I have an unknown LED, I think it was previously run off of ~14vDC.

I want to repower it - I have a 5vDC 3A power supply.

I dont know the LED rating so I've rigged it up to a simple series circuit.

6v cell >>>> 1k R >>>> LED >>>> cell

This gave 5.95v at the cell, and 5.55v across the LED suggesting a 6A LED forward current.

6v cell >>>> 10k R >>>> LED >>>> cell

I retested again with a 10k R which gave 3.77v across the LED again implying a 6A LED forward current.

My questions:

1) does this make sense / is this right? Most LEDs I see have forward currents in mA not A.

2) can I run this 6A LED off my 5v3A supply long term without damage / excessive load?

3) what would be the best series resistor value?

Unfortunately changing the LED is not simple.


  • 1
    \$\begingroup\$ There's a big error in your maths somewhere. 5.95 - 5.55 = 0.4 V across your 1k resistor giving a current of \$ I = \frac {V}{R} = \frac {0.4}{1000} = 0.4\ \text {mA} \$. You figured 6 A! \$\endgroup\$ – Transistor Dec 23 '18 at 13:38
  • \$\begingroup\$ Yes just realised this. Using phone calculator and missed out brackets!! \$\endgroup\$ – user32407 Dec 23 '18 at 13:57
  • \$\begingroup\$ You still haven't fixed your question a day later. \$\endgroup\$ – Transistor Dec 24 '18 at 18:35

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