LEDs and DCC for Kato Hokutosei

[ATShinkansen]

So I’ve picked the Hokutosei coaches as my next DCC project (these are the ones that come with the EF81).  The big catch on it, though, is that the bulbs are incandescent, the only such case among my N scale fleet.  I’ve stripped out the interior light bulbs, as interior lighting is low on my priority list, focusing only on the tail lamps for now.  My big question is if the light boards for these bulbs are useable  and/or worth the time and effort for LEDs.  I’m using Kato FL12 decoders, and the LEDs came off one of my Digitrax DN163K0a decoders.  Also, I know that resistors are needed in the LED circuit but the ones I bought according to the ratings I found look to be a bit on the large size.  Could someone confirm in the picture below if what I bought is correct for this kind of use?  They’re 1,000 Ohm, 1/2 Watt.

 

Thanks

[Kiha66]

1k ohm should be more than enough.  1/2 watt is the proper size, but 1k ohm may end up with the bulbs being a bit dim.  Calculating it out the suggested resistor would be a 470 ohm, while kato seems to use 270 ohm (in the first lightboard I could find).  I've used modified kato boards in the past with the 270 ohm resistor with no issues. 

[kvp]

I would like to add that DCC voltage is usually higher than the DC 12V the models are made for.

 

If you use this link: http://led.linear1.org/1led.wiz

 

With 14V supply, 3.3V led (white) @10mA (mini sun setting), you'll get 1.2 kohms (1200 ohms) @1/4 watts.

 

If you factor in the 1.2-1.4V drop of the diode bride, which the bulb holders miss and they have to be added, then you get 1000 ohm @ 1/4 watts.

 

This means, the tail lights will be ok with 1000 ohms, since they have the protection diode but you can use 1/4 watts resistors, around half the size of the 1/2 watts ones.

 

The interior lights will also need rectifier bridges and resistors to support the leds.

 

ps: Anything below 1000 ohms would be near or above the maximal current of the leds and will shorten their lifespan. (1.5-1kohm sets the current to 10-15mA depending on led color and voltage, below the usual 20mA maximum value)

 

270 ohms @ 14V sets the current to 50mA, well above absolute maximum and the resistor dissipation to 2 watts!

[ATShinkansen]

Okay.  Would the 1/4 watt resistors work if I do away with the board and wire the LEDs directly to the decoder?  The incandescent boards have little to nothing on them compared to the LED boards.  This is what they look like (top and bottom):

[Kiha66]

12 hours ago, kvp said:

270 ohms @ 14V sets the current to 50mA, well above absolute maximum and the resistor dissipation to 2 watts!

 

Interesting, Could the fact that DCC is a square wave and thus the LED is only active 1/2 the time make a difference?  Quite a few of kato's lightboards use this resistance and so far I havent had any issues with them when converted to DCC.  Perhaps the decoders they suggest/I've used have current limiting built in?

[ATShinkansen]

One more thing: which contact is the positive one at each end of the FL12?

[kvp]

10 hours ago, Kiha66 said:

 

Interesting, Could the fact that DCC is a square wave and thus the LED is only active 1/2 the time make a difference?  Quite a few of kato's lightboards use this resistance and so far I havent had any issues with them when converted to DCC.  Perhaps the decoders they suggest/I've used have current limiting built in?

Most leds have a reverse breakdown voltage around 5V. Anything above that with the wrong polarity and the led will burn out. This means that without a diode bridge rectifier or in case of a direction dependent light a half wave rectifier, the leds will get destroyed over time by reverse polarity both in DC and DCC. Internal led lights are usually powered through a full rectifier bridge to get light regardless of the track polarity. This also works well with DCC.

 

Never power a led from an AC source or DCC inverting square wave without a protection diode or bridge rectifier.

 

ps: Some DCC decoders use chopped powering, when the leds are connected to one rail and ground is added from the decoder. This generates a 50% pwm wave but could damage the leds over time and centrals with zero stretching (address 0 mode) could burn out these lights. Wired Digitrax decoders are known to supply the blue wire (rectified power) to avoid this and afaik all Kato decoders supply rectified power through the decoder.

 

Afaik the FL12 (and all Digitrax and most DCC decoders) use low active lights and auxiliary riving and that means the non switched end is the power.

[kvp]

10 hours ago, GCRailways said:

Okay.  Would the 1/4 watt resistors work if I do away with the board and wire the LEDs directly to the decoder?  The incandescent boards have little to nothing on them compared to the LED boards.  This is what they look like (top and bottom):

 

Please keep the boards as they have the protection diodes already on them. I would move the diode to the other two holes with reversed polarity. Add the 1000 ohm, 1/4 watt resistor in the old location of the diode and connect the unconnected new diode pad to the bulbs wire pad (with a solder bridge), and add the new led to the board in place of the bulb. This would create a led parallel with the reverse direction protection diode and both series with the current limiting resistor circuit. Pretty much what you need for a led head/tail light with DC power.

[ATShinkansen]

Infinite apologies for the unacceptable delay, but I wanted to post the finished results: