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The above diagram shows how I'm about the connect the LEDs, info regarding the drop forward voltage been taken from: Calculating forward voltage for a string of fairy lights: multimeter shows "1"

I'm about the connect the following power source: https://www.aliexpress.com/item/MS-50-5-50W-5V-10A-Mini-size-LED-Switching-Power-Supply-Transformer-110V-220V-AC/1323532404.html?spm=a2g0s.9042311.0.0.27424c4dHlPdnQ

My question is, should Vin be 5V for the above formulas? If so, how is the 10A of the power source in taken in consideration with the above formulas?

Please let me know if I missed anything.

Update after Passerby corrections!

So as PasserBy mention these leds are connected in parallel uploaded the updated diagrams and added a 3V DC Motor PWM to fine tune the led light.

Dose it make more sense now? open for any additional suggestions and improvement

Clarification for the motor has input -+ and output -+ as agate to the led wire


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  • \$\begingroup\$ unless you link datasheets your Vf could be wrong as it depends on power rating due to internal resistance. White uses a Blue LED so if same size has same Vf nom. R/Y are close to same unless different materials \$\endgroup\$ Sep 18, 2018 at 19:33
  • \$\begingroup\$ That answer doesn't address the full led string issue, just how to measure a single led. I answered on how to get the full info needed for the entire string there. \$\endgroup\$
    – Passerby
    Sep 18, 2018 at 19:49
  • \$\begingroup\$ @TonyEErocketscientist a phosphor pushed white uses a 450 nm deep blue. A blue LED is usually around 470 nm, and usually has a higher Vf than the deep blue if both use the same GaInN process. That does not seem to be the case here. \$\endgroup\$ Sep 18, 2018 at 21:29
  • \$\begingroup\$ I'd suggest you arrange your LED strings so that you only have to use a safe voltage if this is something you are building for yourself. Setting a maximum voltage of say 48VDC would result in string of no more than 20 LEDs per string. \$\endgroup\$ Sep 18, 2018 at 21:46

3 Answers 3


Fairy lights like the one in the question you link to are not series leds. They are all in parallel. A 5V supply for multiple series leds would not work. I answer how to get the forward voltage and current in that other question.

Since your doing the math, simply adjust each string to reflect parallel leds. This is assuming you are replacing the resistor in the fairy light string, and let's assume 60 leds, at 3.2v at 10mA instead of 3.6v at 20mA, for longer life. You can adjust as you'd like, or if you measure the actual numbers.

R = ( V source - V forward ) / ( If * N leds )
R = (5 - 3.2 ) / (.01 * 60)
R = 1.8 / 0.6
R = 3 Ohms

This is per led string.

Don't forget to get the right power.

P = V * I
P = 1.8 * 0.6
P = 1.08 Watts

A 2 watt resistor would be good for these values.

The 10 Amps are the max you should draw from your supply, so you take your actual total current draw, and subtract that from 10 Amps. If it's less, you're good.

  • \$\begingroup\$ Hi, i think i'm missing something: Red color - (5-1.5)/(.01*200) R= 1.75 P=7 ? 7 watts for lighting it up? \$\endgroup\$ Sep 19, 2018 at 13:37
  • \$\begingroup\$ Red leds with a low forward voltage are not efficiently driven with 5V, unless you put 2 or 3 in series. So that would be correct. It really depends on the actual values of your led string. \$\endgroup\$
    – Passerby
    Sep 19, 2018 at 14:07
  • 1
    \$\begingroup\$ Got it, will measure the forward voltage for each one of them to be on the safe side, will post the final solution for approval :) \$\endgroup\$ Sep 19, 2018 at 14:09
  • \$\begingroup\$ Hello there! I updated the post would love to get your comments, Thank you! \$\endgroup\$ Feb 22, 2019 at 11:58

My question is, should Vin be 5V for the above formulas?

Yes, because that is your source voltage.

If so, how is the 10A of the power source in taken in consideration with the above formulas?

10 amps is the maximum current your source can provide. Add up the currents in all your branch circuits and it must be less than 10 amps.

  • \$\begingroup\$ It appears to me there is 3 strings in parallel. In the equations Vin - (Vf x # LEDs) you get a negative number. Wouldn't Vin need to be like 500V at least for R3? \$\endgroup\$ Sep 18, 2018 at 19:36
  • \$\begingroup\$ Yes, I ignored the XYZ because it makes the problem rather ridiculous. \$\endgroup\$
    – mike65535
    Sep 18, 2018 at 23:28
  • \$\begingroup\$ But that's his problem. He as 3 sets of LEDs that need to be powered somehow. He has 3 sets with 200, 60, and 20 LEDs each. All LEDs are in each set are wired in parallel. The OP wants this doene with a single power supply. His formulas will not work, When you said yes to 5V, look at the formula's result. A negative value. Yes the formulas will use a 5V in but a different formula is needed. So, is Yes still the correct answer? \$\endgroup\$ Sep 18, 2018 at 23:42

Trying to power this number of LEDs in a parallel 5V arrangement is just inane.
Trying to power strings such as you state in the question x,y,z is just plain dangerous.

I would strongly suggest you choose a safe (for you) working voltage (perhaps 48V) and arrange for constant current drive to individual strings of LEDs.

For example, you could use a Microchip CL520 to drive the strings. This allows up to 90V so easily copes with 48V. The chip has thermal shutdown and a fixed 20mA string current. You could put all the current generators at the power supply and simply run the LED wires from there.

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  • \$\begingroup\$ Oh, I see, I interpreted the question as how to calculate resistor values. But let's think about this. The strings of Fairy lights come wired in parallel. So are we not stuck with a low voltage source? I'm going to have to go with this is not a viable solution, \$\endgroup\$ Sep 18, 2018 at 23:28
  • \$\begingroup\$ I see you edited your answer. How are you going to get strings that are parallel wired LEDs to be wired in series? Gong to take more time than calculating R values. But I have concluded this whole thing is wrong. The stings of lights are voltage powered where each LED has its own resistor built in. Not every LED need 20 mA. A whole string may only need 20 mA. It is recommended they be powered with a USB port. \$\endgroup\$ Sep 18, 2018 at 23:53
  • \$\begingroup\$ If the strings of LEDs the OP has are already wired in parallel then they can be considered a single LED at a higher current. You could still connect multiple in series and just create a CC driver with higher current. I can diagram that if you want \$\endgroup\$ Sep 19, 2018 at 0:17

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