Complete newbie led help

[Mrdavidjk]

So i 3d print things for people and one thing i have printed needs 3 white leds in parallel 3v (which i have)
they will be powered by 2 aa batteries in series
My question is what rating resistors do i need,
And also a good starting place to learn about it also

 

[Pommie]

Two AA batteries may not be enough if the forward voltage of the LEDs is 3V. Three would give 4.5V so you'll need a resistor to limit the current. Assuming the current (per LED) is 20mA then each resistor needs to drop 1.5V with a current of 20mA. Ohms law says V = IR or R=V/I = 1.5/0.02 = 75Ω (head calculation so check it). Note, each LED needs a series resistor.

Mike.

 

[rjenkinsgb]

A Pommie says, two AAs is unlikely to work, or for more than a very brief time.

Even three will have a short life, as "1.5V" batteries start out at around that but steadily drop in voltage over their life, to near enough 1V before considered used up.

You could use a little "boost" type regulator module such as one of these:

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That should work from two cells, from new all the way down to 2V input when they are dead.

You can set the output voltage to eg. 3.5V, about half a volt more than the LED voltage so minimum power wasted in the resistors, and you should get constant brightness regardless of battery state.

You could then use a 27 Ohm resistor for each LED & vary the brightness by adjusting the module output voltage slightly.

Make sure you set the output down to 3.5V or less before connecting the LEDs, as it could be set for anything when you get it & things start smoking...