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Discussion Starter · #21 ·
Well, I'm getting 2.88 ish volts on the one ditch light that is functioning, and 2.48 on the side that is problematic. If I jumper the "good" side to the "bad", the "bad" LED lights up in full. So I know it isn't the LED itself. Now I just gotta figure out why I'm getting a .4 voltage drop on the problematic side.
 

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The USA Trains SD70 has blinking ditch light capability. I don't recall seeing any of my 6 Aristo Dash-9 ditch lights blinking.
One of my Southern Pacific Dash-9s has ditch lights that don't work; but never had time to look into it. Been busy on other projects.
-Ted
 

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I have a video showing the Dash-9 in operation. (Greg E. hosts my videos on his YouTube site as well as hosting my "vignettes" (articles) on his own website.)
In the video, the Dash-9 ditch lights can be seen fully illuminated with the train stopped. (I use an Aristo Revolution 15 amp capable base station in PWM mode - so the lights will be on when train stopped)

If you watch the Video on YouTube, the "show more" function will display more commentary with links.


Another video showing the Aristo SP Dash-9s with Aristo Wisconsin Central SD45s:


-Ted
 

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Discussion Starter · #24 · (Edited)
Tore into it - took the cab off to get to the fwd circuit board. There was something finicky going on with the resistor on the board. After a bit of probing, it magically started working. Thinking I may end up having to reheat the solder on the resistor to get a better connection. Also found the connection at the LED itself is a bit flaky, if I wiggle it just the right way it'll cut out. Maybe need to resolder that, but for now it works so I'm just going to run it until it starts getting finicky again then will take the next steps later if needed 🤷‍♂️
 

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You cannot accurately measure voltage across an LED, so the difference between them is not conclusive. Remember they are diodes that emit light. The explanation is involved, but LEDs work by current, and the voltage drop on the LED should be somewhat constant, the variation you measured is most likely the variation in the LED itself.

If they don't blink in operation, then you probably have a bad wire somewhere. The concern I have is the wiring diagram I have shows the ditch lights are powered from a common, so if one is bad, it is wiring or the LED.
 

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Discussion Starter · #26 ·
Now that it works, they are both reading the same voltage across (2.88 ish volts). I was really more so checking input voltage. Are you thinking ohms? If so, I agree - resistance values don't mean much other than to check direction of current flow and to see if the diode is open.
 

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The voltage across any device follows Ohms law... But the resistance of a diode is a tricky thing. The safest way to think of it is an LED has a nominal voltage drop, you need to ensure you have something limiting the current, which is the key to their safe operation. On the Dash 9, it appears to be a 470 ohm resistor off the regulated 5 volt supply.

At worst case that calcs out to be about 11 milliamperes, but it's actually less, since that calculation does not take into account the voltage drop of the LED itself.

Don't know if you already know all this stuff, but this link might be helpful:

Greg
 

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Hi there, I have an opportunity to purchase a UP dash 9 for around $400. I briefly saw it in person but did not have time to check it out. Cosmetically from what I could tell quickly it was pristine. It was said that it runs, but the lights don't work. The wheels did not have much wear, I'd guess 20-30hrs runtime.

My layout is a little on the small side for such a locomotive, but it seems like such a good deal to pass up if the above is true. Thoughts? Are there particulars I should look out for if i decide to proceed?
 
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