I have an LED light bulb and dismantled it to look at the circuit inside.

I made some google search for pcb but couldn't find any relevant info about FST-855 or FST-854: 10 LEDs in seriesPower circuit

After drawing the schematic, I could not figure out what is the purpose of some of the elements. Here is the schematic (sorry for hand drawing): schematic of led bulb

I was expecting to see the electrolytic cap however did not understand the reason for film cap. Can you tell me why there is this "capacitor in parallel with a resistor" in between Live and full wave rectifier?

  • 5
    \$\begingroup\$ A helpful search term: "capacitive dropper" \$\endgroup\$
    – Hearth
    Jan 24, 2019 at 0:50

1 Answer 1


Welcome to Electrical Engineering!

The point of the capacitor is to limit the current through the full bridge rectifier. According to the “824J400V” indication, it is a 820nF capacitor. Its impedance at 50Hz is 3882Ω. It will limit the current to less than 62mA at 240V.

The point of the resistor in parallel is to discharge the capacitor when the bulb is disconnected.

  • \$\begingroup\$ Does that "J" has something to do with the square root of -1 due to some AC property of capacitors? Is that for Joules? How can we infer 820nF from 824J? \$\endgroup\$ Jan 24, 2019 at 14:53
  • \$\begingroup\$ @user458835 824J consists of value and tolerance. Value is 824 (write 4 zeroes following 82. The right-most number always indicates the number of zeroes. The result in picoFarads. So, 820000 picoFarads equal to 820 nanoFarads). Finally J indicates the tolerance: F: 1%, J: 5%, K: 10%, M: 20%. \$\endgroup\$ Jan 24, 2019 at 15:30
  • \$\begingroup\$ @RohatKılıç Thanks for the explanation, I did not know the meaning of the J letter… \$\endgroup\$ Jan 24, 2019 at 17:43
  • 1
    \$\begingroup\$ Series capacitor on live AC is also used on transformerless power supplies. You may find a voltage divider comprised of both resistor and capacitor in order to reduce the voltage without consuming power on the capacitor. \$\endgroup\$ Jan 26, 2019 at 2:27

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