DH165L0 F0F/F0R current limiting

[andy_ban]

Not sure if this is correct forum to ask:

I have DH165L0 decoder, all works fine.
What caught my attention is the F0F/F0R current limiting circuit.
I noticed that single reverse LED is way too bright.
Forward uses two LEDs in parallel.
Decoder uses LM317L with 27 Ohm resistor.
It means that current is limited to about 46mA (1.25V/27) that is quite high for LEDs.
Digitrax website listed current as 15mA.
LEDs are typically rated up to 20mA
I did replace one resistor for reverse with 62 Ohm and now it looks much better, but I did not measure the current.
I also don't know if these amber LEDs are original or replacements.
Any idea what is going on, am I missing something?
Thanks

[peteski]

This is the correct forum. I'm assuming that this is a Digitrax decoder.  Usually the positive common (the "blue" wire voltage is a raw rectified voltage about 1 to 2 volts less than the DCC track voltage.  Usually around 12V DC.  LM317 sounds like a voltage regulator chip.  I did not realize Digitrax used those on decoders (to lower the "blue" voltage?). I have never encountered that in the decoders I use.  But some decoders provide either 3V or 5V common positive voltage.

The standard resistor to use with LEDs is 1k if the common positive. That produces a safe current for most LEDs.

Easy resistor value calculation is the "blue" voltage minus LED forward voltage, divided by the desired current for the LED.  All in base units. Amber LEDs have forward voltage of around 2V.

[peteski]

I have looked at the decoder's specs and it is designed for a specific locomotive model with 1.5V bulb headligths.  I have no idea where you are installing it, but if you will be using LEDs, I would recommend bypassing the "internal current regulator" following instructions in https://www.digitrax.com/tsd/KB257/dh165l0-current-regulator-bypass/ and just use the "blue" positive voltage with a series connected resistor (basically as  it is done with most decoders.  That is the best advice I can give without seeing the decoder in-person and tracing the circuitry.

[andy_ban]

Thank you for your replies.

Let me provide more details:
I am using P2K U28B DCC ready locomotive that I got second hand as a DC.
I replaced the dummy plug with Digitrax DH165L0.
I don't know if this locomotive has original LED bulbs or previous owner installed them.
All I can say is that the rear LED was wired backwards with blue wire connected to the LED minus (I corrected this).

LEDs are actually current devices, they need a current limit at 20mA that is commonly achieved by adding a resistor in series like you described.
If "blue" voltage changes, the LED current and brightness change too.

However, LM317L maybe a better way because it can be wired as a current limiter (instead of voltage), current is calculated by using formula 1.25V/R.
It means that out of factory regulators are set to 46mA, and this is way too much for an LED.

Digitrax manual said that its regulators are rated for LED, but I am clearly see that is not a fact.
I replaced the rear LM317 resistor with 62 Ohm and it's Ok now.
Front uses 2 LEDs wired in parallel and it means that each LED uses 23mA, so resistor should be 31 Ohm.

I will take a look to see if I should bypass regulators and wire resistors like you mentioned.
Thank you