22 Aug
2015
22 Aug
'15
11:17 p.m.
On 08/22/2015 10:23 PM, 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.
...and that's just the nub of it. The success of this depends largely
on the consistent characteristics of every LED in the string. Since
LEDs tend to fail short if submitted to overvoltage, I've often wondered
if a spike in the AC supply would precipitate a cascade failure in the
string. I've looked hard and there are no rectifier diodes in the
string--just the LEDs themselves. Probably saves about 5 cents or so of
manufacturing cost.
I've also seen LED "night lights" from China that employ nothing more
than a safety capacitor (usually about 104) in series with a resistor
connected to two back-to-back LEDs, all across the AC line.
I've wondered what the lifetime of such a setup is.
--Chuck.