Using LEDs: A primer by Michael H. Smith

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Working With LEDs:
A Primer for the Sci-Fi Modeler

By Michael H. Smith - images & text © 2002

Introduction

Many model starship and sci-fi lighting projects involve the use of LED's (Light Emitting Diodes) for main and detail lighting. For main lighting, the LED's can be used as the light source for fiber optics as a cheap, low power solution for lighting round portholes. They could also potentially be used in a light-pipe hybrid application in which an LED fed into a diffused acrylic pipe (a straw for instance) could radiate light for use in engine nacelle lighting. For detail lighting, LED's are useful for position and navigational lights. Finding good documentation on how to choose, size, and correctly wire LED's is difficult to obtain.

Objective

The objective of this document is to help those without a strong background in electronics understand some of the fundamentals of LED's to help incorporate them into their model lighting designs. While some electronic background is helpful, it is not entirely necessary to safely and effectively light a model as the principals introduced here require only basic math and bookkeeping skills.

Choosing LED's

Choices

LED's come in various sizes, colors, voltage and current ratings, luminosity, and viewing angles. While this may seem like too many choices, remember that these were originally intended as indicators for board and circuit designers. Understanding the terminology is not difficult if taken one parameter a time.

[LED Chart]

Figure 1: Typical LED chart

Figure 1 shows an LED chart you might see browsing a catalog such as one from Digi-Key. The physical parameters will usually be keyed to a drawing in the catalog. The lead spacing shown here is in mm. The Electro-Optical characteristics are the key parameters from this chart.

Pd refers to the maximum power (or heat) dissipation of the component. This chart rates power dissipation in mW.

If refers to forward current in mA. Most LED's are rated near 20mA. Less is OK and is dictated by the resistor size (covered in section 4). More is NOT OK as you do not want to exceed the maximum forward current of a diode (breakdown will occur) or the maximum power dissipation.

Vr in volts is the magic number. This refers to minimum voltage applied to the LED at which it will illuminate. You typically want this number less than your supply voltage.

lp refers to the wavelength, an indication of the color of the LED on a spectral scale.

Io is a measure of an LED's luminosity as measured in millicandelas. Luminosity is directly dependent upon the amount of forward current passed thru and LED. An LED configured to pass 20mA will burn brighter than an LED running at 10mA.

Many specs will also publish the viewing angle. This is helpful to know if you are using LED's as marker or navigation lights and you want to see the light from all angles, not just straight on.

As you can see, there are many parameters to LED's but breaking them down, it's a little easier to digest.

Other parameters not mentioned include the lens type. You will need to know this when selecting the color. LED's come in basically two lens flavors - diffused or clear. Clear is a tinted color and you can see through to the LED internals. Diffused is a frosted lens. You cannot see through these but they produce a smooth, even glow.

LED Sizes

LED's come in different shapes and sizes. The most common are round leaded packages, however you can purchase LED's in square and rectangular packages in both leaded and surface mount packages as shown in Figures 2 and 3.

[Rectangular]

Figure 2: Rectangular LED

[Trapezoidal]

Figure 3: Trapezoidal LED

Typical round LED's some in three basic sizes, the size referring to the diameter of the lens. They are:

  • 2mm (T )
  • 3mm (T-1)
  • 5mm (T-1 )

Working with a Power Budget

All DC circuits whether sourced from batteries or an AC/DC adapter must have an electrical budget the designer works from. In simplest terms, assume you have an AC/DC adapter which plugs into a wall. The output is 12V at 500mA. The circuit this adapter powers can draw no more than 500mA of current and must be able to handle or adapt 12V DC. Since a typical LED draws about 20mA, from a 500mA source you can potentially power 25 LED's safely, but with no safety margin. Normally when we design circuits, we factor in some margin, say 50mA. This ensures if all the components in the design are drawing their maximum current, we still have 50mA of headroom before the power source says "enough!"

Remember that budget must also account for any additional circuitry such as timers used to flash certain LED's and DC-DC adapters (the bring the 12V down to a more useful 5V or 3.3V). Don't forget micro-fluorescent lighting as their adapters require quite a bit of current to operate. We'll stick with LED's for this document, however as these principals can also be applied towards more advanced circuitry.

Adding up the components

LED's can be wired in series, but are typically wired in parallel. This means the Anode is of each LED is tied to Vcc and the Cathode is always connected to ground through a resistor (as determined from section 2 above).

When LED's are wired in parallel the currents add. For instance, if you have 10 LED's and each are configured for 20mA, the maximum current would be 10x20mA = 200mA. If you are using 500mA supply, this is well within limits.

Most AC/DC supplies provide 12V. You will need to convert the 12V to 5V or purchase a 5V AC/DC supply. Simple voltage regulators exist that allow you convert 12V to 5V with a minimum of additional components. This is recommended as it still give you 12V in your circuit design to power higher-voltage rated components - like the fluorescent lights. 5V will now become the Vcc of your LED circuit.

Connecting LED's

Anatomy of an LED

An LED is a simple device and figure x shows the parts of an LED in more detail. It's important to know the difference between the anode and the cathode end. Connecting the wrong end will reverse bias the LED. For a short period of time this is OK, but for extended periods, it will destroy the LED - a bigger concern if using the more pricey blue LED's.

[Typical LED]

Figure 4: LED Cross-section

Figure 4 (From Dialight data sheet) shows a typical 5mm LED. The long lead is the anode, which is connected to Vcc. The short lead is the cathode (also has the beveled edge on the lends) which is connected thru a resistor to ground.

[Schematic]

Figure 5: LED Schematic symbol

Figure 5 shows a schematic symbol and typical wiring arrangement for an LED. This LED is powered from 5V and has a 330 ohm series resistor. The current is probably in the 15mA range. The large end of the LED represents the anode, bar represents the cathode. The placement of the resistor is not important as long as each LED has one series resistor. (It is VERY important that EACH LED or series of LEDs gets its own resistor if they are to be wired in parallel.)

The LED Formula

Once VCC has been determined (typically 5V), the LED formula can be utilized to determine the correct resistor. Keep in mind, the value you receive may not be a typical resistor value. You will need to shop around to find a typical resistor value within a couple ohms of the value you receive. Stick with the cheaper 5% or 10% tolerant resistors, 1% accuracy is not necessary for LED's.

[Eq. 1]

Equation 1: LED Formula

where Vcc is the source voltage, VF is the forward voltage in Volts and IF is the maximum forward current in mA. See Example 1 for a typical Vcc of 5V, a typical VF of 2.1V and a typical IF of 20mA

[Example]

Since a 145 ohm is not a typical value, select the next largest value, in this case a 150 ohm resistor would work fine. Working backwards thru the equation, a 150ohm resistor would give us about 19.3mA. This is less than the 20mA maximum, so we're OK. Use this number for your power budget.

Making the Connection

Whether you choose large LED's or small, through hole mount or surface mount, good connections are important. The best way to connect LED's to resistors and ground and power is by soldering them to wires. In the electronic industry, it's common to use flux to remove oxidation from the leadframes of the LED's and the leads of wires. A common flux and solder combination is RMA (rosin mildly active). A 60/40 tin/lead RMA flux core solder is used in various thicknesses - 15 mil is a good compromise between too small and too large.

While Radio Shack may be a good source for certain things - like radios, I recommend a larger electronics distributor such as DigiKey that offer a larger selection LED's and other components.

For a small number of LED's, it's fine to wire everything together, then run the leads back to the source. For a large number of LED's, I like to purchase cheap pre-drilled printed circuit board (PCB) and mount my resistors to that along with power and ground from the power supply. Next, I use 30 gauge wire to connect the lead frames of the LED's to the resistor and then to power. Choose a PCB small enough to fit into the body of the model, then allow enough wire for flexible placement of the LED's.

Summary

LED's are a low-power, low-heat, cost effective method of lighting your models. With a little attention to detail, they can be reliably powered and last several thousand hours without failure. Hopefully, this primer will give you a head start in designing lighting circuits for your models or other projects.

Some other references include:

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This page copyright © 2002 Starship Modeler™. Last updated on 3 May 2002.