Why do the lights flicker?

Hi Folks,
I wonder if anybody with some electrical knowledge can advise me on a problem I am having?
I am sorry it is a bit long winded but I am sure that giving the complete
scenario will be helpful.
I thought it might be a good idea to put lights in my old Triang and Hornby coaches. Not being able to find any replacement coach wheels with a "live" axle I purchased some of the current replacement coach wheels made by Hornby.
As the wheels are all metal and the axle is metal I obtained the ones with two holes in the wheel. I took some single strand stiffish wire, as used in internal telephone extension cable, and poked it through the hole in the wheel, (inside to outside), and bent the end onto the face of the wheel. I then wound the wire tightly about six times around the axle and squeezed the wire onto the axle. The new wheel sets were put into the coach bogie without problem as the old ones were plastic with pin point axles. The wheels were set with one "live" wheel on each side of the track.
Next I secured some pickup wire to the bogie using a small screw and cut a length of the wire to rest on the axle. Under the screw was a small square of phosphor bronze to which I had soldered a wire which was fed inside the coach via the hole through the bogie rivet. The same process was applied to the other axle. This was repeated on the other bogie.
Inside the coach I fixed three grain of wheat lights in parallel connected to the wires from both bogies. In this way the lights were fed electrically from two different directions albeit the same source, i.e. the rails.
I did something similar with another coach but used three LEDs in series, instead of the GOW, using a bridge rectifier and a 1K ohm resistor.
In both cases the lights picked up the current from the track whilst stationary but as soon as the coach started to move the lights start to flicker, even on a portion of straight track. I would have thought that with the lights being fed from two directions the supply would have been constant with two wheels being in contact with the track so if one lost contact the other would keep the supply going.
This is on normal 12v DC; the track was cleaned with a rubber and track cleaning fluid; the wheels are brand new so no muck on them.
Any suggestions to resolve this would be appreciated.
P.S. Is it necessary to oil pin point axles in plastic housing?
Regards
Jack.
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This does not seem like a very satisfactory way of making a connection to the axle. Perhaps just a short length of wire soldered from the axle to the rear face of the wheel would be a better bet, or perhaps trying to find a metal bush to replace the plastic insulator in the wheel.

I suspect that your main problem is the tension of the wire that is pressing on to the axle. Unless it is spring steel, springy beryllium copper or the like it will not make good contact once the axle is revolving. Getting the pressure right is an art, enough for a good contact but not too much to significantly increase the rolling resistance.
I suspect that you will almost always get some flickering, so perhaps a large value capacitor in parallel with the bulbs, or even better the led's, would help smooth out some of the flickering.
Regards Jeff
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Jack wrote:

Because the contact is intermittant. It might only be microscopic bits of dirt, corrosion, etc, but they all add up. Also, some metals don't make good long-term contact between each other, they induce small amounts of corrosion on each other.
Fixes: 1) cleaner electrical contacts. As Jeff said, not sure about your wire wrapped around the axle, it depends what metal it is made from as to whether it works. 2) some sort of "stay alive" capacitor circuit inside the coaches to hold power over the flickers.
If heading down the electronic circuit approach, there are commercial products to do this, or someone (inc me) could post a rough circuit explanation. I would not really want to just put a capacitor alone over the bulbs (or LEDs) as the capacitor would then be feeding back its volts to the entire layout as well as the lamps. A rectifier (diodes) before the capacitor will control this detail.
- Nigel
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Nigel Cliffe,
Webmaster at http://www.2mm.org.uk/
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Just to be clear you have pickup on FOUR wheels, two on each side of the track?

I would glue it with conductive epoxy if I couldn't solder it.

Rechargeable batteries or a super cap would be even better as the light will then stay on when the train is stationary.
MBQ
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On Tue, 7 Jun 2011 02:27:49 -0700 (PDT), " snipped-for-privacy@hotmail.com"

Is that same thing that is sold to repair heated car screen elements in Halfords etc? I always knew it as conductive paint. eitherway the OP can get some from a motor factors and try it.
G.Harman

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On Jun 7, 12:55pm, snipped-for-privacy@yahoo.co.uk wrote:

Different. No use just using paint where structural integrity is required.
MBQ
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wrote:

But I guess you could use ordinary glue to fix the wire and then conductive paint to make the connection, depending how tidy the glue is.
MBQ
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On 07/06/2011 15:51, snipped-for-privacy@hotmail.com wrote:

You would have to be very careful that the glue left enough of the wire exposed and free from glue so that a connection could be made, and enough area to handle the current; bearing in mind that such paints (and conductive glues) have a relatively high resistance compared to soldering.
Regards Jeff
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On Tue, 7 Jun 2011 07:51:24 -0700 (PDT), " snipped-for-privacy@hotmail.com"

I was thinking more of getting rid of the wire wrapped around the axle altogether but wanted to make sure I was suggesting the correct substance. Use the paint to make a conductive path between rim and axle on one bogie and do the opposite side on the other bogie Then make a wiper from phosper bronze strip to rub lightly on the axle. Will still need some sort of capacitor to eliminate flicker completely though.
G.Harman
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Hi there,
Many thanks to those who responded to my problem.
I thought I might give replies to some of the points raised.
I did try soldering the wire to the axle and the back of the wheel but the plastic bush melted before the solder took hold. I have searched the internet, and contacted some of the more specialist model shops, for a replacement metal bush to no avail. I trawled around trying to find metal tubing with the correct inside and external diameters to make my own but all were standard 1mm increases. I even sent an email to Hornby suggesting that they made the replacement wheelsets with only one wheel insulated but all they said was that they would refer it to their developement team.
I agree that getting the tension on the pick up wire resting on the axle is difficult. The wire I used was springy and sold as pick up wire. As you rotated the wheel you could feel the slight resistance of it touching. Even moving the coach by hand very slowly along the track created the flicker.
There was pick up on 4 wheels, 1 each either side, front and back. Thats why I thought there would be some electrical continuity, especially when going over points.
I did try the conductive paint option as my first attempt, but even though I used 3 coats the electrical path was never there when tested with an OHM metre. The wrapped around wire does appear to make a good circuit and would not move as the wheel rotated, so I am not sure if gluing or painting it would make any difference.
My first attempt at the axle pick up was using 2mm phosphor bronze strip but getting the tension right to reduce drag was even more difficult than the springy wire. The coach used was part of the Blue Pullman Set up. Using the phosphor bronze pick ups on the Motor Unit, 1 coach and the Dummy unit caused major wheel spin from the motor and went nowhere. Substituting the spring wire gave reasonable travel but flickering lights.
I think that maybe capacitor idea might be the best one but I have no experience of them. If the offer of designing an appropriate circuit is still there I would be grateful to see one.
Incidently, tonight I tried out the Triang 101 DMU, with A7 headcode, and a 125 HST with headlights, and both of these flickered as they ran. So even the manufactures had this problem.
Again many thanks for your suggestions.
Regards
Jack
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There used to be (maybe still is) a product called super rollers? replacement wheels with a pickup that was wound around the axles, rather than a single contact point.
You could try something similar with PB wire wound like a spring around the axle but not too tight.
Another alternative is pickup wires bearing on the wheel treads or flange edges, as is sometimes done on scratch built locos.
MBQ
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On 07/06/2011 9:09 PM, Jack wrote: [...]

[...]
This suggests dirty and/or oxidised wheels and/or track. To improve the contact, try a minute drop of contact cleaner/lubricant (available at electronics shops).
You might be able to work up a sliding contact fastened to the bogie and bearing on the treads or backs of the wheels. This would improve contact on the carriage itself, but would not prevent the effects of intermittent wheel/rail contact.
I've made up sliding contacts bearing on the rail in the past. I fastened a T-shaped piece of springy bronze onto the bottom of the bogie or locomotive frame on one side, arranged so that the T bore on the rail between the wheel on the opposite side. The screw used to fasten the slider was also used to connect the feed wire. Bend the ends of the T-bar up slightly so it won't catch on joints, etc.
HTH & good luck Wolf K.
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Am not an expert on electrics but do enjoy looking at problems. When I see 2 statements like these :- "Even moving the coach by hand very slowly along the track created the flicker." "tonight I tried out the Triang 101 DMU, with A7 headcode, and a 125 HST with headlights, and both of these flickered as they ran"
Then I would go off at a tangent and look at fundamental stuff for your problem. Try taking your lights on wires with nothing else. Hold them against the track (one either side) then move them along the track. It would give a bit more proof that your track is clean , that you dont have any loose metal about that is causing momentary shorts or a bad connectin along the track (slight wobble of track making and breaking the connection. The other possibility for the flickering is the wheels bouncing although the 'slow push' should have isolated that cause.
Cheers, Simon
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That sounds a good idea. Will try it.
Thanks for that.
Jack
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Jack wrote:

Commercially, DCC Concepts sell a unit for "flicker free coach lighting".
Or, to DIY one. You will need a rough idea of the current drain through the coach lights.
First part; Bridge Rectifier. Four diodes in a square. The input (from wheels) goes to opposite corners, the output to the rest of the circuit follows. Requires four diodes, eg. IN4001, though almost anything will do.
Output of rectifier, one wire is positive, the other negative.
To the positive, connect a resistor in series to limit the in-rush current. 10ohm should do. Between the output of the resistor and the negative wire connect a capacitor to store energy (size below). Between the output of the resistor and the negative, connect the lights. If using LEDs you still need the resistors in series with the LEDs to limit the current.
The question is working out the storage of the capacitor... time = RC where R is the resistance of the discharge circuit. Time is (approx) how long you want the capacitor to be able to run the lights. If using bulbs, then they are usually rated in Watts. Resistance R = V*V/W and V is 12volts. Alternatively, measure the current with an ammeter and use V=IR to work out R. Plug in the results of time = RC and come up with a capacitance in Farad (hopefully a few tens or hundreds of microFarad or uF), and that's the capacitor required. You may find that with bulbs you can only hope to hold charge for say 0.1 or 0.25 seconds. With LED lights you could hold them on for many minutes with a fairly small capacitor.
- Nigel
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Webmaster at http://www.2mm.org.uk/
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On 08/06/2011 23:04, Nigel Cliffe wrote:

DCC is probably the best way to go as the lighting will be on when the train is stationary, otherwise the lights will only be lit when the train is moving and the brightness will be dependant on speed.
There was a system some years ago that superimposed a high frequency ac signal onto the track that was high enough to be 'ignored' by the motors but could be filtered out by the lighting circuit and used to light the lamps.
If you are using leds it would be possible to build some sort of voltage regulator that ran at say 3V, or constant current circuit, which would, keep the brightness the same once a certain threshold was reached. That in association with a large reservoir capacitor should stop the flickering, but of course would only light when the train was moving.
By the way the diodes are 1N4001 not IN4001.
Regards Jeff
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Let me get this right, you recommend spending potentially 100s on a DCC system and decoders for all the locos as the best solution compared to 1 tops per carriage, just to get constant lighting?
I'm a great fan of DCC, but ...
MBQ

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Has the Gaugemaster MODEL HF-3 - Lighting Generator been mentioned n this thread, I dont have one myself so dont know how good or bad they are but a look on froogle may help Just a thought Simon G
Jack wrote:

Let me get this right, you recommend spending potentially 100s on a DCC system and decoders for all the locos as the best solution compared to 1 tops per carriage, just to get constant lighting?
I'm a great fan of DCC, but ...
MBQ
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Jeff wrote:

Err. no.
With a big enough capacitor (or small battery) the lights will stay on for many minutes and be pretty much constant brightness. They will dim eventually, then go out.
I use DCC, its incredibly useful. But its not a cost-effective solution for just getting coach lighting.
- Nigel
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Webmaster at http://www.2mm.org.uk/
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A small battery might work if there was sufficient 'on' time vs standstill to charge it, but there is the issue of considerable additional circuitry to prevent a constant load and total discharge and consequent damage to the battery. The physical size of a capacitor to keep the lights on "for many minutes" would be larger than the coach, not to mention the inrush current when the power was applied!!
Jeff
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