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.
