KATO Sources for Decoder Installation in various locos

[woodone]

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.
Well I was not sure has to what we were working with  I have installed many of the RDC decoders that Digitrax makes. I have one spare decoder stuffed into

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.
Well I was not sure has to what we were working with  I have installed many of the RDC decoders that Digitrax makes. I have one spare decoder stuffed into my decoder stock and will look to see what value Digitrax used with their decoders for this.

John C.

[woodone]

I was trying to find out for sure has to what scale unit we were working with.
I have done several RDC N scale Kato DCC installs using the Digitrax decoder. As a mater of fact I have one in my supplies drawer.
Digitrax is using a very small resistor (physical size) 402?  Marked 751. one on each LED. Same for the interior LED.

[jdcolombo]

Thanks, Woodone.

It's probably a 1/8 or 1/4-watt surface-mount.  The 751 means that it is 750 ohms, which is close enough to 1K for me.  It is interesting that they use a resistor on each LED; Kato uses only one and relies on the diode polarity to light the correct one (if I remember correctly - I don't have an RDC).

I think we're going to replace the resistors on Old East's RDC's with 1K, 1/4-watt surface-mounts and see what happens.

John C.

[peteski]

John, I believe that you are correct about the stick Kato light boards. IIRC, each light board has reverse-parallel connected pair of LEDs in series with a resistor. That is ideal for DC powered models. When converted to DCC the screws (which act like jumpers) are moved in such a way that the LEDs are spearated but still have single comon resistor. That way each LED can be wired to separate function (white and yellow wires). Then the blue wire is also wired to the resistor.  It is an ingenious but also complicated design.

Woodone, I believe that a 750 ohm is a standard value Digitrax uses for their on-decoder current limiting resistors (not just on the RDC decoders). Like John, said, it is high enough to be safe.   But I question the 0402 size.   As the chart here shows, 0402 resistors is rated for only 1/16 W. That is too small for this circuit. A (0805) 1/8W or even  better (1206) 1/4 W resistor would be more suitable.

[woodone]

Peteski,
I just did a measurement of the SMT resistor used by Digitrax on the decoder I have in hand. It is an 603.
And the light boards get their power from the rail not by the blue wire.
What screws are we talking about?
I must have installed over 2 dozen or more of these . I don’t recall seeing those? Maybe a newer run?
Anyway I hope JOHN gets the problem solved.

[peteski]

Peteski,
I just did a measurement of the SMT resistor used by Digitrax on the decoder I have in hand. It is an 603.
And the light boards get their power from the rail not by the blue wire.
What screws are we talking about?
I must have installed over 2 dozen or more of these . I don’t recall seeing those? Maybe a newer run?
Anyway I hope JOHN gets the problem solved.

Kato only made one run of the N scale RDCs.
Yes, the original Kato light boards get power from the track pickup strips in the DC mode. For DCC you have to move couple of tiny screws which act like electrical jumpers and then you solder blue, white, and yellow wired to each board (from the decoder).  The screw jumpers reconfigure the LED board to be compatible with DCC.

But we have probably moved onto a conversation about the Digitrax decoder specifically designed for RDC. I don't recall the specific about it but it sounds reasonable that the LED board still picks up power from the track via the metal strips and then each board probably has the white and yellow wired coming from the decoder.

A 0603 resistor is rated 1/10 Watt so they are really pushing it.  I prefer to only load resistors to about 60% of their rated power dissipation, or less.

I think that OldEastRR's overheating problem will be solved by using a 1k resistor.

[woodone]

Yes,  we are away from the OP. The 1K resistor will cure the problem (We hope).
Has for Digitrax using a 603 sized SMT resistor might be to the fact the there is only about 7 volts being used ( half wave) from the rail?
Therefor the duty cycle is only 50%.

[peteski]

Yes,  we are away from the OP. The 1K resistor will cure the problem (We hope).
Has for Digitrax using a 603 sized SMT resistor might be to the fact the there is only about 7 volts being used ( half wave) from the rail?
Therefor the duty cycle is only 50%.

That is a very good point!