I've also seen C-R series voltage dropping circuits, here & there. Correct me if I'm wrong, but doesn't the series cap dissipate power just as it would, were it a series resistor? I mean, if the LED is passing 20mA, the cap is also doing 20mA - and at whatever the Vdrop is.
Right? If not, why? On Sun, Aug 23, 2015 at 1:17 AM, Chuck Guzis <ccl...@sydex.com> wrote: > 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. >
