I opened up an LED light bulb at my home and sketched its circuit diagram as follows: enter image description here

I don't understand what the initial 470n capacitor connected to the live wire and the final 100ohm resistor connected to the neutral wire does.

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    \$\begingroup\$ 1000 k is 1 M in grown-up units. \$\endgroup\$ Commented Dec 7, 2017 at 11:48
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    \$\begingroup\$ @OlinLathrop It would be 1Meg in SPICE; LTSPICE will interpret 1M exactly as 1m, since it is case insensitive. \$\endgroup\$ Commented Dec 7, 2017 at 12:31
  • \$\begingroup\$ @LorenzoDonati That's a "feature" of Berkeley SPICE from the mid-1970s and it still catches young players from time to time. A consequence of it being written in FORTRAN, I suppose. \$\endgroup\$ Commented Dec 7, 2017 at 13:37
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    \$\begingroup\$ @OlinLathrop Yes, I got that, but I meant to play devil's advocate for the OP: writing 1000k in SPICE is the basic workaround used by people when they realize that 1M is interpreted as 1mOhm. Discovering that it is possible to use 1Meg is an advanced topic :-) \$\endgroup\$ Commented Dec 7, 2017 at 14:01
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    \$\begingroup\$ Except, for some reason, capacitors. Manufacturers seem to be allergic to nanoFarads. You can buy a 10,000 picoFarad capacitor, and you can buy a 0.01 microFarad capacitor, but no one sells a 10 nanoFarad cap. \$\endgroup\$ Commented Dec 7, 2017 at 18:36

1 Answer 1


It's a cheapo capacitive dropper power supply.

The impedance of the cap limits the current without wasting too much power into heat. R1 discharges C1 to protect you from shock when you unplug the bulb. R2 limits inrush current.

  • \$\begingroup\$ R2 also provides a convenient way of measuring current, and relatively safe since it's on the low side, but that's probably not its main purpose. It'll limit surge current to 2.5Arms - this is probably JUST below the single cycle surge rating (Issm) of those diodes. \$\endgroup\$
    – user16324
    Commented Dec 7, 2017 at 11:05
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    \$\begingroup\$ In case you are interested, the rectifier diodes used were 1N4007 with a Non-Repetitive Peak Forward Surge Current (Ifsm) of 30A. \$\endgroup\$ Commented Dec 7, 2017 at 11:27
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    \$\begingroup\$ @BrianDrummond The low side? It's AC! \$\endgroup\$
    – Finbarr
    Commented Dec 7, 2017 at 11:33
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    \$\begingroup\$ Yes, the low side : neutral has a potential close to Earth. Note that I did NOT say the negative side. \$\endgroup\$
    – user16324
    Commented Dec 7, 2017 at 11:45
  • \$\begingroup\$ Screw on bulb usually has neutral connected to the screw and live on the tip (for fingers safety) but others like bayonet/GU10 can be put in both ways... Anyway. Just nitpicking. \$\endgroup\$
    – bobflux
    Commented Dec 7, 2017 at 11:48

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