Re: multi white led circuit

Sorry,
The leds are 5mm 3.7V 27k mcp, 20 milliamp(prefered)
Would like to use 900
ehsjr wrote:
petrus bitbyter wrote:
"ehsjr" <ehsjr@xxxxxxxxxxxxxxxx> schreef in bericht
news:ThPig.1651$oE3.767@xxxxxxxxxxx

redbelly wrote:

ehsjr wrote:



1N4004 Led1 LED20
Ac ---+---[R]---|<----|<--- ... ---|<---+
| |
+---[R]--->|---->|--- ... --->|---+
1N4004 Led1 Led20 |
Ac -------------------------------------+

16 strings of the above. Resistors will
be 120 - Vf*20/I ohms.


Ed, should that be 120 or 170 (is it rms or amplitude)?

Mark


Hi Mark,

You raise an excellent point. He needs to recognize
that the max current rating for his LEDs is 30 mA DC,
which converts to 21.21 RMS. Using 120 is fine in
the computation, as long as he recognizes that.
Otherwise, using 170 as you suggest is the way to go:
R = (170 - Vf*20)/I

Ed





Hmm... Did Jay buy the same LEDs James bought? Otherwise we still don't know
a thing about the LEDs. Nevertheless, most LEDs have different values for
continuous current, average current and peak current. Both last values have
to be taken into account when using rectified, unsmouthed sine.

petrus bitbyter



Right - we know nothing about the LEDs, other than they
are white. The formula using Vf and I rather than specific
figures will work, but you do have to pay attention
to the units you are using if you want to get things exact.
It is simpler, however, to use the continuous max as the peak
limit + or - a few percent. In fact, unless there is some
good specific reason to the contrary, it is better to drive
LEDs well below max, in my view.

But this is going away from the op's question and the
answer, which is predicated on "if you are bound and
determined to do it" where "it" was running a string
directly off AC with no diodes and a cap and resistor
to limit current. The alternative you posted is going
to save his LEDs from excessive reverse voltage. If
he gets sick enough of the flicker, he'll be willing
to take additional steps. Note that despite your first
reply - which was the first answer he got - he did not
post specs on his LEDs. And he still hasn't, despite
your second reply to him. Sometimes, you can't help
people. If he can't or won't post the specs for his LEDs,
peak, average, continuous are all irrelevant to him,
as is a formula that uses any of those specs. The point
I wanted to make to the op was the relative cost/benefit
of using resistor vs capacitor and resistor.

Ed

.

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