# Does wiring a capacitor to decoder help continuity?



## musicwerks (Jan 4, 2012)

Hi,

I am just wondering if we wire a capacitor in series to the decoder...

Sounds crazy, but I wonder if it will help with continuity of loco over dirty tracks...

a) Where to solder the capacitor? what specs of capacitor to use..


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## Ray Haddad (Feb 19, 2014)

The DCC decoder should have a delay to losses of signal. If you are worried about the signal not being present causing the DC to the motors being lost a capacitor in series won't help. The motor on a good engine should have a flywheel which will do more to keep things moving across the gaps.

A capacitor will cause a loss of the signal that is needed for converting it to DC for use by the motor. It will most likely cause your engine to be dead in the worst case or unable to develop enough current to start moving in the best case.


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## spoil9 (Dec 24, 2008)

Agreed with Ray. A cap in series will block all DC power from getting cross it and may interfer a little with the DCC signals.

A cap in parallel would act like a small battery for the DC. As far as values, use at least a 25V cap for safety.


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## musicwerks (Jan 4, 2012)

spoil9 said:


> Agreed with Ray. A cap in series will block all DC power from getting cross it and may interfer a little with the DCC signals.
> 
> A cap in parallel would act like a small battery for the DC. As far as values, use at least a 25V cap for safety.


So to ensure motor continuity, I can install a 25V capacitor parallel across the grey and orange wires going to motor?


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## DonR (Oct 18, 2012)

The old worry wart noses into this thread.

I have concerns about a cap across the motor leads.
It would be OK polarized for front but not right when
backing up. The DC polarity is reversed when backing.
I use caps in my lighted cars, but I have rectified
the DCC current, and it's polarity does not change
when backing. My track and wheels are clean but
the pickup wipers momentarily lose contact at times.

I have even more concerns about the need for a cap. If your
loco lights are not flickering you don't need a cap
to smooth out the current. If they are not steady it's
time to look for what's causing it.

Just normal track and wheel maintenance should be
sufficient for smooth locomotive running on your
DCC layout. If you see a flicker. Stop the engine. Inspect
the track where it occurred and you'll find a small dark spot that needs to
be scraped off.

Don


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## Ray Haddad (Feb 19, 2014)

Don's right. The signal on your DCC line is already a dog's breakfast and adding a capacitor will make the edges of any pulses less well defined. More caps (one on each engine) will increase distortion.

Do not add a capacitor at all. Not on the track side.


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## D&J Railroad (Oct 4, 2013)

I think the OP is trying to keep the decoder running smoothly when running across dirty track or dead frogs. Sorta like the TCS decoders do.
A friend of mine gave me a schematic for installing a cap with limiting resistor and diode to do this. I haven't installed it yet as I've been overcome by other events this past two weeks.


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## Ray Haddad (Feb 19, 2014)

Ken, 

His best option is to either replace his motor with a flywheel equipped motor or get a different decoder. He cannot just add a cap anywhere that will make a difference. They aren't batteries, you know. They are very small in storage capacity.


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## D&J Railroad (Oct 4, 2013)

But that's all ya need for bad track. Just a second or so of continued running to get over the dirt. The Wow decoders have them built in but can run for about 6 seconds on plastic rail.


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## musicwerks (Jan 4, 2012)

DonR said:


> The old worry wart noses into this thread.
> If they are not steady it's
> time to look for what's causing it.
> 
> ...


On hindsight, Don is quite right. Its time I scrape off those nasty plaster powder that is clogging up / rusting up the tracks at the road crossing 

(should have stuck to plastic sheets than using the woodlandscenics method of pouring plaster on tracks)

Also, I have some sections of dead tracksections...that are not soldered.


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## Ray Haddad (Feb 19, 2014)

D&J Railroad said:


> But that's all ya need for bad track. Just a second or so of continued running to get over the dirt. The Wow decoders have them built in but can run for about 6 seconds on plastic rail.


The original question was to install it in series with the decoder. A capacitor will couple voltage across the dielectric but no current. Not a single milliamp will shine through. And that means no motor movement at all.

On the other side of the decoder, you would need a capacitor of a minimum of 1000uf at 25v to carry the motor through for around .5 seconds. The mechanical solution is far better. A flywheel. Trying to do it after the fact by adding a component to an unknown circuit is asking for trouble. I have no idea why anyone would advise it. Just spend the extra on the decoder that doesn't actually promise to keep the MOTOR running but promises to keep the DECODER alive during those errant transitions.

Whatever you guys advise someone to do, remember it is upon your advice that they will do it, not on mine. I advise against it and am now bowing out of this discussion. I mean, I'm only an electronics engineer. What do I know about this.


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## DonR (Oct 18, 2012)

Ray

I agree. A cap across the DC motor leads out of the decoder would
charge only while loco is going forward. When the DC polarization
reverses for backing up, what happens to the cap?

Don


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## Ray Haddad (Feb 19, 2014)

DonR said:


> I agree. A cap across the DC motor leads out of the decoder would
> charge only while loco is going forward. When the DC polarization
> reverses for backing up, what happens to the cap?


If it happens that the user puts an electrolytic of 1000uf across the motor, reversing it will sauce a loud pop and a "fluffing" of the engine shell. Guaranteed. Any other type of cap will be far too small to be of any use. If they use a tantalum, the pop will be much sharper and there will be shrapnel. Hide the women and children.

I've seen this theory espoused on other forums and in other places. They all seem to forget one other minor detail. Any capacitance in the circuit that was not designed into it is going to cause some very intermittent problems. Not worth the trouble.


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## DonR (Oct 18, 2012)

Ray

That's what I thought. You just don't put inverse polarization
across a capacitor.

Don


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## musicwerks (Jan 4, 2012)

Thanks Gents,

I now concur with you gentlemen that its not worth risking / or feasible with attaching a capacitor. It probably better to buy the TCS decoder with built-in capacitor for continuity. 

Well, I know my tsunami T1000 has a capacitor, it didn't do wonders on dirty track too, just a bit to prevent some bad stalling (specially smaller switchers with fewer wheels, my bigger 2-8-8-2 had no problem as the wheels are further apart.

I am sure the engineers in TCS and tsunami must have had done a lot more than merely connecting a capacitor...after all, they run trains and make decoders for a living...

Cheers
Music


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## DonR (Oct 18, 2012)

Music

Most likely the capacitors built into your decoders occur on
the input side after a rectifier. That way the entire decoder
would continue to be powered over those pesky track faults
and would not have the polarity problem that we have
discussed here. What we were pointing out is that you just
cannot put the capacitor across the motor wires for
the reasons given.

Don


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## Mark R. (Jan 26, 2013)

The capacitor on Soundtraxx IS a keep-alive capacitor - but only for the sound section of the decoder. It provides enough power over momentary power losses to prevent the decoder from always cycling through its start-up sequence in the event of a momentary power loss. It provides no keep-alive for the motor control circuit.

IF you want to add a capacitor to provide motor keep-alive, this site will provide all the information you need ....

http://www.members.optusnet.com.au/mainnorth/alive.htm

Mark.


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## DonR (Oct 18, 2012)

Mark R. said:


> The capacitor on Soundtraxx IS a keep-alive capacitor - but only for the sound section of the decoder. It provides enough power over momentary power losses to prevent the decoder from always cycling through its start-up sequence in the event of a momentary power loss. It provides no keep-alive for the motor control circuit.
> 
> IF you want to add a capacitor to provide motor keep-alive, this site will provide all the information you need ....
> 
> ...


Just a scan of the info on the above site seems to say that the keep-alive
is in fact wired into the power input side after the decoder diode rectifiers. Don't
see how a cap could be wired into the circuit anyplace else.

Don


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## Mark R. (Jan 26, 2013)

DonR said:


> Just a scan of the info on the above site seems to say that the keep-alive
> is in fact wired into the power input side after the decoder diode rectifiers. Don't
> see how a cap could be wired into the circuit anyplace else.
> 
> Don


That's exactly where it's wired. It has to be wired after the rectification due to the polarity of the capacitor.

Remember though, while the capacitor WILL power the engine through dead spots, there are no signals being received at that time because, well .... it's a DEAD spot ! However, the decoder will still continue on based on the last command it received (motor / sound / lights).

It will also continue running past that dead track you created for your drop bridge across the open door - so take that into consideration as well ! 

Mark.


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## musicwerks (Jan 4, 2012)

DonR said:


> Music
> 
> Most likely the capacitors built into your decoders occur on
> the input side after a rectifier. That way the entire decoder
> ...


Yeap Don, agree...rectifier and cap is needed. Learnt something new today


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