David asked in Science & MathematicsEngineering · 2 months ago

# How can these led Christmas lights be 3V or 2.5V bulbs but its connected to 120V supply with no transformer? Why doesn't it blow them out?

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• Steven
Lv 7
2 months ago

Multiple LEDs are wired in series and require a current limiting device for that string, which may be just a resistor.  Some LED lamps use a switching low voltage power supply. But it is important to understand that LEDs do not have resistance in the normal sense. They have a "forward voltage" below which they conduct nothing and above which they conduct an unlimited current and burn out, so the current must be controlled by a resistor or controller chip. LEDs also require a rectifier because their reverse voltage capability is not enough by themselves.

• David1 month agoReport

I get it when things are parallel like landscape light 12v system, you have bulbs designed for 12v and just need enough amps from the supply for how many are added.   Take that 12v on 120v and it blows.  So these 3v Christmas light bulbs, there is still 120v across the leads so why don't they blow?

• roger
Lv 7
2 months ago

Because there are  40 of them  connected in  series.    They  drop   3  volts each.  there is also a   limiting  resistor in the string.

• 2 months ago

Each LED is 3V@~10mA= 300Ω.  120V/3v= ~40 LED's in the string of lights.

• M.
Lv 7
2 months ago

Series string to divide the voltage.

• 2 months ago

Because 40 - 50 of them are connected in series.  If you have 40 bulbs rated at 3 volts each in series, the applied voltage would need to be 40 * 3 = 120 V.

• ?
Lv 7
2 months ago

120v is the maximum available voltage. it doesn't mean that's what everything attached to it draws. Your house is connected to a water supply system that moves thousands of gallons an hour, that doesnt mean it all tries to come out your kitchen tap when you turn it on.

• oyubir
Lv 6
2 months agoReport

That answer would have been ok if the question was about current (the classical "I have a 4A 12V power supply, can I use it on a 12V 1A device without blowing it"). But is the worst possible answer for this question. Your answer describes exactly how it does NOT work.