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I'm in the process of designing a PCB that includes 10-15 LEDs. It's battery-powered, so I'd like to use as little power as possible. As such, I've been trying to figure out how little current the LEDs need to still appear relatively bright.

As an experiment, I measured the voltage across the current-limit resistor (R12) for the power LED on my BeagleBone Black. Voltage was 0.75V, so current was only 0.16mA (using Ohm's Law).enter image description here

At first this seemed like great news -- the LED appears extremely bright, so I thought I'd use the same LED/resistor combination on my board. That way, I'd be able to power 10 LEDs at a total of 1.6mA.

However, upon digging around some more, I found the following graph from the LTST-C191TBKT datasheet, which shows that luminous intensity is basically zero below ~3mA.

enter image description here

This seems to be a contradiction ... any ideas why the datasheet claims the LED gives off no light below 3mA, but the LED appears quite bright at 0.16mA?

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  • \$\begingroup\$ Measure the 4k75. It sounds too high. I'll put a small wager that it's 47.5 ohms. \$\endgroup\$ – Andy aka May 6 '15 at 21:24
  • \$\begingroup\$ Just double-checked; it definitely matches the schematic (4k75). I guess I'm wondering if this seeming contradiction can be explained by how I'm interpreting the datasheet -- like maybe the LED manufacturers detail specs at much higher brightness levels than are truly needed for an indicator LED on a PCB. \$\endgroup\$ – user44184 May 6 '15 at 21:29
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    \$\begingroup\$ Did you measure it and if so with what? \$\endgroup\$ – Andy aka May 6 '15 at 21:31
  • \$\begingroup\$ I agree with @Andyaka, 1.6ma is very unlikely to make an LED appear "extremely bright". \$\endgroup\$ – DoxyLover May 6 '15 at 22:47
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    \$\begingroup\$ Actually, it might. Notice that this is SMD with a clear body and no diffuser. The bare LED is visible, and is very small, so it doesn't take a lot of total radiance to appear bright. Plus, given the log characteristics of human eye (1/10 th the power give half the brightness), and assuming his work area is at normal indoor levels, it seems possible. \$\endgroup\$ – WhatRoughBeast May 7 '15 at 1:41
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Small signal LEDs are typically fraction of a watt, with a target 20 mA forward current at the recommended forward voltage. This specific led has a VF of 2.8 ~ 3.8 Volts at 20 mA (specifics depend on the bin, and ultimately on the led).

This spec is for the recommended, 100% continuously on brightness for the estimated life of the led. These specs are for Illumination designs, not simple Indication.

That LED is used as a simple indicator light. In this design, the goal is not to light up as much as possible, its just to show a status.

At very low currents, less than 3 mA, the relative brightness of the LED is so low compared to the brightness at 20 mA, that its considered insignificant. This is a subjective spec. But the actual, objective brightness of the led at less than 3 mA is still visible. It's just not as visible as 5mA, or 10mA or 20mA. At this point, it's no longer used for illumination.

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The graph is normalized to 20mA, but it's a normalized graph, if it was a log graph it might make more sense. I would ignore the graph. Double check the circuit and measure the voltage drop across the diode and the resistor to double check. Also check the AC voltage to make sure that your only getting DC.

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  • \$\begingroup\$ Other way around. 0.75 across the resistor, so 2.55V across the LED, which would be about right for >1mA for a 3.3V @ 20mA LED, in my experience. \$\endgroup\$ – Passerby Mar 17 '16 at 20:24
  • \$\begingroup\$ Yep, I'll edit that \$\endgroup\$ – Voltage Spike Mar 17 '16 at 20:51

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