24 Feb
2010
24 Feb
'10
5:17 p.m.
dwight elvey wrote:
(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.
From: hilpert
at cs.ubc.ca
I have on occasion run single LEDs from line voltage by paralleling the LED
with a diode, then putting the pair in series with a C and R. C is calculated
to provide most of the voltage drop via capacitive reactance, a small R is
still present to limit inrush current. The diode limits the reverse voltage and
permits bidirectional current flow so the cap sees AC.
Power consumption and heat dissipation are reduced compared to a solution using
only R, and a power resistor is not needed.
I haven't tested the circuit over many on/off cycles though, to find out
whether perhaps the peak in-rush current reduces LED life.
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.