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Protecting Led's in Railtruck

1859 Views 10 Replies 8 Participants Last post by  billsharron
Hi Guys,

I am in the middle of battery powering my Bachmann Railtruck. I have decided to scrap the Bachmann circuit board and so will need to protect the led headlights and tailight. I know I need to reduce the voltage to the Leds but have no idea what to use or what size. Resistor? Diode? Do I wire all three in series or parrallel?

There was some kind of LED primer posted but searching on Led just doesn't work. Any help from you electronics experts will be appreciated!!
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What is your battery voltage Bill? You will need one resistor in series with each LED.
Paul Norton posted some nice sites... search on "led calculator"

here's the links:

simple answer: take your voltage and and divide by 0.02, that will give you your resistance in ohms.

use 1/2 watt resistor

You are safe with this calculation.... you can get fancier, but in 99% of the cases this works great.

Regards, Greg
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Not to nitpick Greg, but didn't you mean multiply by .02?
The voltage in the battery when fullly charged is !5.3, Nominal voltage is 14.4. The calculators Greg refers to are great but I have no idea what the headlight voltages are or the voltage of the taillight. Because of this I cannot use the calculator. I have the wiring diagram but it includes no information helpful in this situation. When I am done with the installation I want all lights on both in forward and reverse, so I am still stuck. I was planning to connect the output of my receiver to a resistor or diode then to the three positive leads of the leds, with the negative leads connected to the headlight screw on my receiver with a jumper over to my Rearlight screw. This I figured would allow all three leds to be on regardless of wether I was going forward or reverse.

Greg If I understand your simple method and my voltage is 15.3, I would need a 300 ohm resistor. Is that correct.

Does the resistor always get connected to the positive lead on the led or does it matter??
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I would put a 620 to 700 ohm resistor in series with each LED. The will keep the current to 20ma or less. The resistor can be in either the positive or negative lead since it makes no difference. I am planning on doing the exact same thing with my truck. I have already removed the circuit board.

V = IR .... or if you want to solve for resistance R = V/I = volts divided by current. Current is 20 milliamps, or 0.02... divide voltage by current.... 15 volts divided by 0.02 = 750 ohms....

If you multiplied 15 times .02 you get 0.3 ohms that is clearly not right.

Regards, Greg
If the 3 leds are always on together, I would place them in series and use 1 resistor/ Esp since this is a battery operation, series cuts the current load by 1/3 for 3 leds.
since this is a rail truck, the headlight and taillight can be on together.
I've done this, just can't remember exactly what I used? Sorry, If I get out there today I'll give it a look over! I did use 14.4v and RCS. and did not change out led's for GOW.

your best bet for a battery system is to use the same 20 mA for all three LEDs by wiring them in series. They are polarity sensitive but if you get one backwards, no harm done. They just won't work.

The regular headlights are weak in the extreme, you can change them out to warm white LEDs and get much better results


White LEDs run about 3 volts each, the red one in back about 1.5 volts. This makes 7.5 volts total or about half of your battery voltage. You'll need to drop the other 7 volts in a 330 to 390 ohm resistor, 0.25 watt resistor will work fine.

I wrote and LED primer, maybe or maybe not the one you were looking for

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Thanks to all for the great info!! George I do believe it was your primer. Greg, Your references were very helpful.
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