Km:
you use a 100  oham resistor half watt input side of 7805 its drops small voltages its cheap solution  8)
i use it many time
jackthomson41:
Yeah its better to use heat sink with LM series as they get hot and burnt out, happened to me a lot. Moreover, use 7805 instead of LM317 as there's no need to design the small basic circuit but it will give fixed voltage of 5V. LM317 gives variable output.
Siwastaja:
Note that it's difficult to completely eliminate input voltage peaks and ringing during powerup, or sudden load changes. This ringing originates from cable stray inductance oscillating with the input capacitance of your circuit, and can go up to 2x the supply, i.e., 70V in this case! This happens easily if you have a big ceramic capacitor on the input.

With electrolytic (high ESR) bulk capacitance in place, this peaking is easily damped, but even then, when running any part at the "absolute maximum ratings", it means that even a very slight input voltage peak may permanently damage the part the first time you apply power; or at any point in time later. You have zero margin for such inrush voltage peak!
bogdant:

--- Quote from: Siwastaja on February 21, 2019, 05:34:47 pm ---Note that it's difficult to completely eliminate input voltage peaks and ringing during powerup, or sudden load changes. This ringing originates from cable stray inductance oscillating with the input capacitance of your circuit, and can go up to 2x the supply, i.e., 70V in this case! This happens easily if you have a big ceramic capacitor on the input.

With electrolytic (high ESR) bulk capacitance in place, this peaking is easily damped, but even then, when running any part at the "absolute maximum ratings", it means that even a very slight input voltage peak may permanently damage the part the first time you apply power; or at any point in time later. You have zero margin for such inrush voltage peak!

--- End quote ---
I think it is clear I need to do same redesigne. It is clear the LM7805 will not work at 35V input

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