I own three Proto Heritage 2-8-4 steam locomotives, an NKP, a Pere Marquette (PM), and a C&O.
A few years back, I replaced the burned out incandescent rear lamp in the tender of the NKP with a 3mm Miniatronics Yelogo LED and installed a 3mm Miniatronics Yeloglo LED in the engine head light housing.
A few weeks back, I noticed that the incandescent rear lamps on the other two 2-8-4s (PM and C&O) were burned out. So, I replaced those rear lamps with Miniatronics 3mm Yeloglo LEDs. At that point, I couldn’t help notice that the incandescent headlights in these two steamers were very dim, especially compared to the rear lamps. So, I set about to replace them.
I had no problem with the Pere Marquette because, like the NKP, the LED fit nicely into the headlight shround. However, I ran into problems with the C&O. On the NKP and the PM, the headlight shroud has the lamp fitting directly in the center of the shroud, in line with the female fittings in the boiler that accept the two lamp wires.
But, on the C&O, the lamp fitting is at the bottom of the headlight shroud. Because the two lamp wires have to feed through the boiler weight, this made installation of the 3mm LED impossible because it prevented the headlight shroud from fitting into the boiler shell.
My solution was to install an SMD LED (Surface Mount Device). This miniature LED is slightly smaller than a grain of wheat and is powered by two #36 wires. To get such small wires to fit snugly in the female fittings, I had to solder the #36 wires to small lengths of 22 gauge wire which then fit snugly into the female fittings.
I am starting this thread in order to inform others who wish to replace this headlight how to go about doing it. Not an easy task.
Not easy but worth it because you only have to do it once. Replace with another incandescent bulb and you’ll be taking it apart over and over to replace it every time it burns out.
That’s what makes them so great! Can you imagine what the modellers of 50 years ago would say if they were presented with SMD LEDs? I don’t understand Athearn’s long established resistance to them.
I wish you had done this 2 years ago, or that I had thought of using SMD LEDs…I managed to stuff a 3mm led and a resistor into the smokebox of a Proto C&O Berkshire. It was not a simple task. Lots of filing involved.
I had to use an SMD LED because that smoke box weight was in the way and a normal 3mm LED kept the headlight shroud from fitting onto the boiler shell.
I got around the need for a resistor because I had a DH163LO decoder already in place and that decoder had a built-in resistor on the board.
Hmm, I guess it should be OK - the DH163L0 is set up so that it can be used directly with the P2K 3 volt incandescent bulbs. If they are 15-20ma bulbs then you kind of luck out, although the LED may be super bright, it should still be operating within limits.
Randy, you raise a good point. The SMD LED does not appear to be super bright inside the C&O headlight housing. However, on my Proto NKP 2-8-4, the Miniatronics 3mm Yeloglow is super bright
So, here is my question. Would it make sense to add a resistor on the board in the tender?
That probably would be a good idea. But how much? I know I have one or two of those around somewhere I could probbaly see how much resistence they already have, however, as I thought, there is an easier way. As seen in the instruction sheet
there are two pads on the bottom of the board you cna bridge with solder which disables the on board resistors. Now you just use whatever resistors you would on any other decoder. Both resistors can be fitted in the tender, even the one for the headlight, it doesn’t matter if the resistor is close to the LED or close to the decoder, and if there is limited space up by the headlight, you can just insert the resistor in the line in the tender before it gets to the loco harness. 1K is enough electrically but may still be too bright, you might have to jump to 2.2K or even 4.7K (or 3.3K, in between) to get a bright but not glaring light level. Likely different values for the Yeloglo in the tender vs the SMD LED in the headlight, too.
Edit: the pictures on the instruction sheet were clear enough to figure out how they set it up. There aren’t resistors on board, there are a pair of 317 voltage regulators configured as constant current sources. What I can’t see clearly is the value of the resistor to determine what current it is set for. It’s the two chips and two resistors between them between the 8 pin plug and the edge of the board. They’re SMD resistors so the numbers will be something like 102 for a 1K or 101 for 100 ohm. The current is determined by the formula 1.25/R. If the resistors are 100 ohm, that makes the current 12.5 ma, safe for the LED but brighter than the typical 1K resistor which results in around 9 ma.
OK, I will place a resistor on the front LED and the rear LED. If I am correct, that would mean a resistor on the white wire (front LED) and a resistor on the yellow wire (rear LED).
Randy, do you see a problem with not using a solder bridge to disable the on board resistors on the bottom of the decoder board? In other words, rather than messing with the on board resistors, I would simply leave them alone and add a resistor to each LED.
The formula to determine current confuses me. If the formula is: current=1.25/R, then the on board resistor on the DH163LO would limit current to 0.046296 (1.25/27). So, to get 46ma, wouldn’t the formula be 1.25/R*1000?
One other question. What is the derivation of the number 1.25 in the formula?
You should be able to add resistors as-is, but the constant current output of the decoder makes figuring what resistor to use much more difficult.
The 1.25 comes from the LM317 data sheet, the exact circuit used on the decoder is a standard one that’s even shown in the data sheet - see my previous message.
When the units are volts (the real derivation of the 1.25 is because that this the LOWEST voltage the LM317 will regulate to, with no components on the Adjust line) and ohms, the result is amps. So 0.046 and change is 46 ma. If you add another 3 decimal places in there you are in microamps. .000046 would be 46 microamps.
The more I think about it - I don;t think adding more resistors will work here. Your LED is being driven by a constant current source - that means it will adjust voltage as needed to insure the same current always flows. So if you add a resistor (increases the current requirements of the load), the regulator will just push more voltage to keep the current at the 46ma level, until either the current exceeds the capacity of the regulator and it fries, or the voltage limit is reached (input voltage - some loss in the regulator = max it can possibly output). So I’m afraid you will have to pop it out and solder bridge those two terminals. Or just sub in a different decoder, which means splicing the wires together and removing the circuit board.
Doggone it, Randy, you leave me no choice. I will disable the on board regulator. [8o|]
I only have one chance to do this right, so here is my question. Looking at that photo of the underside of the DH163LO board, I assume that the solder bridge is placed into each rectangular box, one for the front light and one for the rear light. So, the two solder bridges are horizontal, not vertical. Correct?
