DCC voltage measurement

DCC is - to a large extent - a plug'n'play system. You simply assign an address to a locomotive and you're ready to go. And the equipment from different manufacturers is compatible, so usually there's nothing to worry about.

There are however situations when something does not work as expected. In the small N-scale it might a locomotive getting very hot when running at full speed. In the large G-scale it could be a locomotive not running at all in some parts of the garden layout.

That's when you want to know what's happening on your track and the first thing you'll want to investigate is the track voltage. Measuring it properly is not trivial. The DCC signal is an irregular square wave and no popular meter can handle that out of the box.

The best solution would be to use an oscilloscope but those are neither cheap nor portable. This might change soon looking at what is becoming available at JYE Tech. But that's a topic for another post...

There are also solutions dedicated to measuring DCC voltage like the RRampMeter from DCC Specialties. But those are not cheap either, and that's why I decided to build something on my own.

So here it is - my own DCC voltage meter:

OK, I know. It looks like a failed school project but I can assure you: it does work just fine.

So how do you make your own? The detailed instructions can be found here: http://www.wiringfordcc.com/track.htm#a4 but I'll try to summarize them below.

All you need is a very simple electrical circuit that will rectify and filter the square wave. It's really just a few components:

So what you need is:

• The rectifier bridge (the four diodes) - I'm using an integrated circuit here, it's cheap and small.

• A capacitor to filter the rectified signal - any will do as long as it is rated above the voltage you're going to measure. Mine is 35V.
  And you might not want to go too high with the capacity, as the more you use, the stronger switch-on surge you'll cause. 1uF should be enough.

• A DC voltmeter - I'm using a component bought on Ebay for around 2-3 Eur

• Some cabling - my choice is a wire with the "crocodile" clips but it's not the best choice in case you want to have the meter connected permanently to your layout

• A housing - I've used a beautifully transparent Apple mouse box

In my case all those components ended up soldered to a universal PCB and then hot glued to the housing. Works like a charm.

An important thing to remember is that such a meter does not really measure its input voltage. It measures the voltage after the bridge rectifier. You should expect that to be lower by 1.3-1.5V compared to the track voltage.

So what you should do is check what the difference is in your case. This can be done by measuring a DC source and comparing your result to a result coming from another "reference" meter. Then you always need to remember to take that offset into account when performing your measurements on the real track.

Total cost: below 5 Eur.
Satisfaction: immeasurable :)