Why only 270 ohm LED resistor in Kato diesel?

[mmagliaro]

I am back inside my Kato E8 + Broadway Limited special-run SP&S shell hybrid engine.
I am going to put an NGineering MARS light simulator in it, and while I'm at it, I put a little bridge
rectiifier in place to the headlight and MARS will function in forward and reverse.

When I pulled off the Kato circuit board, I removed the little SMD resistor they have on there for the LED feed.
Out of curiosity, I noted that the number was 2701 and I measured it.  Sure enough, it is only a 270 ohm resistor.

12 v across a 270 ohm resistor will draw 44 ma which will promptly blow
up most LEDs.  Even allowing for a 3v forward voltage drop in the LED, we still get 9v and a 33ma current, which is
still way too high for the LED to sustain.

My question:  How on earth does this work?  Is Kato just banking on the fact that almost nobody will run
the engine on a full 12v, ever? 

I'm going with 620 ohm myself.  It's plenty bright enough, and keep the current under 14 mA.

[peteski]

Not sure about your specific example but I have seen 270 ohm resistors used in many older Kato locos.  Usually those were 1/4 W through-hole resistors.  Those were also used with their "lemon yellow" (not white) LEDs.  Those were the T-1 size (3mm) leaded LEDs.

Yellow LEDs drop 2V across them under normal operating conditions.  SO maximum current would be (12-2)/270=37mA

Also, those older leaded LEDs were much less efficient - they needed much more current than today's white LEDs.  And also while I agree that 37mA current is a bit high, even for a T-1 size LED, I suspect that Kato was counting on people not running their models at 12V for extended period of time.  After all, Kato models are speed demons - most people do not run them at more than 6V.    Then, even at 37mA, a T-1 LED would not burn out - it would just get warm.  So would the 1/4 W resistor.   

EDIT:

Mystery solved.  I looked through my stash of light-boards removed for DCC installs and I think I found the same board you have (marked 3010 and with LED on one end and a regular diode on the other end).  It has a larger SMT resistor (270 ohm) and a opaque white T-1 LED.  It lights up white.  I had it powered from 12V for couple of minutes and the LED and resistor only got slightly warm.

I measured the current and it was only 28mA.  The LED had 4.4 voltage drop across it!  I can only summarize that this LED uses some unusual chemistry which results in such a high voltage drop.  But at least the mystery is solved. 28mA does not sound unreasonable for this type of an LED.

BTW, I have identical light boards from older releases of those locomotives. Those still use the yellow T-1 LEDs and those use smaller (1206 size) 560 ohm resistors.  That seems reasonable too.

[jagged ben]

If you look carefully at those Kato boards I think you'll find a couple interesting things...

1) There's only one 270ohm resistor for two LEDs.
2) There's another, unmarked component in the circuit.  Brown, rectangular. 

I've never figured out what that other component does, but I think it's part of your answer.    I doubt that the LEDs are special.

[C855B]

... 2) There's another, unmarked component in the circuit.  Brown, rectangular. ...

Capacitor. Likely there for the motor, not lighting.

[mmagliaro]

Come to think of it, I *have* come across one or two Kato or Atlas diesels in my life that had
blown LEDs.   So they probably are operating them over the line, but unless you run them
at a screaming 12v, you will never see them fail.

Thanks for the replies, folks.

[peteski]

If you look carefully at those Kato boards I think you'll find a couple interesting things...

1) There's only one 270ohm resistor for two LEDs.
2) There's another, unmarked component in the circuit.  Brown, rectangular. 

I've never figured out what that other component does, but I think it's part of your answer.    I doubt that the LEDs are special.

Ben,
Kato has several different light boards. Some have a filter cap which shunts the reverse voltage EMF spikes from the motor when operating on DC.  That prevents the LED in the direction opposite to the travel direction from flickering when there is poor contact with the track.  That cap is wired in parallel with the inverse-parallel-connected LEDs.  This type of design minimizes component count (since only one resistor is needed) and the LEDs naturally protect themselves from the reverse voltage exceeding the opposite LEDs breakdown voltage.  You could call that diode a "dummy unlit LED" 

The specific light board (single headlight hood unit) we are discussing has no capacitor and it is designed to fit other Kato locos with dual headlights.  But because thsi model has no rear headlight, to protect the LED from excessive reverse voltage Kato simply replaced the rear LED with a non-light-emitting small signal diode.  It passes current when the loco is running in reverse, and clamps the voltage across itself and the opposite LED to 0.7V, but it doesn't light up.


The LEDs aren't special. Kato for some reason ended up with a batch of LEDs which due to their internal chemistry had a forward voltage drop higher than the usual 3V for a white LED.  So, they used a lower value resistor.  While they are probably pushing the limit at 12V, 28mA is really exceedingly  high for a T-1 size LED.  Kato seems to change their LED suppliers often (sometimes with less than optimal result).

They also seem to switch between the unrealistic lemon yellow, cool (bluish white) and nice warm white.  Can't figure it out why.  Even the Kato USA people I talked to couldn't figure out why the "mothership" in Japan doesn't stick with one color for the headlight LEDs.

[tehachapifan]

....They also seem to switch between the unrealistic lemon yellow, cool (bluish white) and nice warm white.  Can't figure it out why.  Even the Kato USA people I talked to couldn't figure out why the "mothership" in Japan doesn't stick with one color for the headlight LEDs.

I think they should make warm white LED's an NMRA standard.

[peteski]

I think they should make warm white LED's an NMRA standard.

Golden, sunny or incandescent white?    We need a better color definition. The manufacturers are all over the place.  Degrees Kelvin is a good way of defining the color (which does not require an engineering degree to understand like the X-Y color coordinates) but many manufactures do not provide this spec for their white LEDs.

[mmagliaro]

I am no better than Kato in that regard.  I have used mostly warm/golden/sunny white of one type
or another over the past few years, but I have a fleet of engines with headlights that are all over the color
spectrum.   The "incandescent white" from NGineering are by far the ones I like the best.
I hadn't always known about those.

I have been putting drops of Tamiya Clear Yellow on my "bright white" or other "plain" white ones lately,
and that really improves them.

My beloved SP&S 4-6-2 has a headlight that is positively blinding sometimes.  If I had experimented more,
I would have used a higher-value dropping resistor to hold it back (and it's got tender magnet
wires all places just so and sealed up in the nose now, so the risk vs reward ratio does not
entice me to pull it out).
   
I think I have a 1.8k  in there, so at 6v or so, it's only getting 2 mA,
but some SMD LEDs put out amazing amounts of light with that tiny amount of current.

[peteski]

We all use a plethora of different color temperature of white LEDs for various reasons. Cost, availability, etc.  Still it would be nice if all the LED manufacturers agreed on a single way to describe the hue of the LED.  Degrees Kelvin seems like a good way to do it. That way, no matter what manufacturer's LED you picked, a 2700K white LED would have nice warm glow.  This is done by many light bulb manufacturers (both Compact Fluorescent and LED), but not for discrete LEDs.