Background
DCC decoders are really on-board computers or micro-processors. As such, they need reliable and regulated power to their control circuits. Unfortunately, since the locomotives and cars fitted with DCC decoders roll across tracks, and the tracks are subject to being dirty or power pick ups are intermittent, a common problem with DCC equipment is loss of control, intermittent operation, and sound trouble.
The basic concept behind attaching capacitors to the DCC board is to allow the decoder to ‘ride out’ intermittent power issues. In fact, many decoders come with on-board and separate capacitors. However many of these are not very large and have very limited capacity to store enough energy to make a significant difference.
The Solution
The FIRST thing to do is assure the track is clean and the rolling stock, be it a locomotive or a car, has good contact with the track. Adding capacitors will help the problem, but if the basics are not taken care of, no amount of electronics will help. Even with plenty of capacity, intermittent contact with the DCC system will result in spurious command response, loss of control and general mayhem.
The next thing to consider is adding capacitors. Most decoders already have some kind of capacitor on the logic bus of the decoder. But these generally don’t drive the sound and lights. OK for the microprocessor; not so OK for us operators. So what to do? Add capacitors!
There are two types or approaches to adding capacitance:
Keep Alives: Based on super-capacitor technology. These capacitors are typically rated at 2.7 volts. Therefore they need to be placed in series to assure any single cap does not overvoltage. These are also very sensitive to any spurious voltage spike, and therefore should have some kind of zener diode to bleed off high voltage.
Stay Alives: Basically these are conventional electrolytic capacitors that are placed across the power bus on the decoder. More on that later. Typically these are either 16 or 25V rated, so they operate on the same voltage as the decoder and tracks. They are however considerably larger and without the capacitance of the super caps. Typically the largest capacitance that will fit in a typical HO locomotive is around 3500 uF. This is in contrast to the nearly 1F rating of a super cap. Still, this is enough to get over some of the most common track and power problems found on most layouts. The Stay Alive is sometimes also called a “poor-man’s keep-alive.”
Digging Into a “Stay Alive Capacitor”
A Stay-Alive is basically just a larger cap that powers the decoder power bus. It is using the conventional electrolytic capacitor technology vs the true “keep alive” circuits which use the super-caps. Super caps are limited to about 2.7V. The standard electrolytics are 16V or 25V. I use 16V like the SS ones, but some have made the point that if used on a layout with higher voltage, the cap may let the smoke out (violently). If that is a concern, use the 25V. They are, however, larger. On an aside, I put these stay alives in many of my loco projects, especially ones that have intermittent pick up issues, or trouble getting through a turnout such as a “critter.” The keep alive circuit will keep things going for several seconds, allowing it to traverse many feet of track before running out of juice. Stay alives have at most 1/4 second of power or so. So it’s really for getting the loco over a momentary dead spot. That said, this simple addition as allowed a lot of my vintage locos to play well with sound. For example, I have an Athearn BB U28B that got a second lease on life with the simple addition of this cap. Lights no longer flicker and the sound is great. 🙂
If you are not sure how much difference these will make, check out this video…
Making your own Stay-Alive
The schematic can be found in a number of places, but here it is for easy reference:
Here is another location:
For wiring to the decoder, you will need to tie this circuit into the DC power bus on the decoder. How to find that is a bit of an Easter egg hunt. As such, I would refer you to what I call the gold standard of reference works to do this:
http://www.members.optusnet.com.au/mainnorth/alive.htm
It will guide you on how to find where to wire the KA or SA. It does involve soldering on the decoder, so unless you are comfortable with that, you may want to elicit the help of a buddy with the right equipment.
I use a standard 100 ohm, 1/4W resistor (although I am sure I could go smaller), and a 1A diode (1N4001) (can buy them by the bag on ebay). There is really not much to making your own. Attached are a couple pix of a recent install. It really is that easy. Like with any other component, the hardest part is finding a spot for it.
Links to options for capacitors:
2200uF caps: https://www.ebay.com/itm/293687774325
And the ‘new’ 3300uF caps…https://www.ebay.com/itm/303895574236
[Disclaimer: I have no ties to any of these vendors. Just sharing options!]16V vs 25V Caps
So let’s talk about the use of 16V vs 25V capacitors in model locomotives. DCC specifications allow up to over 20V on the DCC bus. Why in the world would anyone put in a 16V cap. These things blow up if over-voltage!
The reason is size. A comparable 25V cap is considerably larger than its 16 counterpart. Check out this comparison showing a 1000 uF cap in both 16V and 25V form next to an Athearn MP15AC for size comparison. Clearly the 16V will be much easier to stuff in there.
The reality is most HO layouts run at considerably below 16V, generally around 14.5V. And keep in mind the cap is wired AFTER the rectifying diodes on the decoder, each taking about 0.7V off the track voltage. So the actual voltage on the capacitor for your average HO layout will be around 13V, well below the 16V advertised limit on the cap.