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# Could I use a joule thief to power 2 super bright blue leds? Answered

Using this instructable's instructions https://www.instructables.com/id/Make-a-Joule-Thief/, would it be possible to run 2 blue led's off 3.3 volts? I'm using a 3.3 volt logic system and I need to have 2 blue led's light. Also, how do I know what resistor to use for this? And could the led get too much voltage and be blown? Sorry I have so many questions. I've just never used a joule thief!

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## Discussions

a blue led will light on a 3v battery...it should light on 3.3v. Nominally a 4v led might not light up, but I've found they do.

You only need to worry about current protection if you exceed the led's forward voltage, below that it won't draw enough current to hurt itself. the forward voltage is the threshold of damage if not protected from virtually unlimited current since the semiconductor is fully saturated/conducting at that point.

Yes, but since I am having two led's. I need twice the voltage. So my question still stands.

frollard is correct. Make sure that you understand the properties of voltage and current in series and parallel circuits.

The AP wiring for 2 LEDs Way !

May they be blue white yellow or mixed... A

Sure this works too But runs for a shorter time using more battery...   A

Nevermind, everyone. I got it working off of 3.3 volts. I just hooked each led's anode to power and the cathode to a 1k resistor, then grounded the resistor. It seems to work fine. Thanks for your help!

Don't Forget To Be Awesome.

If you connect each of the two blue LEDs by itself to the 3.3V, then connect each one in series with a 33 ohm resistor, just for a small amount of current limiting.

If you connect both blue LEDs in series, they will need about 6.6 volts to light up to full brightness.  Use a Joule Thief, but reduce the number of turns for the feedback winding to half as many turns as the primary winding.  For example, if you have 20 turns for the primary, then wind 10 turns for the feedback winding.  You may have to increase the 1000 ohm resistor, too.  I would start out with 4.7k, and see if they are bright enough.

BTW, if you are connecting this to a 3.3V, high amperage supply, you should always use a fuse close to the supply.  A 1/4 amp fuse would be more than enough.

A lot of joule thiefs are current types, that means they generate
an increasing voltage until current flows, so put your two blues
in series and attach to a JT.

Maybe change some turns.

A