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Please excuse my schematic.

enter image description here

This is a tiny PCB near the DC5.5 Jack port of a laptop power supply. The transformer is broken so I dismantled it to learn some. The transformer reads like "220VAC to 19VDC 1.750A - 2.370A".

My question is : How do these 2 green LEDs light?

Both R1 and R2 read "75B", which according to the internet is 5900 Ohms, which is also what my multimeter reads (5.9K Ohms).

Correct me if I am wrong : the current that comes to the R1 branch is I = V/R = 19 / 5900 = 0.003 Amps (3 mA). Isn't this too low? LEDs are probably 02xx or 04xx (crazy tiny) by the way.

Now the voltage drop accross R1 would be : V = RI = 5900 * 0.003 = 17.7 V

Which means I have 2.3V left for LED1, which seems right actually! I checked with my bench PSU, the LED1 requires no less than 2.3V to light up (after removing desoldering it), but I do not have the greatest equipment to show exactly how much amps that LED1 draws (my PSU shows 0.000A)

R1 and R2 also are very tiny SMDs. How much power do they dissipate? I believeit is P = 0.003 * 17.7 = 0.051 Watts?

Can someone please correct me?

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    \$\begingroup\$ Your calculations look fine. 3mA sounds a bit low but obviously works. Nothing says you have to drive a LED at its rated current. \$\endgroup\$
    – vir
    Commented Oct 1, 2021 at 17:46
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    \$\begingroup\$ My question is : How do these 2 green LEDs light? Because some current flows through them. With modern (SMD) LEDs even 1 mA is enough to make the LED light up brightly. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 1, 2021 at 17:51
  • \$\begingroup\$ I always read LEDs require around 30mA. This is ten times lower than what I expected so I thought something is wrong. Thanks for confirming the calculations are correct. \$\endgroup\$
    – Musa
    Commented Oct 1, 2021 at 17:53
  • \$\begingroup\$ The LED itself will drop about 2V, so the current is more like 17/5900 which is still about 3mA, but 3mA is fine for any decent modern LED. \$\endgroup\$
    – td127
    Commented Oct 1, 2021 at 17:55
  • \$\begingroup\$ You should test it out on a breadboard. Leds will visibly light at very low current. \$\endgroup\$
    – Drew
    Commented Oct 1, 2021 at 17:55

1 Answer 1

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Take a look at this datasheet for a green 0402 LED:

https://www.sunledusa.com/products/spec/XZBGR68W5MAV-3.pdf

Absolute maximum forward current is 10mA, but parameters are specified at If=5mA. If you look at the graph for luminous intensity vs. forward current, you'll see that at 3mA you're still getting ~2/3 of full brightness.

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