In article , Rick Hughes
writes
On 04/02/2014 20:55, fred wrote:
In article , Rick Hughes
writes
On 03/02/2014 23:39, fred wrote:

Actually no, the values I recommended are those given in my second post
in this thread[1] but you have chosen to ignore them.

No ... not intending to ignore value, apologies if you think that.. but
you posted for 7.5mA which would seem too low, I want to get closer to
20mA nominal for LED ........ but accept your warning over 0.47uF
capacitor being too big.

Ok, fair enough.

The general point I was aiming for was that you want to minimise both
the operating and transient current in the droppers where you have
fragile elements (the LEDs and to a lesser extent, the resistors).

Without any real data available for the LEDs I made some assumptions
(perhaps conservative ones) that 100mA repetitive peak would be a safe
max current and that was where my 4k7 pulse/inrush/transient resistor
came from (actually gives 75mA pulse limit) and that is what I think is
a safe value so as not to pop LEDs or overstress the resistor under
maximum switch-on pulse. This is the same for all LED operating currents
(cap sizes) but you can't use 4k7 at operating currents above 7.5mA
(rms) as the resistor will fry.

Reducing the transient limiting resistor will increase the risk of
popping the LEDs so that is why I am not happy about it.

Capitol has suggested 300mA might be a safe peak limit and that may be
closer to the mark but we just don't know.

Trying 300mA peak pulse gives 1k1 as a resistor value so 1k0 may be
something you want to try. The 20mA you mention with 220n is actually
closer to 16.5mA (rms) which will give 272mW in a half watt resistor
which is safe. The peak power at switch on is a hefty 90W which is a lot
or a half watt resistor to take so the res may pop as well as the LED
(or neither . . . ).

I had assumed in your post when you wrote:
"if you go for 20mA then you will limit your pulse resistor size to
about a quarter of the sizes I suggested (power = I sqrd x R), you will
be limited to 1k "

Assumed that this meant use 1K as the current limiting resistor .... if
this is incorrect, what value should be used with a 0.22uF capacitor ?

What I meant was 1k0 is the max you can use (for resistor power
dissipation reasons) when running at 20mA (16.5mA actual) and I didn't
think that would be enough to avoid popping LEDs but if you want to give
it a go then it may work (no guarantees).

I will also try the 100nF & 4k7 option you give .... so I can test both

Do try for the lowest current you can.

Designing this properly is tricky, if I was doing a product design with
something like this in I'd be choosing my LEDs carefully from low
current, high efficiency but pulse capable ranges and operating it at as
low a current as possible, 2-5mA max, to reduce component stress.

--
fred
it's a ba-na-na . . . .