LED question

[peteski]

just wondering...was this not a dual mode DC/DCC decoder that would have run on DC?
i know it is not the best situation to run dual mode on DC unless you are able to program the decoder as such.
but would that not have saved all the work? just really curious.
i hope you get it to run well.

sincerely
Gary

All DCC decoders I have ever dealt with in my almost 3 decades of dealing with DCC have been "dual mode".  By default the analog mode is also enabled.  I have never seen a decoder that only worked on DCC.  Maybe they exist, but I have not seen one yet.

Yes, the Bachmann decoder is "dual mode", but it might not work all that well (especially in a switching loco), and some DC throttles do not output pure DC voltage which can make things not optimal for the decoder.

[mmagliaro]

Peteski, I like your circuit.  I still wonder why it is that white LEDs seem more prone to fail on reverse voltages higher
than their stated 5v "maximum".  Looking at data sheets for other LEDs, a lot of them are rated at only 5v reverse maximum,
regardless of color or size.  But it sure seems like it's the white ones that fail more easily if you push the reverse voltage above 5 for too long.  Do you know why that is?

[jym1]

     Pete, thanks for the circuit-mod. I don't know if I can get all of that done on a board that's only 8mm wide! I'm not into etching so I'd hafta cut all the traces(?) by hand. The problem is compounded by the screws and their bases on the chassis which take up almost 4mm! The screws transmit the power from the track so they're necessary. I'll try the home-made board, but if that can't be done, I'll jerry-rig the components to the original board.
     Gary, I didn't want the decoder because I wanted to have a see-through open-cab.

[glakedylan]

Gary, I didn't want the decoder because I wanted to have a see-through open-cab.



Jym, now that makes perfect sense. good call!


thanks
Gary

[peteski]

I'm familiar with that locomotive.  I have etched a decoder adapter board to replace the Bachmann decoder with any brand 6-pin decoder.



I agree, that it would be tricky to make all those cuts on your board.  The drawing is just an example.  It doesn't have to be made exactly as drawn. You could relocate the cuts to give yourself some more breathing room. I would lay the components (cap and resistors) down on the board, then use a pencil to mark where to make the optimal cuts.

As for the cuts themselves, when I used to make this type of a circuit board, I used a very small dental bur in a Dremel tool to grind the breaks in the copper cladding.



Like this one, about 0.5mm head diameter.

[peteski]

Peteski, I like your circuit.

Thanks, but I can't really take full credit. I saw this type of design on one of the factory-made light boards in some loco.  I don't recall which loco or manufacturer.

I still wonder why it is that white LEDs seem more prone to fail on reverse voltages higher
than their stated 5v "maximum".  Looking at data sheets for other LEDs, a lot of them are rated at only 5v reverse maximum,
regardless of color or size.  But it sure seems like it's the white ones that fail more easily if you push the reverse voltage above 5 for too long.  Do you know why that is?

I believe it has something to do with their chemistry.  Well, most white LEDs are actually blue LED with a phosphor coating.  Same goes for the "true green" LEDs. All those are very sensitive to reverse voltage/current.

While it doesn't explain their fragility, here is some interesting info about blue LEDs.

Here is an interesting discussion.

[Steveruger45]

Pete, By coincidence, while I was checking out my new Kato sd70M’s for future Loksound install whenever those new Kato slide in decoders from ESU hit the streets, I noticed those Kato dc light boards are configured as you have drawn; with one resistor and one small capacitor.

[peteski]

Pete, By coincidence, while I was checking out my new Kato sd70M’s for future Loksound install whenever those new Kato slide in decoders from ESU hit the streets, I noticed those Kato dc light boards are configured as you have drawn; with one resistor and one small capacitor.

So it was a Kato loco where I saw this circuit - figures  . . .   Leave it to Kato to come up with the simplest (minimum parts count) and best design to protect the LEDs.