I am running an NCE Powercab on my 7 x 7 x 2 L shaped switching layout. Recently I bought a new multimeter and I am reading about 12 volts AC at various track Points on the layou. I thought DCC was closer to 20 VAC. Am I wrong?
12 is closer to the actual value. A little low, but that will depend on the meter you are using. The PowerCab is about 12.6 or so to the rails actual. The little plug pack is I think 13.8 volts, and there are some losses in the circuitry. Can’t get out more than you put in. 20 volts to the track would be mainly for G scale.
DCC equipped locos are still the same old locos that use 12 volt motors (except MTH, but we’ll leave them out for now), so you really don;t want more than 12 volts to the motor. This means the track voltage needs to be a volt or so over 12 volts to get a full 12 volts to the motor, because of component losses in the decoders. Most systems have some form of track voltage adjustment, but the PowerCab and others like the Digitrax Zephyr do not, so they pick a reasonable compromise that works on all scales (N scale locos tend to run WAY too fast on 12 volts, and Z scale motors are really often 9V, not 10V). The big NCE and Digitrax systems have track voltage adjustments so you can select the best voltage for your scale, still subject to the power supply used - you can’t run the thing off a 12 volt power supply and expect to get 20 volts on the track.
AN absolute accurate voltage reading isn’t really necessary - what you need is consistency around the entire layout. But the voltage reading alone with no load on the track at the time doesn’t really prove anything. It’s quite common for a power supply that has failed to still deliver the rated voltage at no load, but even hooking up 1/4 of the rated load, the voltage sinks to nothing. And when trying to test the layout wiring by measuring voltage - the voltage drop is the wire is proportional to the current being drawn. So no current means no drop - you could run 50 feet of telephone size wire and measure the voltage on the other end and it will probably be fine. But stick just one loco on there - forget it.
Technical bit you can skip. Even a good meter isn’t "n
I have an NCE PH-Pro 5 amp system. I have 3 RRampMeters, one in line for each of two 5 amp boosters and one that is portable for walkaround measurements. All 3 RRampMeters show 13.5 volts. Those measurements are not under load. If I add an 1156 automotive bulb to the RRampMeter, the voltage drops significantly.
Rich
A multimeter will not provide an accurate volatage reading for DCC, since the waveform isn’t either DC or an AC sine wave. For an accurate reading, you need a RRampMeter (or an oscilloscope). But a standard multimeter set for AC voltage will get you within a volt or so, and that’s usually good enough.
A cheapy meter like the $1 Harbor Freight one will work fine - basically, a meter that doe NOT say “RMS”, because peak and RMS is the same for a square wave.
An RMS meter is calibrated based ona sine wave, it will read incorrectly for DCC. A True RMS meter which can handle sine, square, and triangel waves, will mostly not work either, because while it can handle a square wave, it’s designed for maybe 100Hz tops, and DCC is a much higher frequency.
Want a voltmeter that works like a RRampmeter? Get one of those cheap DC voltage modules on eBay, most of them have a potentiometer to calibrate them. Add a bridge rectifier, best to make it yourself with 4 fast diodes instead of a packaged bridge, most are too slow to handle DCC frequency, but a fast switching diode will work well. 1N5401 I think off the top of my head is a good one.
Your voltage will read two diode drops too low. Adjust the trim on the voltmeter to compensate.
–Randy
Well it was from Amazon not Harbor frieght but for my purposes it eorked fine for what I needed it to do i supppose.
I don’t know why a NCE would be different than a Digitrax. But from what I understand about DCC is that it is 14 to 15 volts AC across the rails. That you can measure with almost any multimeter. I like and use the cheap Harbor Freight ones. Free with a coupon and a purchase. Might be $5 otherwise? As for DC, that does require different calculations. But for me, to K.I.S.S. measuring the AC is sufficient. The Digitrax system allows measuring between ground and either rail. That in DC yields 7+ volts. Add the two rails together and you have “usually” 15 volts DC. If that works for NCE, it might help you? The actual volts in DCC is AC plus DC and it far more complicated than I can figure out. But measured as I mention above is usually sufficient for casual use. A Rampmeter is too expensive for me, but is the best available.
Then you understand wrong. It is different because it is a different company with a different power supply and different specs for it’s power supply than Digitrax.
Exactly. Two different manufacturers, who don’t use the exact same electronic components, nor the exact same circuit design. Outputs will vary.
Just posted on a similar thread on the Digitrax Group. My good handheld meter, all fancy with “True RMS” across the rails reads 2.5 volts AC. Yes, 2.5 on AC! Obviously this is not accurate, sinc emy locos work fine. My even fancier bench meter, which has a lot more features but also can disable RMS - shows 13.4 volts, which is about exactly what it should be - pretty sure that agrees with what my oscilloscope shows. I didn’t have one of my HF meters handy to see what it reads.
For model railroads - cheaper is better. Which is good for this usually frugal PA Dutchman - sometimes though for other things, the fancier meter is needed (but then I guess I can’t really explain why I have a fancy handheld, and an even fancier handheld, as well as not 1, not 2, but THREE bench meters (one was 100% free though)).
–Randy
My NCE Power cab shows about 13.6 VAC with one loco on the tack. About th same at the club with the five amp Power Pro. My Scope verifies this when I do the math. It shows peak to peak.
I have four Harbor Freight multimeters.
Take your voltage as a benchmark.
Don’t forget, the DC power is about 13.9 VDC to the system.
Edit. I wonder why the OP thought 20 VAC?
Rich