Hey Tony's, In the past I have noted that if a loco or lighted car is left across block gaps you will hear a buzzing sound. I asked AJ Ireland [founder of Digitrax] about this once and he told me it was due to any disparity in voltage between the two booster blocks and said it was never a good idea to leave this for long. However I don‘t remember him telling me whether this might damage a decoder, etc. What is your take on this?……………Larry P. in PA

Larry: AJ is essentially correct. Picture the front wheel of an engine truck sitting on 13 volt DCC (district 1) and the rear sitting on 14 volt DCC (district2). The wheels are connected together by the wheel pickup contacts, a very low resistance, say 0.2 ohms. We then have a current loop with a voltage of 1 volt and a resistance of 0.2 ohms. Ohms law says that we will have 5 amperes flowing through the wheel contacts. The decoder leads will be somewhere between 13 and 14 volts.

So, to answer the question, the decoder is not going to see any voltage greater than the higher of the 2 voltages, which it has already been running on, so it is not likely to damage the decoder (exception: if the voltages are OUT of phase, you can get 27 volt DCC which will destroy the decoder immediately).

However, there are bad things happening. First is that 5 ampere current circulating through the wheel pickups. That can damage the pickups, or even worse, melt the truck. In my example, the pickup is dissipating 5 watts, which will certainly cause damage. The second problem is that one DCC booster is forcing current into the other. Depending on the boosters, this can result in booster damage. The fix is easy. Connect an AC voltmeter (in a pinch you can use a 1.5 volt grain of wheat bulb) from one district to another on the SAME rail. The meter will read the DIFFERENCE voltage between the two districts. Adjust the output voltage of one booster (either is OK) until the difference voltage is as close to 0 as you can get it. Using the belt and suspenders concept, you should probably avoid parking any  piece of equipment with wheel pickups across a power district transition.