Adam Aglionby wrote...
Clive Mitchell wrote ...
Not sure but I want one
Not sure of the chip either, but let me describe one simple way it
might work. The 15 LEDs are surely in series, powered by 10 to 20mA
and taking 25 to 45V to operate, depending on their color.
First we can eliminate a common approach: rectify the 240Vac to get
340Vdc and add a 10mA current source. This would dissipate 3 to 6
watts and can be ruled out because it's too much for a plastic DIP.
But the IC must include an internal bridge rectifier to make dc, and
that the LEDs must operate between ac cycle peaks from the charge on
the 10uF capacitor. From dV/dt = I/C we see the capacitor's voltage
will drop 20V in 10ms from a 20mA current, which isn't a very large
drop. Now imagine a current source that's allowed at least 20V of
operating overhead, this would mean we need to charge the capacitor
to 40V more than the LED operating voltage each cycle. Let's assume
white LEDs with a LED-string voltage of 45V. Each ac cycle the 10uF
capacitor would need to be charged to 85V from its low value of 65V.
You can now see how the circuit likely works: use a say 400V MOSFET
to connect the capacitor-charging circuit only long enough to get it
up to 45V higher than the LED string, 85V in this case, and charge
fast enough to insure the ac line voltage hasn't gone much above 85V
when the charging is finished and the FET is shutoff. E.g., for a
150mA charging current, it would take only 1.3ms to charge the cap.
In this fashion, most of the circuitry in the IC works with voltages
under 90V, and only the rectifier and FET portions need high-voltage
ratings. The power dissipated in the chip should also be quite low.
I imaging this IC can work with any color LED, with various numbers
of LEDs in series, and at other ac line voltages, without adjustment.
I don't know the 130k resistor's purpose, perhaps to program the LED
current?
Clive, I suggest that you carefully use a multimeter to measure the
average dc voltage across the capacitor to give us some guidance.
You can also place the meter in series with the LEDs to measure the
average current, and then change the value of the 130k resistor to
see its effect.
Thanks,
- Win
whill_at_picovolt-dot-com