23 Aug
2015
23 Aug
'15
6:55 a.m.
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 <cclist at 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.