Capacitiors or Keep-Alives with Sound Decoders?

[tehachapifan]

Would like to build-in a little more dirty track tolerance with my sound decoder locos, as they really need immaculate track and wheels right now to avoid the hiccups. With space already at a very high premium, what is the best device(s) you have found that adds some reasonable tolerance but is small enough it can still be tucked away somewhere? Some keep-alives I have found online appear to be way too big.

[jdcolombo]

Hi Russ.

Ever since I began doing sound installs, I've used 220uf, 16v tantalum chip caps for keep alive.  In a typical diesel installation, I try to use two of these caps wired in parallel to give me 440uf of total capacitance, which I've found ends all dropouts on reasonably clean track (note the "reasonably clean" - not immaculate, but if the track is truly dirty, 440uf isn't going to fix things). 

I get the chip caps from Digikey.  They are AVX caps, 10% tolerance, and are 3mm thick, 4mm deep, and 7mm long. Here's a link:

http://www.digikey.com/product-detail/en/avx-corporation/F931C227KNC/478-8257-1-ND/4005713

As for installation, I've found that you often can fit a "sandwich" of two caps behind an 8x12mm speaker in a typical diesel installation; another possibility is the center "channel" in a decoder-ready unit (you might have to widen the channel just a bit).  I've also stashed them below the front headlight in an Alco C420 install, hidden by the black plastic "curtain."   I've even stuffed them in a cab once.

The only caution about using these is that tantalum caps will literally explode if they fail.  I had one fail inside a Bachmann 2-8-0 tender, and it managed to melt a spot on the tender side when it did (fortunately, it did not ruin the decoder!).  For that reason, I'm careful to keep the track voltage on my layout at exactly 12v (this is a Digitrax Chief running in the "n" position, with the internal voltage pot adjusted to give me 12v at the rails).  The 12v means that I have a considerable safety margin, and I've found that 12v is more than enough for our N-scale motors to run at crazy-fast speeds anyway.   However, I have (accidentally) run my engines at the HO setting on my Chief, and that didn't cause any cap explosions.  Remember that a keep-alive capacitor is fed by the decoder after the bridge rectifier circuit.  A typical bridge rectifier will cause a 1.4v voltage drop from the input voltage.  So if I'm running track voltage at 12v, the actual voltage out of the rectifier is more like 10.6, which is a 5.4v safety margin for the cap.  The HO setting on the Digitrax Chief puts out about 14v, so after the voltage drop, it would be 12.6, which is probably still fine.  What you wouldn't want to do, however, is stick your engine on a layout with a 16v track voltage (or higher), since that's pushing the safety of the caps. 

I'll try to gather some photos of several different installs and post them later to show how I've used these in diesels.  For now, though, here's a photo of two of the caps next to a dime to give you some sense of size (these were wired with space between them for a steam installation I was doing; you can solder them back-to-back to make a very compact "sandwich"):

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John C.

[jdcolombo]

OK.  Here are some photos of installs.

Older non-dcc ready Atlas/China GP7 frame, in which I've milled out a center channel for wires and the caps (caps are hidden a bit under the kapton tape, but I think you can see where they are - along the bottom of the channel):

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Here is a newer DCC-ready GP7 frame, with two caps soldered back-to-back to form a sandwich that I've mounted behind the 9x16mm speaker:

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Here is a photo of my C420 installation.  You can't see the caps in this photo, because I've stashed them below the headlight in the black plastic curtain - if you follow the green wire that goes into that curtain, that's where the caps are:



In this RS3 installation, I put a single cap at the end of the speaker, with an SMD LED glued to it for the headlight:



The single cap works to prevent dropouts about ALMOST 100% of the time, but using two is better if you have space.

John C.

[peteski]

Would like to build-in a little more dirty track tolerance with my sound decoder locos, as they really need immaculate track and wheels right now to avoid the hiccups. With space already at a very high premium, what is the best device(s) you have found that adds some reasonable tolerance but is small enough it can still be tucked away somewhere? Some keep-alives I have found online appear to be way too big.

There might be some confusion in terms here.  John's "keep-alive" caps are really just to prevent sound dropouts, even on fairly clean track. They can supply probably less than 0.25 second of power to the decoder. They would be more like an "anti-flicker" device I have shown here for illuminated passenger cars.

But in the manufacturer's official DCC-lingo "keep-alive" circuits are very large capacitance (SuperCap) devices which have enough capacity to actually keep the entire loco (sound, motro and headlights) for few seconds of power interruption. Their capacitance is hundreds of times larger than Johns circuit.  Because of that they are much bulkier.

[jdcolombo]

There might be some confusion in terms here.  John's "keep-alive" caps are really just to prevent sound dropouts, even on fairly clean track. They can supply probably less than 0.25 second of power to the decoder. They would be more like an "anti-flicker" device I have shown here for illuminated passenger cars.

But in the manufacturer's official DCC-lingo "keep-alive" circuits are very large capacitance (SuperCap) devices which have enough capacity to actually keep the entire loco (sound, motro and headlights) for few seconds of power interruption. Their capacitance is hundreds of times larger than Johns circuit.  Because of that they are much bulkier.

Agreed.  440uf of capacitance won't keep the motor running for even half a second.  If you need that, you're basically screwed in N scale hood diesels, because we just don't have the room for it.  But 440uf will cure momentary sound dropouts.  Together with the flywheels in most modern diesels, I've found this to be sufficient when the track and wheels are reasonably clean.  By "reasonably": I clean my wheels using a wire brush type wheel cleaner about every six months.  Before an operating session (about every three months), I vacuum the entire layout and then go over the rails with 1200-grit sandpaper.   This routine keeps the rails reasonably clean in my finished basement, and I have literally zero sound dropouts with my capacitor-equipped units even when I'm casually running stuff between op sessions.

John C.

[tehachapifan]

Thanks for the info, guys! What I experience is sort of a mix between a sound dropout and a flicker. They are brief "hiccups" where the whole loco seems to lose power for a split second where the sound also drops out then resumes normally. The sound typically picks up where it left off without going thru a full restart sequence. FWIW, both my sound decoder locos have LokSound decoders, one of which is the Atlas S2. During a hiccup, I notice the S2's opposite facing headlight will flicker brighter (it is set up with Rule 17 dimming). At any rate, if caps have been known to explode if they fail, I may continue go without. Not sure if I want to risk experiencing that!

[jdcolombo]

Thanks for the info, guys! What I experience is sort of a mix between a sound dropout and a flicker. They are brief "hiccups" where the whole loco seems to lose power for a split second where the sound also drops out then resumes normally. The sound typically picks up where it left off without going thru a full restart sequence. FWIW, both my sound decoder locos have LokSound decoders, one of which is the Atlas S2. During a hiccup, I notice the S2's opposite facing headlight will flicker brighter (it is set up with Rule 17 dimming). At any rate, if caps have been known to explode if they fail, I may continue go without. Not sure if I want to risk experiencing that!

Hi Russ.

You won't have any problems with the caps as long as you don't put your engines on a DCC layout set for very high voltage.  I'm sure that the cap that failed in my 2-8-0 was defective from the get-go.  I've since installed over 150 of these caps in my engines, and never had a single problem.  There's always an outside chance of a failure not caused by a voltage overload, but I suspect that's about as likely as a plane crashing into your house.

What I don't know, however, is how you wire the caps to the Atlas S-2 board.  For a regular ESU LokSound Select micro, the positive end of the caps (the end with the stripe) goes to the blue wire; the negative end goes to the unused pad on the "underside" of the board that is directly opposite the blue wire pad - you can see where this is pretty clearly in the photo of the C420 if you follow the green wire back to the decoder.  This pad usually is covered by the plastic wrap, so you need to cut away enough of the plastic to let you solder the negative cap wire to the pad.  Since the pad is right at the edge of the board, it is easy to get to and solder once you've cut away a bit of the plastic.

And two 220uf caps wired in parallel will fix your "stutter."  Originally, I didn't put any caps in my "road" units.  Then I noticed a bit of the kind of stutter you describe: a very momentary sound dropout and light flicker with maybe just a barely noticeable hitch in the engine speed, before recovery.  So I added caps to all my road units, and that fixed the problem (I had already used caps in anything designated for switching duty).   I'm 100% stutter-free now.

John C.

[tehachapifan]

Thanks, John! I may give this a try in a future install. However, per the specs listed in the manual, my DCC system puts out 14.5V.

[peteski]

John,
I mentioned "less than 0.25 second" because I realized that with 470 uF it would be a very short time - I couldn't provide a specifics.

Russ: why not just take one of your locos and temporarily install the caps by hanging them off the decoder board, then test it out (just run the chassis without the shell)?

[tehachapifan]

Good idea! I think I may wait for my next install to give this a try.

[jdcolombo]

By the way - weight helps, too. I've been known to stuff the cab of a hood diesel with tungsten putty or sheet lead to add weight.  If I have room, I sometimes add a strip of 1mm lead sheet below the decoder, on top of the chassis (insulated by kapton tape).   I've found that even small amounts of additional weight are helpful for electrical contact.

John C.