# Curiosity: Finding MY Actual DCC Voltage



## wvgca (Jan 21, 2013)

Curiosity bit me tonight, and I wondered what my actual DCC track voltage was ... the old DC standard was 12V or slightly better as a minimum, and there are many discussions as to "recommended" DCC voltages, different manufactures have different "recommended standards", but in general they range from 14v to 18v, and some systems are adjustable for the different scales ... 
SO, my system is MRC prodigy, non-adjustable and [in my opinion] a serviceable but relatively lower cost system than most ...

First, set meter on AC range, and hook up probes







That reading looked higher than it should be to me at 17.65V, and although DCC is square wave AC signal, I feel that my inexpensive meter was not capable of reading the appx 10Khz signal, reads fine on 60Hz, maybe up to normal 1Khz, but I'm assuming not accurate at the DCC frequency .. 

Next, put a standard bridge rectifier into the circuit, that's what the DCC decoder does so that it can use the power, it converts to DC onboard .. 







That looked a little better, add in the voltage loss through the silicon diodes [some decoders have four diodes, some have integral diode bridges] but voltage loss is the same, appx 1.35V, for a total of 15.8v ..

Next, replace the bridge with a decoder, in this case an older NCE D13SRJ, Jumpers from track to track power in [Red / Black], and jumpers from decoder to meter [Blue / White], this decoder does not have on board dropping resistors for 1.5 micro incandescent bulbs or Lead's, so output is full voltage .






..
Basically same voltage as through a seperate bridge rectifier, so final voltage available to the motor and lights on my layout is 14.65V [after 1.35V rectification loss] .. 
The 0.20 V higger reading with the decoder may be attributed to the decoders on board capacitor, I didn't test the bridge with an added capacitor ..
I then ran two sound equipped loco's on their main line loops, and another sound loco doing some switching, no variation on voltage ...
I 'think" that I may have checked it a few years ago, but was bored and curious. and the usual "Test results valid with MY system and on MY layout, yours may differ"
All is good ..


----------



## tkruger (Jan 18, 2009)

I do not remember the exact number but I did a similar test with may NCE Power Cab. The numbers were around the lines of what you found. That is also the decoder I user for nearly every conversion I do. Prior to the DCC conversion the layout was powered by a pair of MRC Sound and Power 7000. These DC units would output around 16 volts each at full power. There was also a G scale setting that could increase this but I did not use that. To me I always watched my DC amp gauge more than anything. If it was steady and the locomotive was going the speed I wanted then I knew that everything was running well. If it fluctuated up and down but the train was running at speed I new that the rails or wheels were getting dirty. If it was higher than normal the locomotive was in need of a tune. Unfortunately since going DCC the gauges I but in the control panel are of no use.


----------



## mesenteria (Oct 29, 2015)

I'm well out of my depth on this topic, but I have seen posts several times from a very knowledgeable DCC guru on another forum where he says to take digital and analog multi-meter readings about DCC track voltages with a grain of salt. You need a special instrument to accurately measure the true voltage imparted by the DCC unit.

I see, though, that you are mostly interested in a comparison between the two states, and not necessarily in the actual voltages. FWIW, my digital m/m shows a track voltage of just under 16 volts with the base unit set for HO scale rails.


----------



## DonR (Oct 18, 2012)

WVGCA has it right. Curiosity. It really doesn't matter whether you
are seeing an actual DCC track voltage with a special meter,
with a rectifier, or just AC meter probes on the track. Like he
shows, I got 17.65 v on my Bachmann EZ DCC rails. What
the loco motor sees is entirely another thing...same with
the loco lights...the decoder provides the right voltage. 

I used the 17.65 AC track voltage to compute the resistors needed
for my 3 volt caboose incandescents and it came out with the
brightness I wanted. Precision is not really needed
for such use.

You can use the ordinary AC volt meter to check for current conductivity
around your layout. First check it as close to the Controller
output as possible. Then, any variance from that when you read
around the tracks will let you know there is a problem.

Don


----------



## wvgca (Jan 21, 2013)

I spent a few minutes just now with my old B&K scope hooked up ... results very close to my guess of 16v DCC on my layout, [32 V peak to peak], and surprisingly the 17.65 VAC as read by my digital meter looks like it was accurate as well ... kinda... when the DCC waveform transitions from 16 positive to 16v negative there is a noticeable over shoot peak, about 1.5V or so , and I think it's possible the digital meter on ACV was including the momentary spikes .. the MRC system that I run doesn't give a super clean "flat top" square wave ... but I have no experience with other systems [except an old MRC Command 2000] .. and if it matters the DCC frequency on mine is around 8.75Khz with no packet commands ..I did this scope reading at the furthest point from my power supply / booster so I'm good with only a 1.5V spike .. I don't get any loss of commands or weird actions, so there's no need to consider an R/C bus terminator ..


----------



## gregc (Apr 25, 2015)

wvgca said:


> I spent a few minutes just now with my old B&K scope hooked up ... results very close to my guess of 16v DCC on my layout, [32 V peak to peak], and surprisingly the 17.65 VAC as read by my digital meter looks like it was accurate as well ... kinda... when the DCC waveform transitions from 16 positive to 16v negative there is a noticeable over shoot peak, about 1.5V or so , and I think it's possible the digital meter on ACV was including the momentary spikes .. the MRC system that I run doesn't give a super clean "flat top" square wave ...


i was never sure what the AC measurement of a DCC waveform really indicated. So like you i measured the track voltage with both a scope (Tek) and digital multimeter (DMM) set on the V AC setting.

Just to calibrate the scope and voltmeter, i measured the DC wall wart supply voltage. 

scope 14.0 V, DMM 13.78 Vdc -- wall supply voltage

scope 14.0 Vpk, DMM 13.03 -- track voltage with nothing on it (diff of ~1V)
scope 13.8 Vpk, DMM 12.93 -- track voltage and decoder (loco not moving)
scope 13-13.8 Vpk, DMM 12.72 - 12.88 -- loco moving (0.11 - 0.18A)

i hope you agree, the Vac DMM track voltage is very close to Vpk (the delta due to some averaging in the DMM) as well as the supply voltage. One less mystery

(i think your 1.5V spikes on the scope are due to a mis-tuned probe. isn't there a screw adjustment).


----------



## wvgca (Jan 21, 2013)

I just used the onboard 1V reference to verify the scope beforehand .... this is an example of what I mean by 'overshoot' on transition ... as I mentioned before, my sample was taken at the far left hand end of my bus, and I'm okay with the small spike as it's of very short duration ...
the following scope example [with much higher spikes], is not mine, but comes from 
http://www.wiringfordcc.com/dcc_waveforms.htm


----------

