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I am learning a simple LED and resistor circuit. I have the following components:

  • One 5mm blue LED
  • 5V USB power source

From what I've learned so far, I am using the following forward voltage 3V and 20mA of the blue LED. Therefore, the R = (5-3)/0.02 = 100ohm. I think this is the correct calculation. I am also able to put them all together and light up the LED without issue.

However when I use the multimeter to verify my calculation, both current and the voltage reading are completely different from my calculation. From the multimeter reading, here is the schematic and the measurements.

schematic

simulate this circuit – Schematic created using CircuitLab

Can someone point me out what have I done wrong and point me to the right way?

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    \$\begingroup\$ that is not the circuit that you used ... please verify the LED orientation \$\endgroup\$ Commented Dec 14, 2020 at 0:37
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    \$\begingroup\$ @jsotola yes either the battery or the LED must be flipped. \$\endgroup\$ Commented Dec 14, 2020 at 0:43
  • \$\begingroup\$ Did you get the forward voltage specification of 3V at 20ma from a datasheet? If so did it quote a typical and maximum forward voltage. There is usually a range in production tolerance due to manufacturing process variations. You might get a slightly different result with another Blue LED from a different batch. Obviously reality trumps simulation every time! \$\endgroup\$ Commented Dec 14, 2020 at 1:11
  • \$\begingroup\$ Sorry for the wrong orientations. Just fix them. Still have a lot to learn \$\endgroup\$ Commented Dec 14, 2020 at 2:45
  • \$\begingroup\$ Did you measure resistance of resistor? \$\endgroup\$ Commented Dec 14, 2020 at 3:23

2 Answers 2

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The explanation is pretty simple. The forward voltage estimate you used for the LED is wrong. Instead of being 3V at 20 mA, it is 3.57V at 14mA. You will get pretty close to what you want if you just re-calculate using 3.6V instead of 3V.

So, using your equation but with the modified forward voltage we have:

R = (5-3.6)/0.02 = 70 Ohms.

Standard values that might work are 75 Ohms, or 71.5 Ohms or 69.8 Ohms. If you run it at 20mA, make sure it will not burn out. Often 20mA is the max current for an LED, and is much brighter than necessary. Using 150 Ohms at 14mA might be better in practice.

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    \$\begingroup\$ It turns out that particular LED is completely different from the other blue LED from the same batch. I test with couple other LED from the same batch and all measurements come out the same as my calculatio. THANK YOU everyone's tips and help. Much appreciated. \$\endgroup\$ Commented Dec 14, 2020 at 3:02
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This is not an answer.

Please follow convention when drawing your schematic.

Vcc at top, ground at bottom, input on left, output on right.
(you can reverse the input/output if your language uses right to left writing)

It makes it easier to visualize the voltage levels in the circuit and makes it more difficult to make errors such as an LED being backwards as in your circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thank you. Much appreciated. \$\endgroup\$ Commented Dec 29, 2020 at 22:48

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