Making trains run smooth with capacitors (electronic-flywheel)

[Toni Babelony]

Hello all,

I've been wondering about this after seeing this video:

<a href="http://www.youtube.com/v/GBBcFTKVaaE" target="_blank">http://www.youtube.com/v/GBBcFTKVaaE</a>

Now, I'm quite a noob when it comes to micro-electronics, but I do know you have to be careful with capacitors as there are quite a few different types around. My guess is that you need a capacitor with a high capacity and a relatively low breakdown voltage, thus an electrolytic capacitor.

My question is: is it possible to use a normal electrolytic capacitor, or do you need a special AC-designed capacitor for this purpose? I'm not sure, but I guess a normal one is sufficient, as the capacitor would be drained after stopping the train. However, I could be horribly wrong.

[Ben]

You can use a normal electrolytic capacitor - I use some big ones under my tram layout to get a smooth stop - but be careful of the polarity, electro caps don't like to have reverse bias applied to them, and can explode if you do! AC electro caps are available but I've never been able to find ones with high capacities.

[Ben]

Here's a good article (for slightly larger trains)

http://www.trainelectronics.com/artcles/capacitor/index.htm

[Toni Babelony]

Thanks for the information!

Sorry to ask these questions, but my understanding of micro-electronics only go this far:

1) When looking at the attached schematic from your link (which I probably will be using as it looks like it will allow bi-directional travel) I was wondering if the capacity of the capacitor influences the resistor to the LED. If so, what formula should be applied to it?
2) Regarding capacities of the capacitor, what capacity should be sufficient? I have seen 10V 6800µF capacitors, but this to me seems too little.

[Ben]

No problem!

The LEDs are optional. They just indicate the charge state of the capacitors: the resistors are to limit the current through the LED.

I used the circuit below as a trackside inertia simulator, one for each track section, so I can drive my trams 'on the switches'. The circuit is connected between the rails: the capacitors are mounted beneath the layout.

More details...

I used 6,800uF, 16V capacitors and 10 Ohm resistors. Really, the resistors are just there so that if I accidentally short the rails, I don't get a huge current. If you could find small enough capacitors to fit the circuit in the vehicle you could probably omit these resistors.

You can work out the capacitance you need to get a decent braking time from the RC time constant t = RC (see http://en.wikipedia.org/wiki/RC_circuit). For simulating inertia R can be calculated from the current draw of your vehicle, say if it draws 100 mA at 6 V this is an equivalent resistance of 6/0.1 = 60 Ohms. A 6,800uF capacitor will give t = 0.4 s which actually looks pretty good!

I used these capacitors: http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=7111043.

These are pretty big, 16 mm x 30 mm, obviously this will not fit in an N-scale vehicle! If you find small enough ones - maybe supercapacitors - let me know!

[Toni Babelony]

Thanks for the kind reply ben.

These are pretty big, 16 mm x 30 mm, obviously this will not fit in an N-scale vehicle! If you find small enough ones - maybe supercapacitors - let me know!

I've seen a lot of supercapacitors with the right size ((Ø x H) 10.5 mm x 5 mm), but with a a max.V of only 5.5. So that would mean I would have to get four of 'em in one train (two in series). That is quite impossible, so I'll have to settle with a max. V of 5.5 on my layout, which is okay with me (I almost never run my trains at full speed).

Example of the type (I've seen these in 0.047, 0.1, 0.22, 0.33 and 0.47F and more): http://cgi.ebay.de/PANASONIC-EECSE0H224-Goldcap-Kondensator-0-22F-5-5V-10x-/310289387209?pt=Bauteile&hash=item483eb04ec9 Two of these should fit in a regular Modemo tram with some squeezing. I guess the one with the largest capacitance is the best for the most realistic/slowest breaking result, right?

For the diode: can I just use any type over 5.5V?

[Ben]

The highest capacity may not be the best - I recommend you measure the current draw of the tram and aim for t=RC of a second or two. Otherwise your vehicles will coast too far!

Those supercaps look great! Be careful not to overvoltage them though as this can be dangerous. Can you limit your supply voltage to 5.5 V?

Any rectifier diode should do, 1N4004 diodes are standard and can take a few hundred volts. You do want to worry about the current rating which is 1 A for these diodes.

Good luck with this, I am very interested to see how it turns out!

[Toni Babelony]

Thanks a lot!

Tomorrow I'll be looking at Conrad (large electronics store) for the components and a new multimeter, my old one seems broken. I'll buy several different supercapacitors and experiment with the time. It's always better to see how it works out in practice then to see how it looks in theory

As for limiting the voltage to 5.5 V, that shouldn't be a problem. I just shouldn't open the tap too much and watch the volts. Maybe I'll build my own controller in the near future if I get the hang of small electronics.

I'll keep you updated about my advances.

[KenS]

Capacitors are very sensitive to overvoltage; it's a good idea to overrate their voltage by 25-50%.  Also, voltage numbers for power packs may only approximate. For a typical Japanese pack, the max is likely around 12V, so 16V is probably safe, but 25V is safer.  For western powerpacks, which are nominally 16V, 25V is probably safe, but 35V is safer.  And for DCC, which can be as high as 22V (and reportedly some have been higher), 25V might be a bit too low. One issue is that power packs don't always put out a flat voltage; some are "pulsed", and thus the RMS voltage reported by a meter may be less than the peak voltage output (even Kato, which is not strictly pulsed, uses a rippled output waveform).

Now if you're designing for a specific power supply, you can obviously pick a capacitor voltage to your own needs, but the 25% (or more) margin is still a really good idea, particularly if the capacitor is to be located inside an expensive model that could be damaged by a failure.

[Martijn Meerts]

You could also have a look at http://www.digital-plus.de/e/digitalplus_decoder_power.php . It's designed to be used with DCC decoders, but it might give you some ideas. All Lenz 0-scale loco's (and lighted cars) have these built in from the factory, and I can say they work great. I had my V100 drive onto a piece of paper I put on the track, and it just kept on going even if the entire loco was on the paper, and therefor not get any track power at all.

[keitaro]

Interesting so this for example http://www.jaycar.com.au/productView.asp?ID=RY6808&CATID=51&form=CAT&SUBCATID=858

Is small could be put in my motor car and power the motor down slowly as it discharges.

This is smaller than yours linked earlier but still big.

But this would have a one direction limit?

[Ben]

Maybe I'll build my own controller in the near future if I get the hang of small electronics.

I built my own controller - not as difficult as you might think, I used the circuit diagram here:

http://www.rickdavis.co.uk/rail/control-controller.php

[Ben]

http://www.jaycar.com.au/productView.asp?ID=RY6808&CATID=51&form=CAT&SUBCATID=858

This is a bipolar electrolytic capacitor - good, in that it can be used for two directions - but the capacitance is too small for our purposes I think, at 6.8 micro Farad.

Generally the size of a capacitor is a function of capacitance and max. voltage, so a 1 Farad, 50 V capacitor will always be a monster! Electrolytic caps are smaller than other types, supercaps smaller still.

[CaptOblivious]

It's not clear to me in the original movie that that was done entirely with capacitors…or am I mistaken?

Anyway, as others have said, having the super largest cap ever isn't always necessary. And, as a pedantic terminological note, "super caps" are a different beast from very large electrolytics (http://en.wikipedia.org/wiki/Electric_double-layer_capacitor).

If size is an issue, and cost is not, you might consider tantalum caps, to add my two cents.

[The_Ghan]

I've been reading this thread with interest and have a few questions relating to use on N guage - indoor, of course:

1. Is there any benefit of a capacitor on N guage engines, and if so, what are the benefits? 

2. If so, why don't suppliers like Tomix and Kato include them?

3. Presuming a capacitor might smooth out running over points and crossings, what capacitor might realisticly work AND fit in a motor car of a suburban EMU?

4. I'm using DCC, if a capacitor is beneficial, which side of the decoder should it go, track side or motor side and do I need a different type of capacitor for DCC as against DC?

Cheers

The_Ghan

[KenS]

On DC, a capacitor (or more complex circuitry as described above) will charge to track voltage, and supply that to the motor when track power is lost. This provides a form of momentum that can ride across gaps to provide realistic stopping behavior as the capacitor runs down, provided you use a large enough capacitor.

Unfortunately, on DCC it's not that simple.  On DC, a capacitor will hold voltage up to keep the motor running, because voltage = speed.  But on most DCC systems, the output to the motor isn't constant DC, it's PWM (square waves with a peak voltage close to track voltage and a various-duration gaps to lower the average voltage to whatever speed you need).  A capacitor would charge to something like track voltage (assuming the current to charge it didn't just fry the decoder, which is more likely) then degrade from that when the power was removed.  If you didn't fry the decoder, you might be able to control the speed somewhat, as the partial waveform wouldn't be able to fully charge the capacitor.  But it probably wouldn't work very well (although I'll admit I haven't tried it).

But the real problem is charging the capacitor.  Without something inline to limit current, it's effectively a dead short across the motor terminals until it charges, and the current that draws will easily exceed the decoder's output rating, and with a capacitor large enough to be useful, it's likely to do that long enough to damage the decoder.

[keitaro]

i wouldn't mind buying a motor car and try attach a capacitor to one if anyone wants to send a capcitor i'd need to place in an nscale motor car spare.

[Barobutt]

I've never done any electronics work, I've never installed a DCC chip, hell I've only once ever taken the shell off a loco to fix it's paint.  My electrics experience is limited to plugging in my throttle and connecting it to the track.

That said, this interests the hell out of me.  I'd so love my trains to start and stop slower as a real train should.  Would a capacitor make the train start more slowly as well?  I don't plan on going to DCC any time soon, mostly because I have a small layout that really can ever run 1 train at a time.  I would so love to do this, are there any good step-by-step guides that a total scrub like my self could follow?  Is this even worth looking into for someone at my skill level?

[keitaro]

hmm i'm looking at trying out a cap in a steam loco i want to see if it makes a difference i was thinking in the tender as it should fit.

http://au.element14.com/jsp/search/productdetail.jsp?sku=1698331
would that be anygood?? it's bipolar and a touch over the v range. i have no idea how the farad thing works .....

[cteno4]

I seem to remember this being answered somewhere, but what is the ceramic capacitor for in the kato pocket train. It's I think a 0.1uf so it can't provide much in the way of charge to skip over dirty points on the track. 

Any ideas?

Jeff

[brill27mcb]

It acts as an AC bypass around the motor, to reduce electrical "noise" from the motor and commutator that can cause static on nearby radios and TVs. German model locomotives and slot cars I've seen have such a small capacitor or a small choke to reduce transients -- may have been a requirement.

Rich K.

[cteno4]

rich,

thanks that tingled the neurons, it comes back now...

jeff

[KenS]

hmm i'm looking at trying out a cap in a steam loco i want to see if it makes a difference i was thinking in the tender as it should fit.

http://au.element14.com/jsp/search/productdetail.jsp?sku=1698331
would that be anygood?? it's bipolar and a touch over the v range. i have no idea how the farad thing works .....

Unfortunately, capacitors that fit in trains don't store very much real power.

A Farad (which is a lot) is an Amp-second.  A typical modern N-scale motor needs about 30 mA to run the motor.  So you'd need 30,000 micro-Farad to run the motor for a second. So assuming that 22 MFD (micro-Farad) capacitor could provide all its stored power (and it can't) to the motor, you'd get less than 1/1000 of a second of extended operation from it.

You can get caps with 10-20 times the capacity that will fit in an N-scale loco (not without removing weight or taking a very visible space in an EMU). Which means you'd get several milliseconds of operation. I don't think that would be enough to work. Flywheels work much better for N scale.

HO is much better.  While the motors need more power, you can also get supercaps that will fit an HO loco, with massively more capacity. But it's not quite that simple.

You need to prevent caps that large from providing a dead short or you'll likely trip the circuit breaker on the power pack (or worse, damage something), so some kind of resistor is needed between one lead and the track to limit the charging rate. The size depends on the maximum current you want to allow.  Assuming you don't need more than 500mA, to be safe on a worst-case 22V DC system (like an MRC pack), you need 22/0.5 = 44 Ohms (39 and 47 are standard sizes, so you could use either).  But the power dissipated when the motor is running becomes a problem.  If your motor draws 100mA you'll be losing 4 volts across that resistor, which is enough to notice.  You'll also be producting nearly 1/2 watt of heat when running and over 2 watts briefly when charging (a 2W resistor might work), and more when charging, so it would need to be a fairly large resistor (3 watts).  You may be able to optimize the size for a specific motor and track voltage, but it's a bit of a balancing act.

Ohm's law is what matters here.  To limit charging rate, R = V / I (where V is max power pack voltage and I is desired current in amps).  That's where 44 = 22 / 0.5 comes from.  To calculate voltage drop across the resistor, V = IR (where I is operating current of motor) and to figure resistor power rating, take the higher of charging or operating current and the voltage drop just calculated, and P = V I (where P is watts).  Thus V = 4.7 = 0.1 x 47 and P = 2.3 = 0.5 x 4.7.

[The_Ghan]

... and thus it is proven that model railroading is simply a branch of modern electronics !!!

Cheers

The_Ghan