Model Railway capacitors

[Rulycat]

I have this model railway that has lights in the passenger cars. The power is picked up through the rails by ball-bearing wheels which take the power to the light in the car. However, on dirtier tracks, or even relatively clean tracks, the light flickers on ans off as the circuit is broken. Obviously this doesn't look very good. So I had an idea to use capacitors to smooth things out...

I took two 4700uf 25V capacitors (cheap rubbish I had around) and soldered the two + legs together to produce a non-polarised electrolytic. Does as intended. However, given the maximum voltage from the power pack is 25V, is my two-cap arrangement actually 50V 2350uf or do the normal rules of series caps not apply to making a non-polarised one by sticking two of the same legs together?

Also, when the train is moving, the wheels spark a bit and the train stutters along as the caps are trying to recharge as fast as possible and power is being lost from everywhere else. I'm not sure what size resistor to use to slow charging down. With a full charge the caps take about 3 seconds to fully discharge as the bulb dims. I don't know how the resistor will affect this.

Now I accidentally shorted one cap earlier and the other was in reverse polarity and got very hot at 25V. Fortunately the bulb was in circuit and so the cap did not blow up but is most probably damaged. Fortunate as I was holding it with my bare hands.

Below are some pics for your perusal.

[mariushm]

Re: Model Railway capacitors

What kind of light is in the train? led, regular light bulb?

See this page: http://electronics.stackexchange.com...r-electrolytic

Short story: capacity will be half, voltage rating remains the same

But it's really not recommended to do something like this.

I'd suggest just going for half wave rectification, or even full wave rectification, even four diodes are super cheap, or a simple bridge rectifier like this one: http://www.digikey.com/product-detai...5GI-ND/2152818

rectify the ac voltage from the rails, get the dc voltage out, put a resistor in series and then one capacitor... even 100uF should be enough, but i guess it depends if it's a led or a classic light bulb like those 24v car light bulbs..

resistor in series... basically ohm's law:

V = I x R so R = V / I ... 100-250mA should be enough.

If you have 25v AC on the rails, rectified would be 25x1.41 = 35v, less about 1.1-2v drop on the diodes in the rectifier to get about 33 V DC.

So the resistance would be 33V / 0.25A = 132 ohm .. so use 100, 120, 150, 180 ohm ... whatever you have around.

Now if your lamp uses more than 250mA and you're limiting the charge to 250mA, the capacitor won't ever charge and it might still blink. So check the bulb, it's wattage (which is v x current) and set the resistor so that the current going into the capacitor is higher than what the bulb/led uses.

If it's a led inside, then it should already have a resistor to limit the current going through it.

If you want to mod it to use a led, then pick one, check the forward voltage and how much current it can handle (usually 10-30mA for plain leds). If you want to limit the current to 10mA for that led, you just do the math again ... (input voltage - forward voltage) / current ... so assuming the forward voltage is 3v (about this much for white leds) then the resistor would be ( ~ 33 V - 3v) / 0.01 = 3000 ohm ... so I'd just pick the closest.

ps. the resistor that limits the current going into the capacitor would have to be rated for about 3 watts.

[budm]

Re: Model Railway capacitors

Is the power for the train AC or DC? I thought most model trains run on DC from the rail tracks so you easily make it go forward or backward.

[Toasty]

Re: Model Railway capacitors

LGB trains run on DC. AC is used for accessories.

Indoor or outdoor running?

Consider researching an LED in the incandescent color range (<3100K). The bluish-white ones (>4000K) would look out of place. You can then reduce the power requirement and go to a lower capacity cap. You can also go to a Ni-MH battery setup that will charge off the track with a small switch mounted in the undercarriage.

Outdoor requires constant track cleaning and there are several home brew devices that work rather well. Also, their track cleaning locomotive works exceptionally well but is pricey. Old number 2067 ($500 US), new numbers 20670 & 21670 ($700+).

Lubricating the rails with the LGB smoke fluid is an old trick and works well, but can be a dirt attractant. Use of their conductive rail joint grease is imperative if outdoors, as well as using the rail joiner screws.

If building permanent outdoors, consider going to nickel-silver track. Pricey but worth the investment for the cleaning headache.

Oh, don't forget that diodes work well to reduce voltage and don't get hot like resistors. You get 0.7v drop per device when forward conducting.

T

[budm]

Re: Model Railway capacitors

Amber color LED will probably look OK, it does have a little warmt to it.

[Rulycat]

Re: Model Railway capacitors

Thankyou for the help. Some clarification. The setup is DC, 0 to 25V. However, when you turn the throttle the other way, it goes to 25V DC in opposite polarity to drive the motor in the opposite direction. So I need to allow for this. I have been to Maplin today and picked up two 35V 4700uf capacitors, to allow for greater safety margins over the 25V ones I was using. (okay so they're SamWha but in this circuit it's hardly an SMPS) and four 6A 50V silicon diodes to create a rectifier (to rectify the -25V DC to 25V DC) I was going to use schottky1A ones but they only had two in stock and I needed four. (this was a small Maplin shop) Already got 1/4watt resistors but I'll do some maths and see if that's enough and if not do some more maths so I can use several to make up the wattage.

The bulb is an old style filament bulb. I could start buying up new track or another set of ball-bearing wheels (you will notice that in the picture one is metal and one is plastic) but since LGB went bust and Marklin took over everything has gotten very expensive and I can't afford it.

So the plan is to rectify the +DC or -DC to a continuous +DC, which of course I expect will be 0.7V lower. (this is fine) I will be using a resistor to stop the wheels sparking by slowing down cap charging. Will report back next week (When I next get access to the cars)

[Toasty]

Re: Model Railway capacitors

1 cap + 1 bridge rectifier. Easier to wire. Voltage drop will be 1.4v.

T

[Jack Crow]

Re: Model Railway capacitors

R,
Was thinking about something you said.
The lights changing brightness level and the 'sparking'.

Sparks are natures way of saying there is a connection problem.

Got an extra wheel truck that has the pick up points?
Worst case a load resistor and some wires with clips on them?

Might want to make a mobile load of at least the same amount of current the loco draws.

Push this truck across the tracks, loading the psu, while watching the voltage at the truck to see if there is a voltage drop from weak track connectors.

I have been fighting the battle of weak 'bi metalic' junctions so long, I see voltage drops in my sleep.

Or if you look at your track in another way, is there a good reason why the connections stay clean, bright and tight?

Oxygen is bad for metals.

Been noodeling with the idea of an N Gauge layout built into a coffee table under a plexi cover. Using a commercial power supply with four wire sensing to get past the voltage drop problem.

Gross over kill, but that is my style.

Much luck on your project, neat photos.

Jack Crow in Virginia

[Rulycat]

Re: Model Railway capacitors

Jack Crow,

It only sparks when I have the cap across and it is trying to draw loads of power at once. Hence why I will be trying a resistor to reduce the current. There IS a connection problem, inherent because LGB use brass track that oxidises like hell. I do have a component which will fit under the carriages or other vehicles (under a crane vehicle right now) that cleans up the track. In order to keep the train completely smooth this has to be attached at all times. This experiment I will be performing will be to try and work around the problem using non-conventional (and cheaper!) means.

To be clear: It does not spark with just the bulb attached. I can actually connect the cars to the locomotive. They have wiring points to do so. This means the loco and the cars share all pickup points. But it's a pain to uncouple and change cars.

[ben7]

Re: Model Railway capacitors

I think that, because you have no rectifier on the caps - First, the power stored in the caps is back-feeding into the tracks, and causing them to drain quickly. Second, with a abridge rectifier, you don't need to make a special bi-polar capacitor, and worry about it blowing up like it almost did

[Rulycat]

Re: Model Railway capacitors

Well I have now successfully created a rectifier! I used the 4 diodes and they're currently held together in terminal blocks, because I stupidly bought a breadboard whose holes were too small for the diode leads. I have attached the 4700uf cap to this along with a 100ohm resistor, so when I next have access to the car, I will test the car with the new circuit and if it works then make it neat.