I have a locomotive that I converted to TMCC using ERR Cruise Commander. I am in the process of installing LED headlights. The LEDs that I have are 3volt. What size resistor do I need so that I don't blow them up?
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To use LED's with the Mini Commander LC1 and LC2 outputs, you will need a series resistor and diode connected to the LED. The Mini-Commander outputs 1/2 of the supply voltage on LC1 and LC2. Each output has a different polarity, thus choose the circuit above depending on what output the LED is attached to.
The series resistor is calculated as follows:
Example 1:
Source Voltage = 9 volts
Voltage Drop = 3.6 volts (Typical for a blue or white LED)
Desired Current = 20 milliamps (a typical value for White)
To calculate the resistor you need:
(9 - 3.6) / ( 20 / 1000 ) = 270 ohms
Example 2:
Source Voltage = 9 volts
Voltage Drop = 1.4 volts (Typical for a Red LED)
Desired Current = 15 milliamps (a typical value for Red)
To the calculate the resistor you need:
(9 - 1.4) / ( 15 / 1000 ) = 510 ohms
This is from the ERR website.
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Tom,
When we do upgrades, we use Evans Design Leds. Pre-wired and easy installations plus they are forum sponsors.
Bill
Thanks guys!
Bill, I am using Evans LEDs, but they are loose 3mm, 3 volt bulbs. No circuitry attached.
Dave, I am using the Cruise Commander. I assume that it has the same specs as the Mini Commander. The only thing that is throwing me off at this point is the voltage. The transformer (Z4000) is set at 18 volts. The track voltage is 17.84VAC. I am getting a reading of 17.72VDC from the light terminals on the board. Is this correct? Do I have something hooked up incorrectly or is there a problem with the board? I would think that even with the recorded voltage I should be okay if I use the correct resistor.
Tom
Since this loco has dual headlights, I connected the LEDs in series. After I crunched the numbers I came up with a value of 550 ohms. The hardest part was digging through my box of resistors to find the correct ones. Works like a charm!
Thanks, again for the help.
Tom
Post up some pictures!
Won;t the led convert ac to dc on its own without the extra diode in series?
When I tried it without the diode the lights flickered. Installed the diode and everything worked fine. Don't ask me why. Thats above my pay grade. (I used to be a brakeman!)
Tom
The reason for the diode is an LED has a maximum reverse voltage rating in the 5-7 volt range. It's pretty easy to exceed that in our environment and cook the LED. A 10 cent diode protects the LED.
GLAD YOU GOT IT SORTED TOM, I TOO USE EVAN DESIGN PREWIRED LED'S, THEY ARE SO NEATLY PACKAGED THEY ARE GREAT VALUE.
The next time I will remember to use the pre-wired LEDs. It sure would have made my life a lot easier.
Tom
Once you know how to wire them, it's no problem to roll your own. You get the advantage of a custom installation and location of the resistor and diode. If you use any quantity of them, it's also a lot cheaper.
