RS-232 Tx / Rx monitoring LEDs?

drlegendre . drlegendre at
Sun Aug 23 01:55:29 CDT 2015

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> 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.

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