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I have this electrical system: a switch connected to a 120V 60Hz 21W light bulb. I wanted to add two outlets one for a LED, another one for something else like a laptop or a fan.

system

I tried to plug in series like this:

will

bulb

But only the LED comes on.

problem

I tried with a 15W bulb and now both are working. Why is that?

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    \$\begingroup\$ In series? Why? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 16, 2015 at 22:20
  • \$\begingroup\$ That's the whole point. I want it to work in series. \$\endgroup\$
    – Francis
    Jun 17, 2015 at 0:18
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    \$\begingroup\$ Unfortunately that's not how line-powered devices work. What is your goal? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 17, 2015 at 0:54
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    \$\begingroup\$ This is probably a rude comment, but I would strongly suggest that you not just randomly experiment with line voltages. And as to why it "Works" with an incandescent bulb in series with the LED, is the incandescent bulb is just a resistor. The CFL has circuitry that needs the full 115VAC to power up. Series for AC isn't normal. \$\endgroup\$
    – R Drast
    Jun 17, 2015 at 12:49
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    \$\begingroup\$ You need to connect the loads in parallel - the Line (hot) wire connects to the Line side of all loads, and the Neutral wire connects to the Neutral side of all loads, so all loads see the full line voltage, regardless of the other loads. \$\endgroup\$
    – Peter Bennett
    Jun 17, 2015 at 17:43

2 Answers 2

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My guess is

The LED lamp is using a switch-mode power supply, this means it will be regularly turning itself on (and allowing power through) for the fraction of a second needed to to supply the voltage and current the LED needs.

This tiny amount of time and power is going to be virtually useless to an incandescent or florescent and so they'll remain unlit

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As Peter Bennett said,to make the 21watt bulb and LED activate,you have to connect them in paralell.This is because by connecting them in series you increase the total value of impedance,thus lowering current and power.Now the obtained power was too low for the first bulb and enough for the LED.The 15 watt one had enough and turned on togheter with the LED.

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