This may be a more interesting question than it first appears.

If we assume the wall wart is simply a transformer, then the purpose of the=
 diode is obvious, but why would you use an AC output transformer in the fi=
rst place, if you're worried about hum?  Hum is literally all they produce!

If we assume the wall wart is at least rectified, then what exactly IS the =
diode doing?  The wall wart isn't going to be forcing its output to ground,=
 discharging the caps in the "hum buster".

I'd never heard of a "Hum buster" either.   What I see here is an attempt t=
o improve the filtering on the output of an unregulated supply, but I am ac=
tually wondering what the diode was intended to accomplish.

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 mail@crcomp.net
Sent: Wednesday, April 12, 2017 7:57 AM
To: piclist@mit.edu
Subject: [EE] Why the diode in a wall wart hum buster?


Here's what the reverse engineered circuit looks like:

X----->|--------o------------o--------------o  Vcc
    1N4002      |            |
                |            |
            +   |            |
        470 uF ###   0.1 uF ---
         25 V  ---          ---
                |            |
                |            |
X---------------o------------o--------------o  Gnd  =20

The wall wart connects to the X terminations on the left. The caps are comm=
on and their purpose is understood. What purpose does the diode serve?

Thank you,

--
Don Kuenz KB7RPU

Genius is nothing but a greater aptitude for patience. - de Buffon

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