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Flickering headlight opposite direction of travel

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Flickering headlight opposite direction of travel
Posted by tstage on Tuesday, June 12, 2018 11:21 PM

What causes the opposite headlight in the direction of travel to occasionally flicker on a DC locomotive?

I recently replaced the orange LED headlights in a Kato NW2 switcher with Miniatronics Yeloglo LEDs.  After I switched out the front headlight, I noticed the rear headlight (with the original LED) flickering, as the locomotive was moving away from me.

Both headlight lightboards came with resistors.  It's somewhat arbitrary where the opposing headlight flickers along the track.  And it happens in either direction of travel.

Thanks,

Tom

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Posted by j. c. on Tuesday, June 12, 2018 11:54 PM

voltage spikes i use a cap as a drain. 

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Posted by Mark R. on Wednesday, June 13, 2018 8:16 AM

The reverse light flickering is caused by poor track contact. It is really caused by the motor when the power is instantaneously disconnected. This is due to back EMF (electromotive force). To understand this you must first understand that current through a wire generates a magnetic field, and a changing magnetic field generates a current through a wire. The coils in the motor are really a series of very long pieces of wire, and the current generates the magnetic field the motor uses to turn. If a coil is receiving a current, and the current is disconnected, then the magnetic field collapses. This collapsing magnetic field constitutes a changing magnetic field, and generates a reverse voltage in the coil (this is called back EMF). In a motor this would normally be offset by the changing magnetic field that occurs as the coil moves past the fixed magnet(s) in the motor, but not always (depending on the position of the coil when the power fails). Therefore we occasionally, when the power fails and the coil is in a certain position, get a reverse voltage applied to the circuit. This reverse voltage is enough to ever so briefly light the reverse light.

This occurrence is common to any directional lighting system (although it will be more apparent in ours due to its high efficiency and speed), and cures are usually easy. Firstly cleaning the track and wheels of the locomotive should fix the problem 99%. If it does not, then a small capacitor across the motor in the locomotive will provide a permanent fix. The capacitor does not have to be big, say a 0.047uF to 0.1 uF polyester capacitor (often called a "Greencap"). These are small and should be able to be tucked against the motor somewhere.

Mark.

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Posted by DigitalGriffin on Wednesday, June 13, 2018 9:48 AM

Mark and the others are correct.  A small cap would fix the issue.

I would put a low forward loss diode before the positive leg of a cap, then go to the LED.  The negative leg of the cap ties in with the negative leg of the LED.  Be sure to use a bi-polar cap or you'll get a "Kabloey" when you reverse voltage.

The disadvantage of this technique is it will raise the voltage before the LED will light.  So a 3.2V LED will become 3.5V to start glowing due to the .3V forward voltage drop on diode.  But if you have it tied in series with your motor, your train won't start moving before the headlight is on.

If you want a copy of the circuit, message me Tom.  I'll even send you a link to amazon for parts.

Don - Specializing in layout DC->DCC conversions

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Posted by Mark R. on Wednesday, June 13, 2018 12:55 PM

DigitalGriffin

Mark and the others are correct.  A small cap would fix the issue.

I would put a low forward loss diode before the positive leg of a cap, then go to the LED.  The negative leg of the cap ties in with the negative leg of the LED.  Be sure to use a bi-polar cap or you'll get a "Kabloey" when you reverse voltage.

The disadvantage of this technique is it will raise the voltage before the LED will light.  So a 3.2V LED will become 3.5V to start glowing due to the .3V forward voltage drop on diode.  But if you have it tied in series with your motor, your train won't start moving before the headlight is on.

If you want a copy of the circuit, message me Tom.  I'll even send you a link to amazon for parts.

 

Don't need to add caps / diodes in that fashion - as you say, it would affect the motor and / or LED performance. All that is needed is a green-cap across the motor brushes. You know the ones .... a lot of factory engines today already have them. They are there for radio suppression, but that also cures the reverse flickering when using LEDs. We always cut them off when using DCC as they can interfere with the BEMF signal to the decoder.

Mark.

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Posted by DigitalGriffin on Wednesday, June 13, 2018 2:29 PM

The reason I suggested the diode arrangement POST engine was because if there is a power drop due to dirty track, both the LED and other power energy devices may drain the cap if there is a complete circuit.  Putting a diode before the cap & led and after the engine ensures the capacitor only supplies the LED, thus decreasing likelihood of flicker.  

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Posted by Mark R. on Wednesday, June 13, 2018 7:04 PM

DigitalGriffin

The reason I suggested the diode arrangement POST engine was because if there is a power drop due to dirty track, both the LED and other power energy devices may drain the cap if there is a complete circuit.  Putting a diode before the cap & led and after the engine ensures the capacitor only supplies the LED, thus decreasing likelihood of flicker.  

 

But we're not trying to keep the LED lit, but rather absorb / eliminate the BEMF that is powering the LED at the opposite end. We're needing to keep the opposite end LED off, not keep the front LED on.

Mark.

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Posted by DigitalGriffin on Friday, June 15, 2018 10:17 AM
There's more than one thing that causes flicker. My approach fixes all reasons for flicker.

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Posted by gregc on Friday, June 15, 2018 3:57 PM

DigitalGriffin
There's more than one thing that causes flicker. My approach fixes all reasons for flicker.

there's two types of flicker:

when the lamp/led  that should be on goes dark momentarily

when the lamp/led that should NOT be on momentarily lights.

i thought Mark R's explanation and simple solution for the 2nd type of flicker made sense.   (except ???)

greg - Philadelphia & Reading / Reading

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Posted by Mark R. on Saturday, June 16, 2018 8:02 AM

gregc

 

 
DigitalGriffin
There's more than one thing that causes flicker. My approach fixes all reasons for flicker.

 

there's two types of flicker:

when the lamp/led  that should be on goes dark momentarily

when the lamp/led that should NOT be on momentarily lights.

i thought Mark R's explanation and simple solution for the 2nd type of flicker made sense.   (except ???)

 

The second type of flicker is what the OP was asking about ....

Mark.

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Posted by Track fiddler on Saturday, June 16, 2018 9:48 AM

I'll buy what Mark is selling.

I must admit I did not know the technical information he provided. The reason for the lights flickering when they shouldn't be on in the first place.

I had the same exact issue described by Tom with a Kato GP7 purchased at a train show. 

It was a wheel-track continuity issue.

I soaked a doubled up paper towel in 90% alcohol and ran one set of trucks on the paper towel and one set of trucks on the tracks moving the locomotive back and forth. Of course I did this to both sets of wheels ran in both directions.......The paper towel looked like a zebra.

Not only did I get rid of my light flicker problem but I have a creepy crawler for a locomotive now.

P.S.  I forgot to mention the wheels did not look that dirty in the first place. Apparently they were.

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Posted by tstage on Sunday, June 17, 2018 11:35 AM

Mark R.
This occurrence is common to any directional lighting system (although it will be more apparent in ours due to its high efficiency and speed), and cures are usually easy. Firstly cleaning the track and wheels of the locomotive should fix the problem 99%. If it does not, then a small capacitor across the motor in the locomotive will provide a permanent fix. The capacitor does not have to be big, say a 0.047uF to 0.1 uF polyester capacitor (often called a "Greencap"). These are small and should be able to be tucked against the motor somewhere.

Mark.

Mark and others -

Thanks for all the input and explanation.  I first cleaned the track with 99% alcohol and it lessened but didn't eliminate the issue.  Once I cleaned the wheels of the locomotive though, the flicker went away completely.

To apply the small uF cap solution, Mark: When you say "across the motor", I'm assuming you mean soldering it between the two motor brushes?

Would that also eliminate any "pulsating flicker" while moving with a dimmed LED headlight that stays on due to Rule 17 lighting?  I have a Stewart VO-1000 switcher with a TCS M1 decoder and it's been doing this since Day 1.  The flicker is constant (pulsating) and it happens in either direction.

Thanks,

Tom

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Posted by rrinker on Sunday, June 17, 2018 12:46 PM

 The pulsating flicker when dimmed is because of the PWM used to dim the LEDs. Have you tried adjusting CV64 for different dim values? Based on the fact that you use a higher value for incadescent bulbs, I'd say the higher the value, the longer the 'off' time to make the light appear dimmer. They recommend 2-6 for LEDs, so if it's higher (default seems to be 15), you may indeed see flicker when dimmed. LEDs don't have ther persistence of an incandescent filament so you're always treading a fine line between making the LED blink and making it appear dimmer.

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Posted by tstage on Sunday, June 17, 2018 1:25 PM

Randy,

I checked CV64 and it was set to a value of 2 for LEDs.  I increased it to 6 and it did decrease the flickering some but not completely.  It's an acceptable improvement so I will leave it that way.

Thanks for your feedback, Randy.

Tom

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Posted by DigitalGriffin on Monday, June 18, 2018 10:06 AM

If bad pickups was the problem, then my solution is without a doubt the better of the two.

There is always more than one way to skin a cat to solve problems.  There are advantages and disadvantages to each.  

 

Don - Specializing in layout DC->DCC conversions

Modeling C&O transition era and steel industries There's Nothing Like Big Steam!

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