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Getting started with LEDs Answered

Hey Everybody, This is crazy but I ran into this website through a random link in Lifehacker and after what I saw here, I quickly made an account and I bought PRO just to support the site. I am a big fan of communities that exist to enlighten the average brain and to evolve us as beings in creating new things and getting out that genius in ya. Anyway, my current huge curiosity is LEDs. I love them, I don't know much about them except that they are little plastic bubbles that take very little electrical power to get them going. I want to learn more however, I have a project which is to light up my computer desk's edges in my room. Problem is, I don't know where to start. I tried reading a bunch of instructables here already but most of them do not really explain the wiring process. I am a graphics designer and I have had a minor in computer science but never got into electrical engineering. I understand the simple things that go behind an electrical cricuit (like power sources and positives and negatives) but thats the extent of how much I know. I need somebody to help me and get me going so I can start evolving. Could anybody here help me get started? A few explanations, guides, tutorials? Thank you very much in advance.



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Jack A Lopez
Jack A Lopez

11 years ago

Driving LEDs:

There are basically two principles, er "rules", you need to be aware of for driving LEDs.

(1) A LED has a sort of characteristic "on" voltage. When any significant amount of current is flowing (forward) through it, the voltage drop across the LED will be something close to this voltage. Contrast this behavior to the voltage-current characteristic of a resistor. For a resistor a small current means small voltage drop, large current means large voltage drop, etc. Typically you can guess approximately what this "on" voltage will be based on the color of the LED, e.g. infrared ~1.4 V, red ~1.8 V, yellow/green ~2.2 V, blue/white ~3.6 V

(2) LEDs are destroyed when too much current flows through them, so some kind of current limiting mechanism is always necessary. What the limit should be depends on the power-handling ability of the LED. The little plastic LEDs, T1, T1+3/4 can usually take around 5-20mA. Larger high-powered, heat-sinked, LEDs can take 100s of mA. (At the time of this writing.)

So this is the story of LED driver design. You consider rule (1) and ask yourself, "What is the minimum voltage I need to get the LED to turn on?" Also you consider rule (2) and ask yourself, "How do I limit the current?"

The easy design: use a resistor

The sort of easy, obvious answer to this design question is to put the LED in series with a resistor R, and a voltage source, Vsource.

For this design the current through the resistor (also through the LED, also through the stack of cells) is:

I = (Vsource - VLED) / R

BTW, this trick requires: Vsource > VLED

If Vsource < VLED, then I=0, or in other words if you don't have enough voltage in your stack of cells, no current will flow, and the LED will be dark.

The stupid design:

Just hooking up a LED to a voltage source, e.g. stack of cells, a DC wall-wart power supply, etc., is almost always a bad idea. Usually this will result in burning out the LED. The reason why: the current does a "thermal runaway". The story is the conductance of an LED improves with temperature. You turn it on, and some current flows. It gets warm. So then more current flows. Then it gets warmer, so even more current flows! And so on, until it burns itself out.

The only exception to the "do-not-hook-up-a-LED-to-a-voltage-source" rule is when the cells themselves have relatively high internal resistance. For example the little coin-sized cells used in many cheap LED flashlights have sufficiently high internal resistance that this internal resistance can serve as the current limiting resistor.

Switchmode designs:

One drawback to the voltage source plus resistor design is that you need a big stack of cells to make sure
Vsource > VLED
Another drawback is that the current limiting resistor actually wastes power.

Switchmode designs use dark, dark magic, usually involving inductors, to draw power at some voltage and current, and then re-supply that power at a different voltage and current.

Designing a swichmode LED driver is complicated. So if you need one, it is often better to find someone who has built one for you in the form of a module. Or rip a such a module out of a flashlight, etc.

The only switchmode design simple enough for a beginner to tackle is the so-called "Joule-Thief" This desing powers a single LED (you pick the color) from a single battery cell (e.g. 1 AA) and that battery cell can be almost dead. A bunch of people have written instructables on the Joule-Thief.