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Iv a set of fairy lights, Rated Voltage 3v, 15mA, I would like to connect four sets together in series (12v 60ma).I can get uk ac/dc 12V 1A Power. Question1 : Do i need to connect a resistor if so what one. Question2 :Is 12 volts enough. Any advice would be appreciated

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  • \$\begingroup\$ Your question is difficult to read, and I can't tell what you're trying to ask. Clean it up a little, make it clear what you want to know. \$\endgroup\$ – Hearth Apr 28 '17 at 22:17
  • \$\begingroup\$ Felthry, What im trying to say is, can i just wire four sets of fairy lights in series together using ac as the power source, useing the above requirements .Or is there any thing i need to add for it to work \$\endgroup\$ – Main house Apr 28 '17 at 23:20
  • \$\begingroup\$ Stick a current limit device of some sort in there and it should work, yes. You can get LED drivers on Digi-key or Mouser. \$\endgroup\$ – Hearth Apr 28 '17 at 23:22
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Four 3 Volt LEDs connected in series will work diretly from your 12 Volt supply, with no extra components. One such group will draw 15 mA.

You can connect up to 66 such sets in parallel to the same 12 Volt 1 Amp power supply.

Correction: I must have been thinking of incandescent lamps when I wrote the first sentence.

LEDs must always be used with a current-limiting resistor, so you want a group of three LEDs, plus a 200 Ohm resistor to operate from 12 Volts. One group will draw 15 mA. (second sentence is OK)

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  • \$\begingroup\$ Downvoted. Connecting LEDs to a voltage source without a current-limiting resistor is always, always, always a bad idea. Even when it works - that just means you got lucky. \$\endgroup\$ – WhatRoughBeast Apr 28 '17 at 23:12
  • \$\begingroup\$ ARGHHH! I suspect I was thinking of small incandescent lamps - I'll correct it... \$\endgroup\$ – Peter Bennett Apr 28 '17 at 23:32
  • \$\begingroup\$ WhatRoughBeast what would u recommend would a 200Ohms resistor work \$\endgroup\$ – Main house Apr 28 '17 at 23:34
  • \$\begingroup\$ @WhatRoughBeast that only applies to bare LED components, and not even all of them. OP is hacking a consumer product, which must already have that resistor or current limiter. They don't need additional redundant resistors. In fact I've been running two cheapie 12V LED strips in series on 24V continuously for 5 years! (Got a bargain on a top brand power supply). Rushing to say "always" and rushing to downvote is usually a bad idea :) \$\endgroup\$ – Harper - Reinstate Monica Apr 29 '17 at 1:35
  • \$\begingroup\$ @PeterBennett - OK. I've reversed my downvote. Good answer. \$\endgroup\$ – WhatRoughBeast Apr 29 '17 at 4:15
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Your fairy lights are likely 2 AA battery powered string of parallel leds and a single resistor. Super common.

You could, in theory, wire four of these in series, including the resistors, and it should work just fine. You would be better off wiring them in parallel, and use a USB power supply with 3x 1n4001 diodes in series to drop 2.1V from the 5V supply.

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If they are LEDs, as they very likely are, then no 12 Volts would not be enough. But it is not likely the set of four LEDs or your supply will be exactly 12v.

You would have to first measure the LED string's forward voltage and find out if they are over or under your supply voltage.

The supply voltage must be higher (not equal) than the LED string's forward voltage.

Once you know the voltages you can calculate the resistor value or use a 50 cent CCR (constant current regulator) like the On-Semi NSV45015WT1G.

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