# Selecting resistor for led

It’s been a while since I’ve done this so can someone please double check my work for me here? I have an NTE brand 5 mm clear blue LED. It has a forward voltage of 4 V and a forward current of 30 Millamps max. I’m using a small lipo battery that goes up to 4.1V. So to calculate my resistor size is simply (4.1-4)/.03=3ohm resistor...or close to it. I feel like I’m missing something here. Is that correct?

I feel like I'm missing something here.

Yes, you are. The maths is correct, the underlying assumptions are not. 0.1V is far too small a margin between the nominal LED forward voltage and the supply voltage. Suppose you are out by 0.1V; if it's 0.1V higher then the current will be 60mA, if it's 0.1V lower it will be 0mA.

I'd probably want the LED Vf to be no more than 80% of the supply voltage, so a supply of at least 5V and an appropriate resistor. The more tightly regulated the supply voltage the tighter you can have the margin between supply and LED Vf. A battery or any kind does not have a tightly regulated voltage.

Also, don't operate the LED at its maximum current, probably best to aim for no more than maybe 80% of its maximum.

Your calculations are correct (except you don't necessarily need to run the LED at full current) but in the real world there is no good way to run a 4V LED from a 4.1V battery.*

Typically, You'd like to drop around half the voltage across the resistor and half across the LED. The more voltage you drop across the resistor the closer you get to an ideal current source. (With higher-power LEDs you generally use a switchmode constant-current supply so you're not wasting power in the resistor.)

Your battery voltage isn't always 4.1V. It might be slightly higher when first charged, and then it starts falling when you use it. The LED voltage may not be exactly 4V either, and it will change (slightly) with temperature. And if there's an Arduino, transistor, or MOSFET, etc., controlling the LED you'll get some voltage drop through that device.

* You can do it if you have a step-up DC-DC converter.

30mA absolute max means that you shouldn't use it over 20mA.
0.5mA might already be enough if you use it as indicator light.

Blue LEDs already start working at about 2volt, and have an average working voltage of ~3.3volt@20mA.
Try if a 100ohm resistor gives you enough light.
Leo..

Blue LEDs are typically around 3.2V forward voltage. That NTE 4.0V is a maximum, meaning that most
devices will be less, and if you reduce the current to 20mA, its likely to be significantly less than 4V, as some of the forward voltage is wasted in spreading resistance once you push a device to its max.

Try 33 ohms or so.