View Single Post
Old 11-13-2003, 09:29 AM
Russo 85300tD Russo 85300tD is offline
Registered User
Join Date: Feb 2002
Location: Richmond, VA
Posts: 70
Replace Lamps with LEDS

How to replace those pesky incandescent bulbs with LEDS.

By replacing the incandescent bulbs in your dash you can improve the illumination and customize your vehicle instrument lighting. You also will probably never have to replace an LED like the incandescent bulbs.

LEDs are Light Emitting Diodes, a modern replacement for the miniature incandescent lamp. They emit light in a variety of colors from invisible Infrared to bright white and most recently ultra violet. The color is some times specified in wavelength of light and is usually in nanometers (nm). The visible colors are typically Red, Yellow, Orange, Green, Blue, and White. Each color of LED has a different voltage or forward voltage drop needed to have it turn on and emit its characteristic color of light. This voltage varies from about 1.7 volts for a RED LED to approximately 3.6-4.0 volts for WHITE and BLUE LEDS. The forward current thru the LED determines both the brightness of the light and power dissipated by the LED. Brightness is typically specified in mcd or milli Candela. Most standard output LEDS have a light output less than 1000 mcd and are not well suited for illumination applications. There are “High Output” and “Super Bright” LEDs and their light output is usually 2000 mcd to 10,000 mcd. The more light output the better for illumination applications. High output LEDS typically cost $ 2.00 to $ 5.00 each in quantities of one or two, and are available from many electronic suppliers, including Radio Shack. The typical forward current for optimum light output is usually specified to be approximately 20 milliamps (0.02 Amps). Increasing the current significantly above this level results in very little increase in light output however does increase the power that the LED must dissipate as heat. The increased power dissipation will result in a shortened life of the device. The typical operation life time of a LED at the proper current is 50,000 hours or more. Below is a list of the forward voltage drops for different colored LEDs. These are actual measured values from my stock of parts and are pretty typical.

RED 1.7 volts
ORANGE 2.0 volts
YELLOW 2.3 volts
GREEN 2.5 volts
BLUE 3.6-4.0 volts
WHITE 3.6-4.0 volts

LEDS are connected by 2 leads one is the Cathode (-) and the other is the Anode (+). If you hook them up backwards you usually don’t get smoke, just no light output.

To use the LED with a 12 volt power supply REQUIRES a series voltage dropping resistor. If you hook up a LED to 12 volts without the series voltage dropping resistor you get a very short bright burst of light and then smoke. Your LED is now a DED (Darkness Emitting Diode). The series resistor can be connected to either the + or the - lead of the LED. It does not matter which one, but you have to have a series resistor connected to limit the LED current.

Now the only “tricky part” of this whole process is to figure out what value resistor goes in series with the LED to produce the correct current through the LED. First we know the electrical system of our car is supposed to be + 12VDC. In reality the voltage on the electrical system can run as high as 14 to 15 volts depending on the condition of your voltage regulator, battery and a host of other factors. We will assume the highest voltage normally to be 14.5 volts and it will be called Vmax. To calculate the size resistor needed, we subtract the forward voltage drop for the color LED we are using from Vmax. This gives us the amount of voltage that needs to be “dropped “across the series resistor. Since we know the current we need through the LED, it is a simple case of applying Ohm’s law to determine the resistor value in ohms. See the example below

Using a blue LED with a forward voltage drop of 3.6 volts and a forward current of 20 ma we subtract 3.6 volts from 14.5 volts the maximum expected voltage in the vehicle. 14.5 – 3.6 = 10.9 volts. This is the voltage we need to drop across the series resistor. Using Ohms law (Voltage= Current x resistance) or R= Voltage / Current (14.5-3.6 / 0.20) = 545. We find a 545 ohm resistor is needed. The closest standard value is 560 ohms. The next closest value is 510 ohms. A 510 ohm resistor is color coded green, brown, brown. If we choose the 510 ohm resistor we will find that the current thru the LED will be a little higher than 20 ma at 14.5 volts (10.9/ 510) = 21.3 ma. If we use the 510 ohm resistor and the voltage goes down to a low of 11.5 volts the current through the LED will go down and be (11.5-3.6 / 510) or about 15.5 ma. The perceived brightness of the LED will probably not change with the voltage changes. Our only other concern is the power dissipated by the series resistor. Power = current 2 x resistance so (.02*.02)*510= 0.2 watts. With this information we know that we can use a ¼ watt 510 ohm resistor in series with our blue LED to have it operate properly over an operating voltage from about 11.5 volts up to 14.5 volts. The only thing that needs to change with using different color LED is the value of the series resistor as long as the voltage stays approximately the same.
The value of the resistor is actually not that critical in this application. For example if we substitute a RED LED for the BLUE LED and still use a 510 ohm resistor the current thru the LED at 14. 5 volts is:
14.5- 1.7= 13.0 volts 13.0/ 510= 0.0254 Amps or 25.4 ma. This is an acceptable value of operation for a red LED. The power dissipated by the resistor will be (0.254)2*510 = 0.33 watt so a ½ watt resistor should be used to be safe.

If you need to increase the light output you can wire two or more LEDs in parallel. Remember if you put two or more in parallel each LED will draw about 20ma. You will have to change the size (OHMS) and wattage of the resistor to provide the correct current for the LEDs. LEDs can also be wired in series, however to get uniform brightness special LED driver chips are usually employed.
1978 300 D 258,000 + miles sold 1995
1983 300 D Turbo 152,000 miles
Thank God for the man who put the white lines on the highway
Reply With Quote