If I used 500 ohms it'd drop 15V at 30mA - which wouldn't let me get
30mA into the LED string with low line in - and it'd dissipate 450
milliwatts when there was 30mA through it.
The trick to why it works with 100 ohms in there is that the LM317
doesn't start waking up until the voltage differential between the
input and output is a volt or so, so until then it acts pretty much
like a saturated NPN series pass transistor and the voltage across the
load will follow the input voltage minus a volt or so.
Then, as the voltage across the load rises, the current through it
will increase until the voltage dropped across R2 increases to 1.25
volts, when the LM317 will start limiting the current into the load to
30mA until the voltage across R2 falls to less than 1.25 volts later
on in that mains half-cycle.
Doing it that way only drops 3 volts across R1 and causes it to
dissipate 90mW with 30mA through it.
Also, the value of C1 was chosen to allow it to charge only to the
voltage required to get 30 mA into the string at low line, with the
LM317 doing the required limiting to keep the current in the string
relatively constant from low line to high line.
I've posted some photos to abse:
news:ng0ug7hf2jrhh49sn1gppa0betc05juu55@4ax.com
I didn't post any of the output voltages into the LED string, and I
want to get this on its way, so I'll do it later on this afternoon
when I have more time.
>Also, I'm a wimp where it comes to running LEDs near their ratings,
>so I'd use 51 ohms or more for R2 to keep the current lower than
>30 mA.