SOUND DECODER INTERMITTENT POWER
12/33/15- This article restates info that I added to my D51/Orient Express article, but it's worthy of its own article. While the previous article in this section, "Troubleshooting Locomotive Intermittent Power" discusses basic problems that affect getting reliable power into a balky locomotive, this one discusses maintaining glitch-free power, primarily for sound decoders-- assuming that the trucks work reliably for getting power to the train's innards.
When a train briefly loses track power going over a turnout or bad section of track, you may not notice the quick light flicker, or a motor briefly losing power. When it happens to a sound decoder, it's hard to ignore because you can hear it.
Motor flywheels don't help to avoid the sound glitch. They help the locomotive coast through the bad section of track to reestablish good power contact, but because the sound decoder briefly loses power and you'll hear the glitch.
The ideal way to avoid this problem is to have perfectly-laid, clean track, and reliable, well-maintained track power pickups on your trains. That's the ideal and something that we should strive for, but difficult to achieve in practice.
Beyond that, there are two main ways to reduce the occurence of sound decoder power glitches: Improving power pickup and installing a power buffer/electrical flywheel/onboard rechargeable battery.
Multiple Power Pickup Points: You can improve power pickup by sharing/coupling power pickup across as many power pickup points (the wheels) as possible. This way, if one wheel loses power pickup, another one is filling in. This is as simple as running power plugs and sockets, or a 2-conductor coupler between all the cars that have power pickup wheels. This can also solve the problem of a locomotive that runs poorly due to bad power pickups.
Looking good! Power-sharing plug and socket doing double duty as a coupler.
Ugly, but gets the job done: Separate power-sharing plug and socket.
The downsides are that all locomotives and cars that share power need to be hard-wired for it; it can be a hassle to plug and unplug cars when you want to uncouple them, and unless it's built into the couplers, it's an ugly solution. But it's simple, reliable, and low-risk. There are no electrical components to age and eventually fail.
Keep-Alive Circuit: This is like an onboard rechargeable battery that can bridge brief losses of power to keep your sound decoder running without glitching. The key component is the capacitor, which stores DC power and releases it when power is interrupted. The downside is that capacitors are an additional component to install, take up space, and have to fit within the car. The more power-storage capacity, the bigger the capacitor. If insufficient power is stored, the circuit will have no useful effect. The familiar aluminum electrolytic capacitors with high capacitance tend to be very big.
Six Tantalum capacitors fitted in the space grinded out (in vain) for a 2200uF 25v Aluminum electrolytic capacitor that was sooooo close to fitting, but didn't..
Some types of capacitors can store energy more densely, resulting in a smaller component to install, but they're generally more expensive, more volatile and can cause ugly damage if they fail (see my Orient Express article).
Not so lucky the second time!
Which is Better? If you don't have problems with glitches, neither. Simpler is best. If your track and stock wheel pickups provide reliable power to the sound decoder, or you can ignore occasional sound glitches, count your blessings.
Shared power and keep alive circuits are something you should use when you do have problems that you can't ignore and can do the modifications safely, with minimal sacrifices.
The specific locomotive or car will dictate whether it can be done, and point to the best solution. Most n-scale locomotives don't have a lot of room to install large capacitors or a sound decoder and speaker, so a shared power solution may be the only option. This can also be a solution for a locomotive that drives poorly because of poor power pickup. Hard-wired shared power is a fairly common solution used by manufacturers for steam locomotives with attached tenders.
A sound car may have plenty of space for capacitors, a sound decoder and speaker, in which case a Keep-Alive circuit might be a good solution. You may have to block some windows to hide the circuitry, but that may be a perfectly acceptable sacrifice.
Keep-alive circuits do have some risks, especially DIY circuits that use bargain-priced Tantalum capacitors. It's very tempting to use them because they can be fitted in N-scale trains much more easily than the large (safer) aluminum electrolytic capacitors. There are plenty of sites on the Internet that warn against using Tantalum capacitors, and it would be wise to heed them. Witnessing a Tantalum capacitor combust will inform you of the potential damage they can inflict. Capacitors degrade with time, voltage spikes do occur, and there's nothing that forces Tantalum capacitors to combust only on first use, while on a test track. This should make you cautious about using them, and decide whether they're worth the risk.
If you buy a commercial Keep-Alive device, you should be able to assume that the design offers protection against catastrophic failure; if not, then you can blame the manufacturer! Sweet.