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Electronics : Using LED's

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: Using LED's
« on: May 22, 2018, 12:50 PM »
Using LEDs
Lead Author: Bar'uun Strieg
Edited by: MMCC Education Team

This guide will serve as a basic primer for working with LEDs.  More detailed and specific tutorials will follow.

LEDs (Light Emitting Diodes) are great tools to use on your kit to add that little extra “wow” factor.  They are small, bright, and require very little power to run for a long time (think multi-day troop event like a Con).  LEDs come in several sizes and multiple colors to fit whatever design you may have.

LEDs can also be somewhat delicate and if you give them too much power, you WILL burn them out, rendering them useless.  This tutorial will help teach you how to avoid that.

Most LEDs have two wires attached to their base.  One of these wires is longer than the other one.  For the majority of LEDs the longer wire is the positive power “leg”, referred to as the Anode.  The shorter wire is the negative, or ground “leg”, referred to as the Cathode.  It is important to only have the positive voltage for your project attached to the Anode.  If you reverse this, the LED will either not function at all, or will burn out.  The negative voltage, or ground, will be attached to the Cathode leg.  Some LEDs have legs of equal length.  In this case, the Base of the LED will have a flat spot that indicates the Cathode leg.  If in doubt, consult the packaging of the LEDs, or ask questions before attaching power to help avoid burning out your LEDs.

As stated earlier, if you provide too much power to an LED, you will burn it out, rendering it useless.  You can avoid this situation by using the appropriate resistor in your LED circuit to effectively lower the amount of power flowing through it.  The key is to know how much power your controller device is putting out, then determine how much voltage can be applied to an LED, and finally putting an appropriate resistor in the circuit so that you do not give the LED more power than it can handle.

This website is a great resource for how to determine what resistor to use.

Excerpt from the above link: “LEDs have a characteristic called “forward voltage” which is often shown on the datasheets as Vf. This forward voltage is the amount of voltage “lost” in the LED when operated at a certain reference current, usually defined to be about 20 milliamps (mA), i.e., 0.020 amps (A).   Vf depends primarily on the color of the LED, but actually varies a bit from LED to LED, sometimes even within the same bag of LEDs. Standard red, orange, yellow and yellow-green LEDs have a Vf of about 1.8 V, while pure-green, blue, white, and UV LEDs have a Vf of about 3.3 V.”

That said, let’s use a simple circuit to illuminate one LED using a 9 Volt battery as power.  We know a 9V battery will put out about 9V.  We’ll use a white LED that has a forward Voltage of 3V and 20mA (.020 Amps).  The formula used to determine what resistor will be required is shown below.

R = Vs – Vf / i
R = The resistor value needed
Vs = supply voltage
Vf = LED forward voltage drop in Volts (found in the LED datasheet)
i = LED forward current in Amps (found in the LED datasheet)

So for the LED and power supply listed above, we have the following equation:

R = 9V – 3V / 20mA, which can be shown as R = 9 – 3 / .020

The resistor we need is a 300 Ohm Resistor.  Since resistors do not come in EVERY value needed, we would go up to the next highest resistor in a common value.  In this case, that would be a 330 Ohm resistor.

This website has an easy to use calculator where you simply plug in the known values for your supply voltage, the LED forward voltage, and the LED forward current.  The calculator then tells you what resistor to use and even gives you the resistor color code that corresponds to the resistor you need.  The website also provides a link to another similar calculator useful for multiple LEDs.

Using these tools (or any number of similar websites), you can determine what resistor to use in order to complete the circuit you are wanting to build.
Once you have the resistor you need, you can build the circuit.  Unlike the LED, a resistor is not polarized (it does not have a positive and a negative side).  It doesn’t matter which wire from the resistor you attach between the power supply and then to the LED.  It also doesn’t matter if you put the resistor between the power supply and the LED on the positive side or on the negative (ground) side of the circuit.  It will function the same in either case.

Now that you have this information, it should make building circuits for LEDs easier for you.  Knowing how to determine what resistor is required for a circuit will help insure that you don’t burn out your LEDs and will keep them illuminating parts of your kit for that “wow” factor you are looking for.

Stay tuned for more electronics tutorials that will be added in the near future.

--MMCC Education Team--

« Last Edit: Dec 16, 2019, 07:00 AM by Hik'aari » Logged
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