Details of FL12 decoder?

[David]

Ok, the FL12 decoder is a Kato branded Digitrax 2 function (only) decoder. Only the bottom are two pads that act as pickup (red and black) and on the top are 4 pads acting as seperate +/- terminals for the 2 functions. How it works with Kato's DCC regular trains is rather neat: a brass strip is bent in on both sides, creating something of a sprung stand with two points. This is set against the eletrical pickup strip. The LEDs are wired with resistors and have their +/- ends going to pads that happen to rest on those sprung strips. Since LEDs only work in one direction, DC current lights the correct LED. The decoder gets pushed between these - now the brass strip applies current to the 2 pickup pads, and the 4 LED pads make contact with the 4 output pads of the decoder.

I'd like to figure out what the exact layout of the 4 pads on the FL12 decoder are,however taking apart one of them enough to get at the lightboard and follow all the traces is a bit of a challenge and may take some guess work. Obviously the left and right side have the + pad of one function and the - pad of the other function. What is not obvious is which is in front and which is behind, and which is function 1 (head lights) and which is function 2 (tail lights). Also of note is that I can see 2 tiny 270ohm SMD resisters on the board. It doesn't make much sense, but I wonder if these are for the LED outputs, or for the decoder.

So does anyone already have a link to a diagram or explanation of the decoder, or should I put together a rig and figure it out through trial and error and post the results here? The ultimate reason I want to know this is so that I can do some hardwired installations in non-DCC ready cab cars.

[CaptOblivious]

This page does a lot of wired installs with the FL12. You should be able to figure out what each pad does from the photos, esp the one at the bottom of this page:
http://www.dccmodel.com/dcchome5-33.html

[The_Ghan]

Cap'n,

I know you've been through this before, but could you explain - in really plain English - the benefit of the diode over a resistor?

Cheers

The_Ghan

[David]

This page does a lot of wired installs with the FL12. You should be able to figure out what each pad does from the photos, esp the one at the bottom of this page:
http://www.dccmodel.com/dcchome5-33.html

Thanks. That's actually the same model of lightboard I have on my plate, it's used in the Kato 115-1000 (I did the motor car yesterday with a DZ125). I was wondering how I'd go about using those leads or if I was going to need to solder a wire to the brass pickup strips.

[CaptOblivious]

Cap'n,

I know you've been through this before, but could you explain - in really plain English - the benefit of the diode over a resistor?

Cheers

The_Ghan

Ghan,

Both are devices for limiting current. The amount of current a resistor passes is linearly proportional to the voltage: At low voltage, the resistor passes low current (= dim light); at high voltage, the resistor passes high current (= bright light). The amount that a CRD (not just any diode, this is a special kind of diode) passes is, within a certain range, fixed: At very low voltage, it passes very low current, but at low voltage, the CRD will pass the same current as at a high voltage (=bright light even at low throttle settings).


The disadvantages to CRDS are: Hard to obtain (but, apparently, not in Japan), and can be expensive.

So, the question you should be asking yourself (because I'm asking it of myself) is: The guy in the link is using CRDs in his DCC conversion. But what is the advantage of a CRD over a resistor on DCC, where the voltage is pretty much fixed, and doesn't vary much between DCC systems? I can't see one…

[inobu]

I think it is a blocking diode that is directional. The DC lighting boads are designed to pass voltage based on polarity
The white and red leds share the same power path. Each LED is behind a diode that allows
One voltage polarity to pass which illuminates that LED while the other diode is blocking voltage on its leg.
Inobu

[brill27mcb]

Isn't an LED (D stands for diode) itself directional? Why would it need another diode to do that function?

Rich K.

[CaptOblivious]

All diodes are directional, LEDs included, yes. The CRD is there to limit the current to the LED. My point was only: THere is no advantage to using CRDs instead of resistors in DCC installations, as near as I can tell, but I wondered if someone could show me I was wrong.

[The_Ghan]

Cap'n

In another thread I've mentioned using a motor decoder in end cars to control the head/tail lights using the motor leads instead of the function leads.  The advantage is that, with all decoders set to the same address, the train will behave exactly like a DC train inasmuch as changing the direction of travel will automatically change the lights.  Not very prototypical though, really,  The use of CRDs might be appropriate for such an application.

Cheers

The_Ghan

[inobu]

Isn't an LED (D stands for diode) itself directional? Why would it need another diode to do that function?

Rich K.

I thought he was blocking like a DC lighting board but looking at the schematic it does not match the actual wiring. Based on the data sheet the cathode is on the blue side so its not blocking and should illuminate.

The only conclusion that I could come to is he mimicked a setup for the original incandescent bulbs which would make sense based on the current surge on initial power up. Using the LED changes makes it moot. 

Inobu

[KenS]

All diodes are directional, LEDs included, yes. The CRD is there to limit the current to the LED. My point was only: THere is no advantage to using CRDs instead of resistors in DCC installations, as near as I can tell, but I wondered if someone could show me I was wrong.

Well, I don't know how a CRD behaves, but with a resistor you're basically converting excess voltage to heat, and thus potentially reducing the number of trains you can run on a given supply. With multiple light boards that can add up, although just back-of-the-envelope I don't think it would have an significant effect without a LOT of lightboards.  If the CRD reduces current without discarding a large share of power as heat, that could be a reason to use it.

[CaptOblivious]

Ghan,

Quite insightful. (although I wonder if mucking with Vmin and Vmid CVs might be a cheaper means to that end?).

Cap'n

In another thread I've mentioned using a motor decoder in end cars to control the head/tail lights using the motor leads instead of the function leads.  The advantage is that, with all decoders set to the same address, the train will behave exactly like a DC train inasmuch as changing the direction of travel will automatically change the lights.  Not very prototypical though, really,  The use of CRDs might be appropriate for such an application.

Cheers

The_Ghan

[inobu]

Well, the resistor used in a LED's circuitry operates in a current limiting capacity which is fixed. Meaning the resistor's value is calculated, if the applied voltage changes then the current changes. A resistor with a different value must be used to maintain the target current value. So a resistor does not regulate current.

The CRD regulates current not voltage so it should be the better choice for a DCC configuration. Not all DCC boosters operate at the same base voltage which has a correlation to the DC voltage produced by the decoder.

Some decoders LED outputs leads have 13 volts and some have 3v. CRD should resolve that issue because it is limiting the current to a fixed value for the LED.

Inobu

       

[The_Ghan]

Cap'n ... in a logical world I would agree with you.  However, I wouldn't trust myself to rely entirely on a software solution.  Ie: accidently reset the decoder and you fry an LED.  With my late nights, red wine and rush to complete whatever it is that I'm working on (so that I can concentrate on the wine) I'd at least have a resistor in series.

Cheers

The_Ghan

[David]

Fried one with a short, only have one more sitting around.

Any suggestions on where to order from for best overall price? Kleins is out of stock, and Plaza Japan seems to have a good price and 15 available.

[CaptOblivious]

Inobu, that's a good point! I guess I don't take my trains to different layouts (not knowing anyone nearby who has one), so that wouldn't have occurred to me.

Cap'n ... in a logical world I would agree with you.  However, I wouldn't trust myself to rely entirely on a software solution.  Ie: accidently reset the decoder and you fry an LED.  With my late nights, red wine and rush to complete whatever it is that I'm working on (so that I can concentrate on the wine) I'd at least have a resistor in series.

Cheers

The_Ghan

I know the scenario all too well! (Only in my case, it's more like I'm in a rush so that I can concentrate on the feeling of the wine putting me to sleep ;D ) I meant, the constant brightness could be achieved with a resistor and jacking vmin and vmid up to meet vmax, so that it never dimmed, just turned off. :D

[inobu]

It comes down to the point that the CRD regulates current only and the resistor limits current based on a formula of applied voltage.