At 04:57 PM 9/28/01 -0500, you wrote:
>At 05:56 PM 9/28/01 -0400, Spehro Pefhany wrote:
>>At 04:48 PM 9/28/01 -0500, you wrote:
>> >At 05:26 PM 9/28/01 -0400, Herbert Graf wrote:
>> >>POP I would assume! :) TTYL
>> >
>> >I think it would be "bang", without a reversed diode across the LED.
>>
>>Could be much worse- it could seem to work until a line transient hits
>>it. Maybe I'll give it a try and report back.
>
>
>Wear your safety goggles :)

Not so bad, the LED has a reverse breakdown of something like 30V.

I use the following circuit; Ther would normally also
be a high-valued resistor in parallel with the capacitor
if nothing else is connected to the circuit (for obvious
safety reasons).

                                   Generic green LED
                                         //
L------[270R]------[1uF/250V]------x---|>|----------x
                                   |                |
                                   x---|<|----------x
                                      UF4005        |
                                                    |
N---------------------------------------------------x

This should give an average forward LED current of
22mA.

With the UF4005 (hey, I had them around) removed, the LED was
distinctly more yellow in color and dimmer. It also got
quite warm. After 10 random on/off cycles it was still working.
After 15 more it failed. Just the kind of design fault that
is most bothersome! If I'd picked a more conservative value for
the capacitor it might have worked even longer.

Best regards,


Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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