[geeks] Voltage regulators don't!

[der Mouse]

>> Next up, I'll try adding some ceramic disc caps here and there
> This is what I sugest.  The big electrolytic filter capacitor can't
> filter high frequencys.

True - but where is the HF coming from?  The misbehaviour persists even
after I unplug the mains side of the transformer, and thus it's running
off true DC from the cap (30000uF holds enough power to run a 7805,
even into a 150 ohm load, for a substantial time, at least several
seconds, before falling below the dropout voltage of the 7805).  That's
why I was hesitant to suspect HF.

But it seems my hesitancy was wrong; it turns out that a .01uF ceramic
disc cap across the input settles it down very nicely.  Since my own
body capacitance was enough, I suspect I could get away with much lower
capacitance; .01uF is simply what I had at ready hand.

Perhaps odder is that some of my 7805s don't need anything of the sort.

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