Each LED will flicker at half the supply frequency, but each will emit light
on opposite halves of the AC waveform. Thus, if the two LEDs are physically
close together, the flicker from each will be mitigated by the other.
Arfa
This whole argument is silly but, pardon me, that is NOT what you said Arfa
(thus my reason for jumping in) and you appear to be combining two distinct
posts.
Message-ID: said:
That said, if two LEDs are used in inverse parallel as you suggested, rather
than paralleling with a conventional diode or just relying on the reverse
blocking of the LED, flicker is not an issue, as each LED will produce light
on opposite half cycles.
If this (back-to-back) is done, the "flash" frequency (periodic rate if you
prefer) will be 100 times / sec. Correct? Two flashes per complete sine?
Still only 1 flash per each diode per full cycle of course but most certainly
flash pulsing will not be halved under any conditions. Doubled in fact.
And just for kickers, the forward conduction breakdown from each diode, in
turn, ought to be lower than the reverse destruction breakdown voltage, in my
experience anyway. Therefore a single dropping resistor would be effective.
But at a later post said:
Which is exactly what I said. If running them on AC, they are fine as long
as the series resistor is of sufficient value to limit the forward current,
and the reverse voltage on the opposite half cycle does not exceed the
rather low PIV that LEDs tend to have.
So far so good
That aspect can be mitigated by
having a conventional diode in inverse parallel, but this will result in a
flicker at half the supply frequency that Phil can't see, but some of us
can, or by putting another LED in inverse parallel, which will conduct on
the opposite half cycle producing light, and clamping the reverse voltage
across the first diode to the value of the forward voltage of the second
diode, different, of course for different colours.
Now what do you mean by that? Piggying a standard .6 V drop diode across
a dual, reversed set? That would circumvent 1 of the diodes getting a
sufficiently high forward voltage bias. In that case, we are back to half-wave
operation at the line frequency (whatever that may be). BUT, it (the flashes)
will never be at half the line frequency (which Phil defines as the repetition
rate).
OR do you mean having ...
Just a single LED with conduction in a particular direction but which is being
shunted by a reverse diode so that the LED's being protected from reverse
over-voltage? If so, then the same conditions as just outlined, still apply.
But, whichever viewpoint you deem more accurate/intended, indeed even if you
reject both, I repeat the section of your post for emphasis ....
That aspect can be mitigated by
having a conventional diode in inverse parallel, but this will result in a
flicker at half the supply frequency that Phil can't see, but some of us
^^^^^^^^^^^^^^^
No sir. Never *half* the supply frequency. That would mean 25 or 30 pulses/sec
for standard 50~ or 60~ supplies. Power might well not be consumed for the
full input cycle - a fact which we acknowledge by using the term 'duty factor'
or 'duty cycle'. Irrelevant what Phil sees -)
Reducing to absurdity: A simple transformer fed from a 50~ source with a
secondary output sufficient to produce forward conduction will produce 50
flashes/sec. Not half. Not 25. Now, if we'd full-wave rectified, we'd get 100
FPS from a single diode.
I'm outa here.
