Can an (Unmodified) Lightboard with LED Remain When Installing Decoder?

[tehachapifan]

I'm converting an old, non-DCC-ready Kato SD40 to DCC and already successfully installed a LokPilot. Working on lighting now and realized I could maybe keep one of the original end lightboards (that take power straight off the frame) in for the numberboards vs. reinventing the wheel. Would this work on DCC power OK? Could it damage the decoder? I realize it would always be on with track power but that's OK.

[jagged ben]

I would just cut the traces and solder the LED to a function. 
That said, I think your idea should also work and it should not damage the decoder.

[tehachapifan]

I would just cut the traces and solder the LED to a function. 
That said, I think your idea should also work and it should not damage the decoder.

Thanks for the reply! I normally do the trace cut thing but I'm being a bit lazy on this loco. Plus, these old, bulky Kato drives don't leave a lot of room for running wires.

[peteski]

Not sure what you would think that the LEDs on the original light board (and powered directly from the track) would damage the decoder.  That LED circuit should be isolated from the decoder, and only connected directly to the frame halves (the track power). Decoder is not  involved. Or maybe I'm misunderstanding your question?

[Scottl]

@GaryHinshaw has a detailed thread on modifying the original Kato board for Lok Pilot and using the factory lighting. 

[tehachapifan]

Not sure what you would think that the LEDs on the original light board (and powered directly from the track) would damage the decoder.  That LED circuit should be isolated from the decoder, and only connected directly to the frame halves (the track power). Decoder is not  involved. Or maybe I'm misunderstanding your question?

This would be the first time I ever attempted to leave any other type of (complete) circuitry in place with a decoder install and never really saw anyone else do it (trace cutting surgery aside). I didn't think it would be an issue but wasn't absolutely positive the decoder wouldn't see it as a short or something. Are we saying there is such a thing as a stupid question?

@GaryHinshaw has a detailed thread on modifying the original Kato board for Lok Pilot and using the factory lighting.

Yes, I've done trace cutting on installs many, many times.

[mu26aeh]

Here's an example installing decoder using existing DC boards

http://n-scale-dcc.blogspot.com/search/label/Loco%3A%20Kato%20SD40%20or%20SD45

[tehachapifan]

I did throw the loco on the test track and the unmodified lightboard works on its own with no connection to the decoder. So, this seems to work and cause no obvious issues with the decoder. Yes, the light stays on continuously but that was to be expected.

What a study on communication this thread was!

[peteski]

This would be the first time I ever attempted to leave any other type of (complete) circuitry in place with a decoder install and never really saw anyone else do it (trace cutting surgery aside). I didn't think it would be an issue but wasn't absolutely positive the decoder wouldn't see it as a short or something. Are we saying there is such a thing as a stupid question?

Absolutely not!
I didn't think that my reply implied that the question was stupid, and I explained in more detail that the factory LED circuit is totally independent from the decoder, only sharing the same source of power (from the track, through the truck pickups, to the frame halves). Just as if you were to plug in a lamp and a radio into the same AC outlet. They both get power from the same outlet, but are unaffected by each other.

The factory light board works from DCC power (and is non directional) because DCC is roughly a 12V signal which switches polarity many times a second.  The factory installed LED is actually flashing very fast (since it only lights up on the part of the DCC signal that supplies correct polarity to the LED).  The persistence of human vision sees the LED as flowing steadily. You might want to replace the original resistor with a higher value one if all that LED does is illuminate the number boards as the LED might glow too brightly for that.

[tehachapifan]

All good.

The concern for me was that the separate (unmodified) LED board would potentially create (thru its circuitry) a bridge between frame halves that doesn't typically exist with many/most DCC installs and if this would make the decoder cranky.

A separate concern I didn't really bring up here is if an old school lightboard will operate in the long term on DCC power straight off the frame. I have vague recall to there possibly being a need for additional components to protect the LED.

Edit: Looks like you added to your reply when I typed mine. Turns out the original LED still illuminates really dim, so it will work perfectly for numberboards.

[peteski]

All good.

The concern for me was that the separate (unmodified) LED board would potentially create (thru its circuitry) a bridge between frame halves that doesn't typically exist with many/most DCC installs and if this would make the decoder cranky.

A separate concern I didn't really bring up here is if an old school lightboard will operate in the long term on DCC power straight off the frame. I have vague recall to there possibly being a need for additional components to protect the LED.

The only bridge between the frame halves that would affect the decoder (and everything else sitting on the track at that time) is a dead short. It would hopefully trip the breaker in the booster and shut the power off.  When the loco sits on the track the frame halves become electrically part of the track.  Any devices connected in parallel to the frames (and to the track, as in another locomotive or illuminated passenger car) are all part of the same electrical circuit.

As for the LED safety of the DC light boards powered from DCC signal, that depends on the DC board's design.  There multiple variations. Some will be safe from damage (to the LED) and some might need additional component (like a diode).  Others already have the extra diode installed. I would have to see the one you have to be able to tell.  And as I mentioned, you might find it too bright and want to replace the factory resistor with a higher value resistor.

[tehachapifan]

...Any devices connected in parallel to the frames (and to the track, as in another locomotive or illuminated passenger car) are all part of the same electrical circuit....

This is the exact same conclusion I came to last night but wanted to double check that I wasn't missing something before I applied power. I've found that you don't get a lot of warning or second chances with a DCC installation error!

...As for the LED safety of the DC light boards powered from DCC signal, that depends on the DC board's design.  There multiple variations. Some will be safe from damage (to the LED) and some might need additional component (like a diode).  Others already have the extra diode installed. I would have to see the one you have to be able to tell.  And as I mentioned, you might find it too bright and want to replace the factory resistor with a higher value resistor.

This is an old-school lightboard (the little ones that go on each end of a (non DCC ready) frame and it only has the LED and a resistor on it. These old Kato LED boards were always pretty dim, by my standards, and is still really dim on DCC. So, it's perfect for numberboards.

[peteski]

This is an old-school lightboard (the little ones that go on each end of a (non DCC ready) frame and it only has the LED and a resistor on it. These old Kato LED boards were always pretty dim, by my standards, and is still really dim on DCC. So, it's perfect for numberboards.

If it's that old, then it probably has a yellow LED on it (not white).  White LEDs were introduced later. In that case it is safe to use with DCC power.  Those LEDs have a high enough maximum reverse voltage not to be damaged by the 12-14V DCC power.  If it was a newer white LED then I would have recommended adding a protection diode (and a higher value resistor) as white LEDs have rather low maximum reverse voltage, which if exceeded can let the smoke out of the white LED (while not affecting other devices connected to the DCC track or frames)

[tehachapifan]

Yep, it’s a big, old school, dim and very yellow LED. I usually can’t wait to replace those but, in this application, it’s perfect!