DCC voltage control

In my previous post I described a simple home-made meter that can be used for a proper measurement of the DCC voltage. There are two reasons why you may need to take that measurement:

• your voltage may be either too high resulting in locos getting hot or not running at all (and possibly dying)
• or your voltage may be too low resulting in locos not running at all or running slowly.

I needed the meter since one of my older N scale locomotives was running hot after conversion to DCC. The loco came in a starter set with a power supply that provided a maximum of 14V. Using my meter I was able to identify that my DCC voltage was almost 18V.

I'm not really sure whether the locomotive was in any kind of danger. But it was getting hot and that worried me. So I decided to solve that potential risk. And how could I lower the DCC voltage on my layout?

The simplest way to lower the voltage is to insert a resistor into the circuit. That however does not work well in our case. The voltage drop on a resistor depends on the electrical current and that changes all the time when running the locomotives. In this case the more locomotives would run, the lower the voltage on the track would become.

We need a constant voltage drop instead. And the solution is in using diodes which do exactly that, independent of the electrical current. The diodes however pass the current in one direction only and therefore we always need to use a pair to ensure the full DCC signal. The circuit diagram looks as follows:

So we basically attach some diodes in series on one of the wires connecting our command station to the layout track. Each diode generates around 0.6-0.7V drop and any number can be used depending on the need. Of course the number has to be the same in both directions.

I've decided to lower the voltage to around 15V and that resulted in 4 diodes in each direction. Using my poor soldering skills I created this device which works perfectly:

What kind of diodes do you need for this? Any rectifier diode will work but you should consider two aspects:

• The diode should be 'fast'. The DCC signal is not really high frequency, so it should not matter that much, but it's your choice and the difference in price is very small, so always go for 'fast' diodes.
• The diode's maximum current must be more than what you plan for your layout. My diodes are rated 6A and that's way more than I'll ever need with my N scale trains.

Of course I know that some modern DCC systems allow us to change the track voltage. Even the Z21 I use can do that. But I'm using my command station for two different scales and it's easy to forget to change the setting when going back from my garden layout to my N scale layout. And that's why I prefer to have a hardware solution which ensures that I don't kill my decoders when I make a stupid mistake.

And here's the reason behind my DCC voltage experiments - the very "hot" locomotive: