# Where goest thine capacitor, fair PollySwitch?



## JackM (Jul 29, 2008)

Boy, I so rarely quote Shakespeare.

I took Thomas the Tankard apart in order to give it a quick lube job and install some polyswitches. I also had to replace a couple semi-fried wires off the track pick-ups, so definitely better late than never.

With only two axles to pick up the track power, Thomas doesn't do very well at low speeds but I'd like to not have to keep giving him the nudge to get him going. The newer granddaughter is a few days short of *2*, so I'd like to improve performance as well as prevent frying the internals (although the witches would love to read said entrails). I have a nice 35v, 1500mFd cap ready to install as well as the polyswitches.

Thus bids the question (ya know, I really hate Shakespeare): where does the capacitor go - before or after the polyswitch?

Is it pickup-polyswitch-capacitor, or pickup-capacitor-polyswitch?

JackM


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## toddalin (Jan 4, 2008)

I've no idea where the capacitor would go for dcc. Seems like it would be a part of the existing circuitry and has nothing to do with the polyswitches.

Put the polyswitches between each front wheel pick-up and the circuit board, before the point where the front axle and rear axle wires come together, or between each rear wheel pick-up and the circuit board, again, before the point where the front and rear axle wires come together. You could do both axles (four switches), but it is _largely_ redundant.

Really makes no difference, but I prefer the fronts because if the track is dirty and the front wheels loose contact, under the additional load the polyswitch on the rear is more likely to open up.

If the polyswitch is on the front, there is the chance that the track is just a bit cleaner when the rear wheels pass over the bad spot forcing less current through the polyswitch on the front with less tendancy to open up. If and when it does open, that set of wheels will loose contact and the train gets "jerky" running on one set of wheels/trucks. Remove the engine from the power for a few moments to let it cool down and it's good to go again.


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## JackM (Jul 29, 2008)

*Erase that thought*

I think maybe I was on the wrong track with that capacitor.

I put a fairly low cost Dallee sound board in Thomas to give him some chug, whistle and bell sound reasonably close to what he should sound like. The board has a plug which can be used to wire in a capacitor (suggesting 470mFd) to help prevent sound dropouts from track glitches, dirt, etc. But apparently my thinking was that the cap was connected to the track pickups. Wrong. It would be connected to the board output.

I already have a 470mFd cap on that output plug and it hasn't prevented the sound from dropping out frequently. I can try replacing it with the 1500mFd cap and see if it improves the sound.

This puts me back to square one as far as the question of where I could insert a cap to help prevent a jerky trip, if indeed it can be done. Time to take a look at all my NCE info and see if there's an solution to this problem.

And thanks for the advice, Todd, about polyswitch placement. That's what got this going in the first place.

JackM


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## toddalin (Jan 4, 2008)

JackM said:


> I think maybe I was on the wrong track with that capacitor.
> 
> I put a fairly low cost Dallee sound board in Thomas to give him some chug, whistle and bell sound reasonably close to what he should sound like. The board has a plug which can be used to wire in a capacitor (suggesting 470mFd) to help prevent sound dropouts from track glitches, dirt, etc. But apparently my thinking was that the cap was connected to the track pickups. Wrong. It would be connected to the board output.
> 
> ...


It seems like it could be done in the same way it is done for track power to maintain polarity with two caps. Dave Bodner explains how here. Note that the caps are quite large. As to whether this would have any effect on the dcc bit stream..., I don't know.

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

BTW, I don't know where you get your polyswitches or how much you pay, but I use these. I double them up for my diesels (two in parallel for each side of the front trucks), but they are so cheap ($1.00 per engine for all four and less $$$ in quantity) to protect the wiring, it doesn't matter.










http://www.allelectronics.com/make-a-store/item/cpx-100s/resettable-circuit-protector-1a-2.5a/1.html


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## Greg Elmassian (Jan 3, 2008)

Normally the "keep alive" capacitor goes in parallel with an internal power supply on a decoder/sound board.

For boards getting power from the rails, there is typically a full wave bridge rectifier right after the track power inputs.

These caps also also typically have an inrush limiting resistor, and sometimes a diode to give you full "outrush" if you will.

Greg


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## Dan Pierce (Jan 2, 2008)

For track power the only choice would be a non-polarized capacitor as the polarity on the track reverses with engine direction changes.

For DCC, many decoders have a keep alive place and in some you can add supercaps which let an engine run for more than a second.

Aristocraft/Crest had a board that could do this, is it still available via Navin at the new crest electronics in NJ?


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## toddalin (Jan 4, 2008)

Dan Pierce said:


> For track power the only choice would be a non-polarized capacitor as the polarity on the track reverses with engine direction changes.



No Dan, it just takes two caps and a couple diodes. You have to follow the link to Dave Bodnar's site that I provided.


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## Dan Pierce (Jan 2, 2008)

I would never place 2 caps in series with reversed polarity!!!!!
The diode drop is applying reversed low voltage to one cap.
You would not need the diodes with a NP cap and space for only 1 to be added.

Also, a minimum of 35WVDC should be used as capacitors have a 20 percent tolerance which means that a 25 volt cap can be 20 to 30 volts and 20 is too low for our trains.
And yes I know the tolerance is mainly for storage capacity, but one should assume the worse case always.

Sure you can cheat. many do cause 'it works', myself I would never do this.


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