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I began officially studying Lessons in Electronics and as practice, I decided to detail an LED strip I've been using around the house. My results raised some questions around the LED voltages on the strip.

led strip

The strip is supplied with a 12V 5A power supply. The green and blue channels have a 180ohm resistor and red has a 360ohm resistor. As I understand it, the color component in each LED is hooked up in series and the whole segment here is in parallel.

enter image description here

If that's true, this means the green and blue LEDs are getting 1.91V (red is getting 1.84V), but most of the top google results have the minimum forward voltage for a 5050 LED's green and blue segments around 2.8V. I don't know the datasheet for this particular 5050, but I assumed it's similar. The strip works fine, so now I'm wondering why. Did I misunderstand the circuit design or is the lower voltage meant to reduce heat / increase lifespan? If it can work at ~1.9V, what is the purpose of the minimum forward voltage rating?

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    \$\begingroup\$ No credit for partial answers. Show your work. What formula did you use to get 1.9 Volts? \$\endgroup\$
    – Passerby
    Commented Jun 30, 2022 at 19:44
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    \$\begingroup\$ @Passerby This was my fault. I used the default-led profile on falstad in that diagram and hovered over the led components to see those values. Looks like the default profile assumes 1A draw at 2.2V apparently. \$\endgroup\$
    – Ben
    Commented Jun 30, 2022 at 19:55

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I don't know how you determined the voltages, but a typical value for LED current is 20 mA.

20 mA through 180 \$\Omega\$ is 3.6 V. Subtract that from 12 V and you have 8.4 V. Divide that by 3 and you get 2.8 V per LED.

Do the same math for the red LEDs and you get 1.6 V per LED, which sounds reasonable.

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    \$\begingroup\$ Ahh I see. I'll need more repetition to get better at this. I mentioned in the above comment that I got the voltage from the falstad circuit analysis which was set up making some poor default assumptions. \$\endgroup\$
    – Ben
    Commented Jun 30, 2022 at 20:00
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    \$\begingroup\$ @Passerby I don't see where that is stated. The OP said 12V 5A, so I went by that. Looking at the LED data sheet it looks like 14.5V would give the maximum rated voltage of 3.6V for blue and green at 20mA, and 12V would give the minimum of 2.8V, so it looks like it's all within the correct range. \$\endgroup\$
    – GodJihyo
    Commented Jun 30, 2022 at 20:02
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    \$\begingroup\$ 1.6V for a red LED is still lower than that datasheet and wikipedia says most red LEDs are around 1.7-2.0 volts. I guess my question is how firm are these "minimum" foward voltages if the LED operates below them? \$\endgroup\$
    – Ben
    Commented Jun 30, 2022 at 20:13
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    \$\begingroup\$ @Ben What would happen is they would just draw a bit less current. If the LED voltage drops were 1.8V each, multiplied by 3 is 5.4V, (12-5.4)/360 = 18.33mA. So they'd be a little bit dimmer. That's the thing about using a resistor, it gives a bit of wiggle room at the expense of variable brightness. A string of LEDs fed by a resistor can all have different brightness and vary with supply voltage. If you want a constant, even brightness you feed them with a constant current source. \$\endgroup\$
    – GodJihyo
    Commented Jun 30, 2022 at 20:35
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    \$\begingroup\$ @ben leds will light up to fractions of a milliamp. Of course they will only pin pricks of light at that point. The voltage is determined by the current pulled by the diode. It's all a balancing act. Resistors are the least precise way to limit the current through. The typical/nominal values are measured at a specified If. You want to look at the If vs Vf graph of a datasheet for the actual nominal curve for the full range. \$\endgroup\$
    – Passerby
    Commented Jun 30, 2022 at 21:33

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