Circuit?

How are you wiring the LED's?   Are you using some sort of a bridge rectifier?  LED's and DC sometimes don't play well as the voltage varies and is not constant like DCC voltage were a rectifier turns the DCC voltage into straight DC voltage that is constant.  A picture of your wiring setup will help to give you a better answer.

Thanks for the reply: I am

Thanks for the reply:

I am using LED drivers to get the 20ma I need, I made PC boards for each lite front and rear as I mostly own steam engines.

I got the idea from the site below at the bottom of the page and it does work well, except for the flickering.

Although I had the flickering when I was using the 1.5v bulbs and a bridge rec.

T.C.

http://www.trainelectronics.com/LED_Articles_2007/LED_104/index.htm

I suspect...

Looking at the website and given the fact that you are using LED drivers instead of rectifier diodes like a 1n4001 ect. and a 470 to 680 ohm resistor, the flickering may have something to do with the BEMF of the motor even though we are talking about straight DC power. It is also possible that the driver is allowing a small amount of current to flow backwards that is allowing the driver to activate the LED in the reverse position.  Basically it's backfeeding on it'self.  There are more qualified experts on here that can give you a definite answer to the problem.  

need larger cap

instead of 0.047 uF cap, you need more like a 47 uF cap.  However, larger electrolytic caps are polarity sensitive.   Oddly, the negative terminal on an electrolytic is marked.

each LED circuit requires a cap.   You can wire the cap in parallel across the LED and current regulator, not the diode, the negative side connected to the LED/track.

this also means, that the LED may remain light for a while if you reverse direction.

Thank you for the reply's I

Thank you for the reply's

I will try the larger cap wired as you say,  although I may try a smaller cap first as the idea is to keep it small.

I know this is not small by today's standards but it's something I can build with my "ten thumbs" and hide inside the engine.

I found this while looking for a fix, I think it's pretty much what you guy's are saying only he doesn't explain what "capacitor across the motor " means? I took it to mean from brush to brush, but that got me nowhere? Hopefully your way will fix the problem.

T.C.

Reverse Light Flickering, Causes and Solutions 

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.

dynamics of a motor

there are two things to consider

just as a capacitor stores charge, an inductor maintains current flow.   When the circuit causing that flow is disrupted,  an inductor maintains a current.   If the resistance is high (e.g. air) a large voltage spike results (Ohms law).   This is how the coil of your car generates the high voltage to the spark plugs and why diodes are wired across relay coils and motor circuits (i.e. h-bridge).   It is also the mechanism used in switching power supplies and how they can produce an output voltage greater than their input.

so his point is that when you stop the motor, it maintains a current that can momentarily maintain a light/led.   adding a small cap provides a place for the current to go and minimizes the voltage.  Current (Amp) is coulombs / sec.   Capacitance, Farad, is a measure of Volts / coulombs.   So the collapsing magnetic field of the coil will push a limited number of coulombs into the cap, charging it to some voltage dictated by its size.  (isn't electronics fascinating)

i would think the simpler explanation (as opposed to collapsing magnetic field) is that a DC motor acts like a generator producing a back EMF whenever it is turning and that when power to it is stopped, it continues to turn for a little bit and acts as a generator producing a voltage and current capable of maintaining a light/led.

I think it is not simple fixing this issue

I think it is not simple fixing this issue.

Motors in DC have commutators.  As each of the poles changes polarity, a commutation spike is generated. This sends current forward through the LED so it lights up.  Anything simple added to inhibit this forward current ( while running in reverse) will inhibit the normal forward current.

Be very careful with capacitors, large electrolytic's can explode.  I tried this with a non-electrolytic but it had no affect.  Since electrolytic's cannot be connected in reverse, you could try two identical caps connected in series, but  connect negative to negative, then the two positive leads to the track, (wear safety glasses)

I'l bet the OP has a simple DC power throttle, the PWM types generate their own reverse current thousands of times per second keeping the LED headlight on constant in Forward and Reverse.

The gentleman who wrote this is incorrect "The reverse light flickering is caused by poor track contact"

Ouch. so much poor info from Dennis461, it hurts the eyes!

patently wrong!

i agree with peter 

the polarity of electrolytic capacitors cannot be avoided by connecting them back-to-back in opposite directions  and they certainly should not be connected directly to the track

don't understand why a PWM throttle would generate a reverse current the keeps headlight on in forward and reverse but only moves the loco in one direction. 

The OP posted.. "Anyone have

The OP posted..

"Anyone have any advise or suggestions ?"

I also am looking for some magic from the internet, and something that will fit inside a tender or locomotive.

The last link below has a 3300UF capacitor and bridge, which is not directional :-(

As for dirty track and intermittent  power, my  older open frame motor loco (Athearn Blue Box) LED's turn on before the loco starts moving.

(Preferably from someone with first hand experience with headlight on DC, not DCC)

I have tried everything I've read and still have the flicker, capacitors, voltage regulators, current limiters.

now for some links to this continuing problem..

  https://electronics.stackexchange.com/questions/21928/can-you-make-a-non-polar-electrolytic-capacitor-out-of-two-regular-electrolytic

http://cs.trains.com/mrr/f/88/p/105549/1227432.aspx

http://www.trainelectronics.com/LED_Articles_2007/LED_102/index.htm

As for dirty track and

the voltage the motor starts turning may be higher than the LED and rectifier voltage

have you tried what was suggested above.   A single diode in series with an LED and current regulator with a large (470 uF) capacitor (with proper polarity) across the LED and regulator, not the rectifying diode?

If the size of the electrolytic capacitor is a problem, you might try tantalum capacitors which are smaller (bear in mind that the positive side of a tantalum cap may be marked)

Yes Greg, thanks for asking.

Yes Greg, thanks for asking. I built that circuit, it did not help.  I hesitate to post today's progress as it may hurt someone's eyes .

I am going to re-evaluate the issue after trying a new throttle design.

how long did LED stay lit?

when you tried the circuit above, what size capacitor did you use and after being lit, how long did the LED stay lit when removed from the track?

when you say hurt someones eye, are you suggesting that the circuit burned up?

it's a shame to give up on something for the wrong reasons

Thanks for all the

Thanks for all the suggestions, Yes I am running DC and I tried the cap's and got nowhere. (still have all my fingers) still have the flicker.

The clean track seemed to help but doesn't 100% fix the problem. I'm not an electronics design guy so I cant design a fix. I can build it if I have a schematic and parts list .

Thanks again T.C.

One problem, three different issues...

Hi,

it seems there are multiple issues causing the same problem... Dirty track causes flicker. Some motors may cause (or contribute to) flicker. There may be more... For example a PWM power supply (!)

LEDs are rather responsive, they can change their brightness hundreds of times per second (which is used in optical data communication). As such, any change in the operating current (which is a function of voltage) will change the LEDs brightness.

As stated, a battery or a capacitor will act as a buffer and will reduce flicker. There is some time constant involved, even if you might not realize it...

There was a schematic posted earlier. It is quite obvious there are basically two independent circuits (one forward and one backward), each consisting of a diode, a current regulator and an LED. Put a modest capacitor parallel to the current regulator and LED (you won't have issues with polarity apart from attaching the cap in the correct orientation the first time). On both circuits. This will increase your time constant, reducing your flicker.

But you will still have to deal with track cleaning and you might need to put a circuit on the motor to reduce motor emissions. (What's the english word for "Entstörung"?)

Have fun!

PS: connecting two electrolytic capacitors back to back (as described above) will create a "bipolar capacitor" which is an electrolytic cap that is indifferent to the direction of the voltage. However, the overall capacitance is the inverse sum of the inverse value of the individual capacitors (that is the overall capacitance is very low).

The formula is Csum = 1/(1/Ca + 1/Cb). So, to get 0.5F you'll have to place two times 1F, negative to negative. However, looking at the circuit diagram posted earlier I see no reason for this arrangement.

Edit: corrected the formula...

OK so if what your saying

T.C.

large cap across LED and regulator

add a large (1000 uF or more)  electrolytic cap across the LED and regulator, negative side on LED.

you can't add a large electrolytic across the motor terminals since it's polarity sensitive and you're just trying to fix the flicker, not build a keep alive.  right?

you can test it by lifting the loco off the rails and seeing how long the LED say lit.   Since the board is outside the loco, you can test it by connecting it to the rails and seeing how long the LED is lit after disconnecting it.

Adding caps

Hi,

first of all, the flicker will not go away, it will get less. The thing is, a capacitor has a time constant that is defined by the capacity over the current charging/draining it. You'll end up somewhere in the sub-second range. Which reduces flicker a lot but does not cure it completely. You'll still have to deal with the sources of the flicker, like dirty track, "dirty" motor emissions, the PWM your DC throttle may use, ...

Secondly, adding a 1000 uF cap is probably not a good idea - for one thing the cap needs to be at your track voltage plus a safety - for HO I'd assume a 25V cap would be called for. Which is a huge beast. Another problem with large caps is the charging current. With the capacitor mostly empty it acts like a short-circuit when DC voltage is applied (*). With a cap of 470uF or above you need to control the charging current. Which adds extra resistors, probably diodes, ... I'd go for something like 50 to 100 uF.

(*) In this case a short-circuit with a big cap will mean sparks between the wheels and track, which makes the dirty-track problem worse. With a big cap it might even trip the throttle's short-circuit protection or may damage the delicate pickups and wires...

You could consider using a low-current LED (which uses ~2 mA instead of 20 mA) to increase your time constant (and thus reduce flicker even more) with the same capacitor. Again, this will reduce the issue, not solve it.

Hope that helps...

I really want to thank you

Terry C.

there's something to encourage you

flicka, I meant to write flicker

TC,

Let me know of the PWM throttle makes it worse.

I've built a small circuit to virtually eliminate the flicker when an analog DC throttle s used.  Not an ideal solution as the LED must run on at a slightly higher voltage which will be a problem for my slow moving diesels.  I'm testing the circuit with a Varney steam loco/tender with old 3-pole motor.

Simple,  add a TCS KA3 in

Simple,  add a TCS KA3 in line of the LEDs. For example, all the red wires of the LEDs should go into either the blue or black side of the KA3 then the other wire goes into the track/wheel pick up. Then the white wires will go where needed so that they turn off when in reverse or in forward etc etc. It is the way I always recommend people to wire the Monster LEDs who have DC and they have all done it that way without any problems. 

TCS KA3 is designed to keep

TCS KA3 is designed to keep lights ON. And, in a DCC system.

We are working to keep a headlight OFF in a DC system.

But, send me one and I'll try it out, thanks.

The 10c solution

Hi TC,

Have you tried wiring in a rectifier type diode like 1n4001 as well in the wire going to the backup headlight? It will drop the volts to the back-up headlight by another 0.7V. 

Wire it in with correct polarity in the line going to the headlight, you will have a headlight when travelling in reverse and the rectifier part of this diode should kill any back bleed current to the backup headlight when travelling forwards. Wire it in with the wrong polarity and you won't have headlight at all when backing up. Try it on the backup headlight first before going after the front headlight. You've got nothing to lose apart from 10c for a cheap diode.

For Dennis - Why is the light coming on before the loco moves such a bad thing?? Why wouldn't the driver on a real locomotive fire up the headlight before moving off so that the driver could see what is "out there in the dark"? Or am I missing something?