KATO Sources for Decoder Installation in various locos

[OldEastRR]

I have this RDC -- wait, now I have two -- with decoders installed and they are driving me crazy. The problem apparently is the wiring to the dual LEDs at each end. The light circuit from the decoder drives two LEDs instead of one -- simultaneously powering one LED from each end alternating with the other two on direction change. The resistors on the factory boards overheat massively, now having ruined two RDC shell ends and parts. But what makes it insane is they'll overheat when the decoder is set for one direction, but not heat up in the other direction!!
I'm wondering if KATO or anybody has instructions on how to change this system from the DC set-up to the DCC one by hard-wiring. Not some YouTube thing but ones direct from KATO or some decoder mfgr.  I know Digitrax sells a complete set of decoder plus new LED boards (have no idea how you "drop in" that on a KATO RDC) but I've already got decoders in the RDCs.

[peteski]

Didn't we already cover your Kato RDC installs?
https://www.therailwire.net/forum/index.php?topic=41455  You called it a "win" at this end of this thread.

The Kato RDC DC wiring is pretty much unique.  You will not see the lights hooked up that way on any other model.  IIRC, there are 1 or 2 screws on each of the light boards. Those act as electrical jumpers and will have to be moved from DC to DCC locations. Then the wiring has to be modified slightly (a wire has to be added).  I don't have one of these handy so I can't give you any specific details.  But remember, the difficulty is only with the way RDCs were designed by Kato, and only because Kato got fancy with having the headlights and markers functional.

[peteski]

Easy way out (as you already know) is to simply buy the Digitrax decoder specifically made for this application.

[OldEastRR]

The screw-jumpers are a nice idea, but they don't seem to keep the resistors from overheating. Because it's just a 2-function decoder, the "front light" command will light up one red LED at one end and a white LED at the other. In reverse, the "rear light" comes on which powers the other 2 LEDs.  Only one LEDset can be on at the same time. Can the front and rear light commands each run 2 LEDs instead of one like this, or do I need to add another resistor to each circuit?

[peteski]

The resistors should not overheat (unless Kato uses resistor values that are too low and your DCC voltage is on the high side). But I can't give you any exact info unless I dig out one of my RDCs and take it apart.  Or if you tell me what is written on the resistors or what are the color stripes on them, that might help. Or take a photo which shows this info and post it here.

[OldEastRR]

The SMD resistors are "271" which is a 270 ohm w/ 5% tolerance. This is the same one that I see on all their E8 lightboards, maybe all KATO light boards. But THOSE don't overheat and melt the shell on DCC, do they?  Anybody have that happen to a KATO E8 or anything KATO on DCC?

[peteski]

Ah, so those are rather low value.  I don't have any factual info as to why Kato uses rather low value resistors, but my hypothesis is in https://www.therailwire.net/forum/index.php?topic=42126.msg547159#msg547159 . In that instance the resistor is 560 ohm with a white ED, but it will also apply in your case (with RDC light boards and the 270 ohm resistor).

With full 12V applied at all times (since it is DCC), with 270 0ohm resistor the current will be fairly high for the LEDs (around 37mA for red and yellow LEDs). The resistor will be dissipating quite a bit of heat.  For DCC operation I would replace the 270 with a 680 ohm resistor. That will still pass 15mA through the LED which should be plenty bright, but will run much cooler. 750 ohm resistor would still pass 13mA but dissipate still less power (run cooler). If you were to replace the headlight LED with a white LED then I would use a 1000 ohm resistor (since the white LEDs are much more efficient than red and yellow).

[woodone]

Just so I am clear about this- This N scale? What decoder are you using?
If N scale , I have used the decoder from Digatrx and have had no problem with heat.

[OldEastRR]

Ah, so those are rather low value.  I don't have any factual info as to why Kato uses rather low value resistors, but my hypothesis is in https://www.therailwire.net/forum/index.php?topic=42126.msg547159#msg547159 . In that instance the resistor is 560 ohm with a white ED, but it will also apply in your case (with RDC light boards and the 270 ohm resistor).

With full 12V applied at all times (since it is DCC), with 270 0ohm resistor the current will be fairly high for the LEDs (around 37mA for red and yellow LEDs). The resistor will be dissipating quite a bit of heat.  For DCC operation I would replace the 270 with a 680 ohm resistor. That will still pass 15mA through the LED which should be plenty bright, but will run much cooler. 750 ohm resistor would still pass 13mA but dissipate still less power (run cooler). If you were to replace the headlight LED with a white LED then I would use a 1000 ohm resistor (since the white LEDs are much more efficient than red and yellow).

Oh.. aha.... I DID change out the yellowish LEDs for pure white ones, maybe that's the problem. A 1K ohm resistor per circuit board would seem to be the answer.

[peteski]

Oh.. aha.... I DID change out the yellowish LEDs for pure white ones, maybe that's the problem. A 1K ohm resistor per circuit board would seem to be the answer.

Yes, that will reduce the heating. I would still replace the red LED resistor with the one I mentioned. The red LED is also lit when the headlight is lit and its resistor will generate heat too.

[OldEastRR]

There's just one resistor on each board, as only one light at a time on it is lit.

[peteski]

There's just one resistor on each board, as only one light at a time on it is lit.

Ah, like I said I have not looked at my RDCs for a while. Then as you already know, a 1k resistor works well and much cooler.

[woodone]

There's just one resistor on each board, as only one light at a time on it is lit.

On your OP you stated that there was a front light and a rear light. Am I correct?
And am I correct in that this is N scale? which decoder are you using?

[jdcolombo]

On your OP you stated that there was a front light and a rear light. Am I correct?
And am I correct in that this is N scale? which decoder are you using?

Hi Woodone.

I put the decoder in for Old East.  Yes, this is N scale.  The decoder is a 5th-generation Lenz Silver Mini.

I think Peteski identified the problem.  The very-low value resistor is sinking too much voltage; Old East has a Digitrax Zephyr system, which puts out about 14v to the track.  The Lenz is probably supplying about 12.5v to the lighting circuits after the voltage drop for the bridge rectifier, and that plus the low-value resistor is likely the problem.  Kato probably figured that no one would be running these lighting circuits at 12v (the RDC would be going 200mph at that voltage, given Kato's motor) and picked a low-value resistor so that the lights would look right at a much lower voltage (e.g., 6v).  So I'm inclined to think that the low-value resistor is the problem.

The way the lighting works with the RDC is that both the front and rear have two LED's - one red, and one "white."  The original "white" LED was more yellow than white, so we replaced that with a 3mm sunny white LED from Richmond Controls.  I didn't even think about the dropping resistor value when I did the Lenz installation.  The Lenz is wired so that the white wire lights the white LED in front and red LED in rear when the unit is moving forward, and the yellow wire is just the opposite (red front light and white rear light when the RDC is running in reverse).  Each light board has one resistor that feeds the active LED.

I think that replacing the resistor with a 1K would solve the problem; we'll see.  I wonder if the Digitrax decoder that is made for the RDC outputs 12v to the lighting circuits, or if it has its own internal voltage limiting.

John C.

[peteski]

I think that replacing the resistor with a 1K would solve the problem; we'll see.  I wonder if the Digitrax decoder that is made for the RDC outputs 12v to the lighting circuits, or if it has its own internal voltage limiting.

John C.

Digitrax decoder designed for the Kato RDC actually consists of 3 boards: the decoder itself and 2 hard-wired LED boards which replace the Kato boards. The resistors are whatever Digitrax chose. I also have one of those installed in one of my RDCs but I don't recall the resistor values.