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I want to make a lamp with LEDs, those have to be powered from a socket (230V). I have a cord with a switch and two wires (ground, voltage), which supports up to 1300 W. The switch for turning it on/off.

I want to use this cable with switch and the socket to power 3-6 LEDs (I think serial,) but what kind of resistors do I need, if this all is possible?

(I'm kind of new to this) I read that LEDs have "voltage drop" and "thus changing the amount of current running through it" so V= IR doesn't apply.

With LEDs, I refer to Leds

  1. Is this somehow a good idea (safe, doesn't catch fire, etc.)
  2. What kind of resistors do I need
  3. Taking a 9V battery how do I calculate the voltage drop and the resistance needed?
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    \$\begingroup\$ NOT a good idea from 230V. \$\endgroup\$
    – user16324
    Commented Jun 29, 2020 at 17:54
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    \$\begingroup\$ Just put an LED light bulb for 230V in the socket and be done with it. \$\endgroup\$
    – JRE
    Commented Jun 29, 2020 at 18:08
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    \$\begingroup\$ You really, really, really do NOT want to use your mains lamp cord and switch system to run an LED. You could do this with an NE-2 bulb. But not an LED. Don't do it. As far as trying to understand what kind of resistor to use with 9 V, you should experiment. We could write equations and wave our hands around, but the best way to start is to just "do something" and see. Get a \$1\:\text{k}\Omega\$ resistor, to start, and use that in a series loop with one LED and your 9 V. Then change the resistor to smaller or larger values, by increments, and see. Measure and report back and we'll help. \$\endgroup\$
    – jonk
    Commented Jun 29, 2020 at 18:25
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    \$\begingroup\$ The link that you provided leads to a page that has the price and description. Next to the "Description" tab there is a "Features" tab and a "Document" tab. Click the features tab and read what is there. Then click the "Documents" tab where you will find a "Datasheet" linked an "LED Tutorial" link. Click those links and read the material that you find there. \$\endgroup\$
    – user80875
    Commented Jun 29, 2020 at 18:32
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    \$\begingroup\$ have to be powered from a socket (230V) and asking lots of very basic questions means that you should not be doing this. For anything at 230 V you must know how it is done and what the risks are. First experiment with LEDs and batteries and educate yourself. Is this somehow a good idea No. \$\endgroup\$
    – Bimpelrekkie
    Commented Jun 29, 2020 at 19:11

1 Answer 1

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Forward voltage drops for LEDs range from about 1.7 v (red) to 3.0 v (blue). If you plan on arranging the LEDs in series then you will need one resister calculated as follows. Assume you will power the LEDs with 9 volts of DC power. Then the voltage drop from 3 red LEDs would be 3 X 1.7 volts = 5.1 v. If you are powering this from, say a 9 volt battery, you will have 9 - 5.1 = 3.9 v that would need to be dissipated by resistance. Generally you should supply no more than about 20 to 25 mA of current to the LEDs. Anything above 30 mA will likely burn them out. Therefore the resister you will need above (assuming 3 red LEDs and 20 mA current with a 9 volt battery) would be R = V/i = 3.9/20 mA = 195 ohms. SO, in general for a series circuit of n (red) LEDs, V volt battery the resister calculation would be (V - 1.7n)/2010^-3. (20 mA is 20^10^-3 A). If you have a blue LED substitute about 3.0 for 1.7 in the above calculation.

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  • \$\begingroup\$ You can find the actual voltage drop for your LED from it's spec(data) sheet. It will be listed as V_f. The datasheet will also tell you the forward current rating (I_f). These are the values to use in the calculations that @boo gave you. If you buy your LEDs from someplace like Mouser/Digikey/Farnell, they should have links to the datasheets right on the ordering page for the part. \$\endgroup\$
    – BrianB
    Commented Jun 29, 2020 at 21:14
  • \$\begingroup\$ Thanks for your answer. As I stated above I will try to use 9V and start from there. I will study your calculations and use them with the answer of @BrianB and see how far I can get. \$\endgroup\$
    – Helper
    Commented Jun 30, 2020 at 12:38

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