G Scale Model Train Forum banner

1 - 11 of 11 Posts

·
Premium Member
Joined
·
189 Posts
Discussion Starter #1
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!!
 

·
Super Modulator
Joined
·
20,517 Posts

·
Premium Member
Joined
·
189 Posts
Discussion Starter #5
Del,
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??
 

·
Premium Member
Joined
·
658 Posts
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.
 

·
Super Modulator
Joined
·
20,517 Posts
Dave:

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
 

·
Premium Member
Joined
·
3,883 Posts
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.
 

·
Registered
Joined
·
213 Posts
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.

cale
 

·
Registered
Joined
·
811 Posts
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

http://www.girr.org/girr/tips/tips8/bachmann_railtruck_tips.html

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


http://www.girr.org/girr/tips/tips7/white_led_tips.html
 

·
Premium Member
Joined
·
189 Posts
Discussion Starter #11
Thanks to all for the great info!! George I do believe it was your primer. Greg, Your references were very helpful.
 
1 - 11 of 11 Posts
Top