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Discussion Starter #1
Hello,
I am using a Digitrax DN166PS Decoder for my scratch built locomotive. I bought some LED’s from a website called evandesigns.com. Each LED came with a resistor leading into a large rectifier with a capacitor connected to its output in parallel with the LED. All of these components take up too much space in an incredibly small locomotive.
It is my understanding that decoder function outputs are DC. If so, would it be possible to remove the rectifier and just use the resistor? If I do this, what should I do with the capacitor?
Thank you for all of your time and help. I am new at this whole thing.
Eric
 

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I'm a little confused but maybe this is the park. I bought a tsunami 2 pnp decoder. It has several 12v dc outputs for lights. With an led and resistor it worked. The led also worked without the resistor for like 5 seconds and then burned out. Lesson learned. So it would be easiest if you know the voltage out on the decoder, have a known led in hand ... Then you can calculate a good resistor value and wire it all up. Another way is to guess ... pick a large resistor then work down until it seems about right. But this may not work well since you might destroy all the LEDs or resistors you have on hand. (I got that the pnp had 12v for lights to allow for incandescents...)
 

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Discussion Starter #3
I'm a little confused but maybe this is the park. I bought a tsunami 2 pnp decoder. It has several 12v dc outputs for lights. With an led and resistor it worked. The led also worked without the resistor for like 5 seconds and then burned out. Lesson learned. So it would be easiest if you know the voltage out on the decoder, have a known led in hand ... Then you can calculate a good resistor value and wire it all up. Another way is to guess ... pick a large resistor then work down until it seems about right. But this may not work well since you might destroy all the LEDs or resistors you have on hand. (I got that the pnp had 12v for lights to allow for incandescents...)
Interesting. I was under the impression that the voltage out on the decoder reflected the voltage of the track. If not it makes this a bit easier. I’ll take another look at my decoder documentation. Thanks!
 

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Some decoders have built in LED resistors, others are 12 and you supply the resistor. I would try a 1k resistor. You don't want the diode or the cap connected, so you can remove them. Any idea of the ohms of the resistor used?
 

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Interesting. I was under the impression that the voltage out on the decoder reflected the voltage of the track. If not it makes this a bit easier. I’ll take another look at my decoder documentation. Thanks!
The voltage IN is always what the track is running, but the point of a decoder in the first place is to 'meter' outputs to achieve the functions the way the operator wants them to work. For the motor, the 15 volts AC is first rectified, and then metered out in small increments of parts of a volt so that something like inertia in CV3 will work.

Sound and lights are the same; voltage to the amplifier, and thence to the speakers, affects volume, while variable voltage to the lights affects how bright they are, either by 'dimming' when stationary, or so that the user can adjust brightness, maybe to delay the demise of the filaments in incandescent bulbs.
 

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The lighting power out of a DCC decoder is 12v DC...
You would need a resistor in one wire to the LED.
NOTE: The BLUE (light common) wire from the
decoder is POSITIVE, thus it goes to the 'long'
wire on each LED. The resistor to the 'short'
LED wire... connect to White decoder wire for Headlight,
Yellow decoder wire for rear light.

Disconnect any capacitors before connecting to
decoder.

Don
 

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Discussion Starter #8
The lighting power out of a DCC decoder is 12v DC...
You would need a resistor in one wire to the LED.
NOTE: The BLUE (light common) wire from the
decoder is POSITIVE, thus it goes to the 'long'
wire on each LED. The resistor to the 'short'
LED wire... connect to White decoder wire for Headlight,
Yellow decoder wire for rear light.

Disconnect any capacitors before connecting to
decoder.

Don
My decoder documentation states that “If the track voltage applied exceeds the operating parameters of the locomotive and it’s LEDs/lamps, damage to the locomotive or decoder may occur.” And says nothing about the output voltage to the lights, which is why I believed the voltage carried over. Is the 12V output a standard for decoders? I just don’t want to assume it’s 12 and then blow my lights if it’s 14 or so like the track.
Thank you,
Eric
 

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Discussion Starter #9
Some decoders have built in LED resistors, others are 12 and you supply the resistor. I would try a 1k resistor. You don't want the diode or the cap connected, so you can remove them. Any idea of the ohms of the resistor used?
The lights on the site say that regardless of color, they are 3 volts at 20 mA each. This tells me for a 12 volt supply I need a resistor of 450 ohms? The single LED’s have an 820 ohm resistor and the three lights In parallel for the firebox have a 150 ohm resistor.
 

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Eric

Yes, the Decoders are made to NMRA standards, so all have
the 12 V DC light output. The 'over voltage' mentioned in your
manual refers to track voltage. This also is a NMRA standard.
Every DCC controller puts approximately 14 V sqarewave AC
on the track at all times. (the speed controller does NOT vary
the voltage). To vary the speed of the loco motor, the controller
sends digital information to locomotive decoders. The decoder
uses the digital information and varies the DC to the motor
for speed control. (The decoder rectifies the AC track voltage
to provide that motor power and light power)

So observe these factors and you can confidently connect
your LEDs.

Don
 

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Discussion Starter #11
Eric

Yes, the Decoders are made to NMRA standards, so all have
the 12 V DC light output. The 'over voltage' mentioned in your
manual refers to track voltage. This also is a NMRA standard.
Every DCC controller puts approximately 14 V sqarewave AC
on the track at all times. (the speed controller does NOT vary
the voltage). To vary the speed of the loco motor, the controller
sends digital information to locomotive decoders. The decoder
uses the digital information and varies the DC to the motor
for speed control. (The decoder rectifies the AC track voltage
to provide that motor power and light power)

So observe these factors and you can confidently connect
your LEDs.

Don
Don,
This is awesome and puts me at ease. I know exactly what resistors to get now.
Thank you so much,
Eric
 

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The 20 ma is for max brightness, probably not what you want. Try it out, 820 ohms should work just fine at 12v dc. make sure the LED is connected as Don R suggested. If you can't figure out which lead on the LED the anode/cathode, one way it will light, the other way it won't (and it won't harm anything). it makes no difference which lead has the resistor, just don't think you can get away with using a single resistor connected to the blue common lead, that will give you fits.
 

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I've used those also. Their biggest problem is the brightness. really need an adjustable current instead of straight 20ma. its just too bright.
 
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