24 Feb
2010
24 Feb
'10
10:01 a.m.
From: hilpert at cs.ubc.ca
---snip---
The diode across the LED serves two purposes. There is that that you mentioned
about the capacitor and the other is to keep the LED from burning up.
When the voltage reverses on the LED, it is back biased. The LED
zeners at about 10 to 12 volts someplace with almost the same current
as it had when forward biased. A simple I*E calculations shows that
in this mode it is using about 10 time the power as forward biased.
When I first did this I forgot the diode and noticed that the LED
was glowing orange. Checking with my finger confirmed that it was
quite hot.
(Yes, I mentioned both purposes: AC current flow and limiting the reverse voltage.)
I had a similar experience as you when first experimenting with this with just
a simple R. I wondered about Vr for the LED but didn't find it specified
anywhere in databooks, so I found out what happens as you did. I didn't measure
the breakdown V though, interesting you found it as you specify.