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recently I've been wondering how so many leds can be powered by a single supply.

For example, my christmas lights have 1000 leds on them which are powered from the mains. They definatly arn't connected in series which would require ~3.3KV, so how are they powered if each one doesn't have any sort of resistor on them?

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If we take a typical string such as this one made by Ningbo Laihe Christmas Gifts Co.,Ltd, the power supply is 30V (for isolation and so it can use low voltage wire). The total power is only 9W. That's about the power used by just two C6 15V incandescent lamps (series string) that were common when I was a child.

The 1000 lamps could be arranged into series strings of 8 LEDs with a single resistor (perhaps molded into the base or cord) in series with each string. There could be a resistor in series with each lamp of 1/8 the value, but that would be wasteful.

Then put 125 of those series strings in parallel to operate from the 30V source. The 30V would be produced by a switching power supply adapter similar to a phone charger or other modern mains-operated switching supply.

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  • \$\begingroup\$ This would make sense also as I forgot to mention they are in a pattern of red green blue orange, then repeats. Thanks! \$\endgroup\$
    – user248702
    Dec 11 '20 at 20:09
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    \$\begingroup\$ If the bulbs are removable, pull out one and see how many lights go dark. This will tell you what the series/parallel combination is. \$\endgroup\$
    – Mattman944
    Dec 12 '20 at 2:56
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They're all connected in parallel, and need a low voltage high current supply. Each LED gets a ground and a voltage wire. If they are individually addressable (as some LED Strips are), each LED also gets an additional Data wire, which is passed on to the next LED in the series. Using this method we can control any number of LEDs using only 3 wires, with a low voltage, the only limiting factor being the current.

Also LEDs do not necessarily require 3.3V to operate, many operate on far lower voltages, and some have current limiting resistors built into them.

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  • \$\begingroup\$ I believe the leds are connected in parallel, but do not appear to have a built in resistor. They are not addressable. If they were connected in parallel without any resistor, this would cause uneven currents \$\endgroup\$
    – user248702
    Dec 11 '20 at 20:08
  • \$\begingroup\$ The power supply has a single resistor in series that controls the current through all of them at once. The leds do not require a resistor each. Because the LEDs were built by the same manufacturer, their effective resistance will be the same, and the current in parallel will also be the same. \$\endgroup\$
    – Adi
    Dec 11 '20 at 20:14
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    \$\begingroup\$ It seems extremely unlikely to me that all of the LEDs are connected in parallel. As you said, a high-current power supply would be needed but it is also true that heavy wiring would be needed to pass the required current without a voltage drop. I've never seen any decorative LED lights with 14 gauge wiring. \$\endgroup\$ Dec 11 '20 at 20:53
  • \$\begingroup\$ @Aditya Chavan I've read somewhere that leds are not perfect and despite being made by the same manufacturer, a 1% difference to their resistances can cause a 30% change in current flow. So one led (or a few) will 'hog' the current \$\endgroup\$
    – user248702
    Dec 12 '20 at 7:47

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