23 Aug
2015
23 Aug
'15
5:23 a.m.
From: cclist at sydex.com
On 08/21/2015 08:36 AM, dwight wrote:
I would think the reverse voltage sum of the diodes is enough.
Different diodes also can handle different voltages. Since the sum
of the forward voltages is enough to handle AC, I'd suspect the
reverse voltages each would handle is quite small as well.
The problem is when the current limiting is done with a resistor
that in the forward direction drops a lot of voltage.
The diode has to handle the voltage until breakdown when reversed.
If the resistor was handling 1 Watts, with the right break down,
the LED could be taking .5 Watts. This is more than most are designed
for.
Dwight
I was going to add something but it has already
been said several
times. I will add that if using a LED on an AC like signal of high
voltage, one should use a diode. I recommend using a shunt diode
rather than a series diode when high voltages are being dropped by
the resistor. It reduces the need for a high voltage diode but makes
the resistor hotter. Some red LEDs glow orange when not protected
from 12VAC. You can ask how I know. Dwight
Ever take a close look at a string of Christmas-tree LEDs? Most are
composed of a string of LEDs hooked directly across the AC line--no
rectifier diode to be found.
Some seek to reduce the 60Hz flicker by employing two strings to
illuminate on both half-cycles, reducing the flicker somewhat. Since
my eyes react to the flicker (it's like ants crawling over the string),
I found that simply employing a full-wave bridge rectifier can reduce
the appearance of flicker tremendously.
--Chuck