Flickering LEDs?

[CPRail]

Hi Group,

I've done two installs for my Dad using the factory LEDs, and whenever we turn off the headlights or set them to DIM, they do this weird flickering. We did a Genesis 2-8-2 last night and I know we downloaded the current files and updates for the LM-3S. Is there something else I need to set-up?

[KPack]

On the function button for "dim", did you adjust the slider at all?  If so, leave it at 100%.  When the "dim" function is selected, 100% means that the lights are at the brightest dim setting.  Lower than that will dim the lights much more and lead to flicker if using LEDs (that is how LEDs dim). 

[Alan]

As Kevin stated, turning down the brightness setting eventually leads to flickering meaning there is a minimum usable brightness setting. Increase the current limiting resistor value if you want the LED to dim more than the lowest flicker-free setting. Be aware this will also lower the LED brightness at the normal setting so it may take some trial and err to find a value that produces acceptable results at both settings. If the resistor solution does not prove suitable for you then place a small value capacitor (1µF good starting point) across the LM light output and the blue wire. This will solve the flicker issue regardless of dim brightness setting. With the capacitor in place expect a split second lag time between pressing a button and the LED changing brightness or turning off. Make sure the current limiting resistor is downstream from the capacitor.

Blue wire ----------------------------------------------
                                    |                                   |
                              capacitor                            |
                                    |                                   |
LM light output -------------------> resistor --> LED

[CPRail]

I've tried having DIM at 100% and lower - same result. The flickering is also there when just turning off the headlight.

Admittedly, the Genesis 2-8-2 is older tech (DCC 9 pin plug with an orangy yellow LED), but the Bowser C630M is relatively recent. These are the only 2 engines with this issue.

I do not have this problem with my Bowser SD40-2s which have the 21 pin plug and LEDs, nor anything with actual bulbs (like an Athearn RTR SD40, etc).

[Alan]

LEDs do not flicker natively so we know the problem lie in the circuit driving the LED. First step in any troubleshooting is to closely inspect your wiring. Double check every wire routing, connection, etc. It wouldn't be the first time an errant strand of a wire touching something has caused a problem. Process of elimination may also help. Disconnect the motor leads. LED work correctly? Bypass the plug. LED work correctly? Take the LM out and connect it directly to track power. LED work correctly? And so on.

Can you accurately describe to us in words the flicker effect i.e. evenly spaced pulses, random erratic pulses, LED gets dimmer as it pulses, etc.? Or better yet, post a video. Does the LED work properly when in full brightness mode? Does the HC report any warnings while LED is in dim mode?

[CPRail]

I'll see if I can get my Dad to answer some of those questions (they're his locos and he's retired so he can play with them while I'm at work.)

Stay tuned for further exciting developments.

[CPRail]

It took forever (the old man went away to the West coast for most of the winter), but we finally tested the 2-8-2 on my layout which is powered with the official RailPro power supply, and no flicker in the headlights.

I attribute it to his use of an ancient Troller power pack for the juice on his test track.

At least it isn't something we screwed up in the install/downloads.

[Alan]

Problem solved! Glad it was that easy for you.

DC power packs have no place on a RP layout other than maybe running incandescent bulbs in your structures. I'm guessing running on DC power pack probably shortens LM life too.