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I'm looking at using an IN-S85AT5UW in an upcoming hobby project. I'm pretty sure it says that with the white LED, I'll get about 180 mcd and that the maximum continuous forward current is 5ma.

That feels surprisingly low, so I thought I'd get a sanity check. It also kind of makes the forward voltage range a bigger concern I think.

I have a 5V supply and if the LED has a typical forward voltage between 2.6 and 3.4, then I'll have to drop 2.4V to 1.6V across a current limiting resistor.

So, somewhere between 320 and 480 ohms? That seems like a pretty big range. If I get an LED with a forward voltage of 2.6V and I have a 320 ohms resistor, then I'll have a 7.5ma current . . . which is 50% higher than the rated forward current.

Do I pick conservatively and accept the loss in brightness? Do I pick a resistor based on the nominal forward voltage of the LED (3V) and hope I don't burn out the LED? Are the datasheets written so that if you select components at nominal values, the component will not burn out over its rated min/max range?

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    \$\begingroup\$ The quality on these LEDs is indicated by the low test current and also the unusually wide tolerance at low current. This means the tolerance is much wider at 20mA. Test and select R on test or get better LEDs \$\endgroup\$
    – Tony Stewart EE75
    Apr 1, 2019 at 17:04
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    \$\begingroup\$ No smoke just put your finger on it till it gets warm with 100 Ohms in series. then measure current and voltage. Big spender 10 x 0.01c \$\endgroup\$
    – Tony Stewart EE75
    Apr 1, 2019 at 18:05
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    \$\begingroup\$ To give some current limiting as you increase voltage and also to measure current from its voltage drop. \$\endgroup\$
    – Tony Stewart EE75
    Apr 1, 2019 at 18:50
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    \$\begingroup\$ AS long as it does not burn your finger pressing on it or exceed 2V over 100 ohms. My 5mm indicators are 20,000 mcd at 20mA and you can see them at the end of a long tunnel. \$\endgroup\$
    – Tony Stewart EE75
    Apr 1, 2019 at 18:56
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    \$\begingroup\$ When you have to select on test every batch of LEDs, when the colour is too cool, when it fails ( infant mortality) long after handling because it has no ESD protection. etc \$\endgroup\$
    – Tony Stewart EE75
    Apr 1, 2019 at 19:00

1 Answer 1

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The maximum forward current is not 5mA, that just happens to be the test current that they guarantee the light emission at.

The absolute maximum forward current is 25mA at 25°C, so derate this for your maximum ambient temperature.

Chances are you can just design for a nominal 5mA if that yields appropriate brightness for your application.

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  • \$\begingroup\$ Thanks!! How would I estimate the luminosity at 20ma? Would it be roughly 4 times as bright? \$\endgroup\$
    – D. Patrick
    Apr 1, 2019 at 15:55
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    \$\begingroup\$ Yes, but visually it won’t be so dramatic a difference due to eye response. \$\endgroup\$
    – Spehro Pefhany
    Apr 1, 2019 at 15:59
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    \$\begingroup\$ P.S. if you don't need the extra brightness, running at a lower current (like 1-10mA) saves power, allows it to run cooler, and will make the LED last much longer. \$\endgroup\$
    – Spehro Pefhany
    Apr 1, 2019 at 17:56

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