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Here at Showcase we get many questions about how to light our signals with our LEDs, in specific, what resistor value should I use.
We owe a great debt of gratitude to Jim Hinds of Richmond Controls. He is always quick to help with any questions our customers may have and he seems to enjoy sharing his knowledge. Thank you very much, Jim. Below is a Question from a customer and the Answer from Jim.
Customer Question: I've had experience with LEDs before, but none so tiny as these. I want to know what is the minimum voltage I must use to get them to light. And, at the minimum voltage must I still use a 1,000 ohm resister in the line?
Answer: LEDs are quite different from LAMPS, and you can get into a lot of expense and trouble if you think about LEDs the way you think about LAMPS. Avoid thinking about the voltage applied to an LED by itself.
To be specific, for LAMPS, you apply rated VOLTAGE, and the lamp draws whatever CURRENT it wants. Just the opposite, with LEDs, you supply CURRENT, and the LED takes whatever VOLTAGE it wants.
Different LED colors exhibit different inherent voltage drops. That voltage is the LED's choice, not the user's choice. Typically, Red LEDs exhibit an inherent voltage drop of about 1.8 volts. Yellow LEDs exhibit inherent voltages of about 2.2 volts, green LEDs exhibit about 2.5 volts, and blue and white LEDs exhibit inherent voltages of about 2.9 volts. That VOLTAGE is the LED's choice, NOT the user's choice. The inherent voltage drop is determined by the two different materials that are used to build the LEDs (which are diodes), and that choice of different materials determines the LED color and its inherent voltage. The user only gets to choose the current, which determines the brightness.
Here's how I would like for people to think about powering an LED: Have a voltage available which exceeds the LED's inherent voltage. For a three-color LED, you need a bit more than what the green LED wants. Remember that this inherent voltage is different for different LED colors, so you can't logically expect to apply one specific voltage to different colors and have the LEDs survive and emit reasonable amounts of light.
Think about always having an LED in series with a current-limiting resistor, with the overall voltage applied to the series pair. The LED will automatically take whatever voltage it wants, and the resistor will get rid of the voltage above that. The user should never worry about how much voltage the LED wants. Only worry about how much current will flow through the LED. The resistor needs to be selected to provide the amount of current that gives the brightness that the user wants.
To be practical, I'd suggest that the minimum applied voltage you'd ever want to have is 5 volts. Without the resistor, this amount of voltage can be expected to instantly blow out the LED. But applied through a resistor which limits the current, you can get whatever brightness you want while safely powering the LED.
Things work fine with 5 Volts, or 12 Volts, or 24 Volts, etc. Start by deciding how BRIGHT you want the LED to be. That means you have to select an applied current. My recommendation for a BRIGHT light is a current of 3 mA, or 2.5 mA for a somewhat subdued brightness. The equation for determining the resistor value is R = Vr / I, where "Vr" is the RESISTOR VOLTAGE, and "I" is the current through the series resistor and LED. But the resistor voltage is simply the available voltage minus the LED's intrinsic voltage. For example, for a RED LED given a current of 2.5 mA, the resistor is R = (5-1.8)/.0025 =3.2 / .0025 = 1280 ohms. (You can do these calculations in your head if you recognize that the quantity (1 / 0.0025) equals 400, so 3.2 * 400 = 1280 ohms. Remember that "I" is in Amps, while we are specifying values in milliamps, or thousandths of an Amp.)
In the same situation, for GREEN, R = (5-2.5) / 0.0025 = 2.5 * 400 = 1000 ohms. For yellow, R = (5-2.2) / 0.0025 = 2.8 * 400 = 1120 ohms.
If you have 12 V available, these become (12-2.5)*400 = 3800 for green, (12-2.2)*400 = 3920 for yellow, and (12-1.8)*400 = 4080 for red.
Being practical, these are not readily available resistor values. Common values are 1200 (for the 5 V case) or 3900 (for the 12 V case). The difference in brightness within these ranges will not be detectable.
For a Showcase 3-color LED (Item #LED0001), it works great to install one resistor in the common positive lead, and it will serve to limit the currents to all three LEDs, which are assumed to be activated one at a time.
Customer Question: What should be the power supply for L.E.D.s ? Is 8-12 Volts D.C. O.K. or less needed?
Answer: All you need to power LEDs is a DC voltage greater that the minimum required to light the LED. You ALMOST ALWAYS want to have a series resistor to limit the current.
Different colors of LEDs require different minimum voltages, as explained in the above answer to the first question. If you attempt to match the applied voltage to the LED's required minimum voltage, however, you are almost certainly asking for trouble. Doing this can severely limit your ability to control the LED's brightness and may result in accidentally burning out the LED.
My recommendation is to have a 12 VDC power supply to power all of the stationary LEDs on a layout. A great source of these is the Mean Well brand sold by Jameco Electronics (www.jameco.com) . These are Switching Power Supplies which are highly efficient, give off very little heat, have many safety certifications (UL, CSA, VDE, etc.) and are available in sizes around 1 Amp. A 1 Amp supply will be able to power hundreds of LEDs if used correctly.
I recommend starting with 3 mA per LED and seeing whether the resulting brightness is pleasing to you. To change the brightness, use less resistance for more brightness, or more resistance for less brightness. Regardless, memorize and honor Kosik's Law: Never use less than 500 ohms to limit the LED current on a model railroad layout unless you really know what you are doing.
With a 12 VDC supply powering a visible LED and aiming for a current of 3 mA, the necessary resistor is around 3.3K The wattage rating of that resistor doesn't matter when the current is this low. A rating of 1/4 Watt is usually the cheapest. You can buy any quantity of 1/4 Watt 3.3K resistors from places like All Electronics, Mouser or Digi-Key.
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