Why DCC locomotives appear to short out on track

CMX HO Scale CMX Machine HO Scale

Out of curiosity why does an engine encountering dirty track cause a PSX to trip? I know, I’m not using my CMX as often as I should, but I still wanted to understand what’s happening electronically. I have set the PSX CV55=1 and CV65=32 enabling a delay of 4ms. Thanks for all the help.-----------------Bob A. in New Hampshire


Most of today's engines have very large capacitors on the input to the decoder. The purpose of these is to keep the decoder operating during short losses of power (e.g. dirty track). When the engine looses contact with the rail (dirt) the capacitor starts to discharge to keep the decoder active. When rail contact is restored, the capacitor re-charges. Here is the key piece of information: a capacitor at the start of the charge electrically appears as a short circuit with the current only limited by other resistance in the circuit. So, if you discharge the capacitor a little bit and then charge it, the difference between the charging voltage and the capacitor voltage is small, so the current spike is not too large. As you discharge it more and more, the voltage difference increases and the current spike to charge it becomes larger. Eventually, it can trip the PSX. With the settings you have, the PSX is telling you that you are getting charging currents of at least 4 amperes for at least 4ms. You can set the PSX to ignore these, but do you really want to keep pounding your engine with current spikes?

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If you have a CMX car, here is my suggestion. Get in touch with us and order the 25ml DeOxit D100L (it is a little pricey, but a little goes a long way). Add this to a pint of 91% isopropyl alcohol (usually available at a drugstore). Use this mix in your CMX car, it works wonders. I do volunteer work for several museum layouts, and the trains run vastly better after we started using this mixture. The DeOxit chemically cleans the rails to ensure an electrical contact and the alcohol dissolves the gunk. I use this on my layout and it runs beautifully. I have a small N scale layout that comes out at Christmas time. This year, it would not run, so I used some of the DeOxit solution to clean the track and engine wheels. Voila! Perfect operation including very slow yard speeds! One minor issue is that you will loose about 10% traction right after you clean, but this will go away within a day.