A typical digital multimeter I understand measures RMS AC voltage if capable. RMS voltage * sqaure root of 2 = peak voltage or essentially, RMS is the average voltage over the positive side of the AC sine wave. I think I understand that DCC track voltage at the track is composed of an AC voltage plus a small DC offset. With my Lenz system set on 12V for CV7, a digital multimeter will provide a value greater than 12 volts for a reading taken at the track. The supply voltage from the supply measures 17v AC at the input to the Lenz system. Tony, at Tony's Train Exchange says it is not that simple, that my meter isnt' telling the whole story, but there is a device that I can buy that will tell the story. I agree, but I would like to undertand why... Will someone help me understand what my RMS meter is telling me and why a Lenz 12 volt setting doesn't provide a 12V AC reading at the track, but something over 12 volts. Is my meter showing me the 12V AC plus the small DC offset voltage? I think this would add up to about what I am seeing. Some help on this electrical quandry would be appreciated. I should know this, but I am not thinking clearly today. Thanks.
True RMS Voltage I did a google search (RMS voltage) and tried to learn about this. I believe you are looking for a True RMS voltage/volt meter. This website is a good explanation. Note the comment at the very bottom of this url page. I don't know the answer to your question, but I think that statement is relative to the issue. Your 12 volt vs 17 volts has to do with the cosine of 45 degrees is 0.707 and what your meter has the capability to read. While the RMS voltage is 12, the peak voltage is about 16.8 or 17 volts. More info. Car audio theory; same type of thing.
Flash, I found the answer... something I had never assumed... I read the bottom of page 5 and the top of page 6 to understand why my digital multimeter was showing an incorrect value.... http://www.tonystrainexchange.com/download/rramp-appnotes-df.pdf Ultimately, the higher frequency of the DCC signal is screwing up the RMS value calculation in the digital multimeter which is assuming a frequency of 60Hz.
Thanks Hutch, you managed to jog my memory...as I was reading your first post, .707 popped into mind (I've been out of school a LONG time and carry a EET). I read the links and enjoyed them. You can pick up a used oscilliscope on e-bay quite reasonably and this will "show" you the waveform and amplitude if interested. This might be a little overkill though.:shade:
Chris: With a Tonys RRampmeter you 'll get the true value which shouldn't exceed 12 volts. It will usually read a value from 11.7 to 11.9 volts. I have a RRampmeter and occasionally check the voltage on the mainline as well as the amps used by the entire system. Stay cool and run steam......
Thanks Bob, it is nice to know that the proper measurement tool, will produce what Lenz had intended in the manual. I think that might be a handy tool to have one day, I will get one with the next order from Tony. Good to hear from you. Have a happy Fourth of July! I need to get off my butt and go do some wiring on the layout.
Standard multimeters are made to read smooth DC and/or sinewave AC (mains power). DCC is a square wave AC (with added variations). Hence the need for 'specialist' meters. That said, knowing the exact and correct voltage is somewhat unnecessary for most people (how many cars have an oil pressure gauge these days?). If you note the AC voltage reading for you standard meter and DCC system when it is 'good' (eg. new and near the controller), then if you have a problem like a bad joint or suchlike, you will likely see a lower than normal reading where the problem is. I won't buy an RRampmeter unless and until I have a specific need for it - my multimeter is sufficient for now.
A RRampmeter not only measures the voltage across the rails but also the total amps consumed by the layout. It is an invaluable tool to keep track of these important variables. Stay cool and run steam......
What is the frequency of the DCC signal? Most DMM's will give a reasonably correct AC RMS voltage (depending on the quality of meter) up to around 1kHz but give upwards of 50% lower voltage reading over that frequency. Meters like the Fluke 187/9 series maintain correct RMS voltage readings to around 200kHz. But they will set you back around US$450 a piece. The RRampmeter certainly sounds like a great tool to have on hand.
I dont believe DCC has an identifiable frequency, more like two basic frequencies. As I understand it, AC is constantly changing in a sine wave pattern where the peak voltage will be much higher than the RMS value, it takes an entire half cycle for the voltage to switch from one peak to the opposite polarity peak. However a DCC signal switches polarity as fast as possible and in theory the peak voltage is very close to the average voltage. According to my understanding a simple bridge rectifier and a DC meter would be useful for measuring DCC. The voltage displayed by the DC meter will be the average of the DCC voltage minus the voltage drop of the bridge rectifier (about 1.2 volts?).
Again I'd question that. If you want the total current the layout is drawing (near enough for most practical purposes) you can measure the AC or DC supply with a multimeter upstream from the DCC unit. (Depends on how it's supplied of course, and maybe not practical on an all-in-one (mains in, DCC out) unit.) For individual sections/locos the point I made for voltage also applies to current - take a few 'good' readings with a mm, then use these as a baseline when you have a problem. (In practice if you don't take baselines even with a RRampmeter it won't really be any more use than a mm.) Also the difference in readings from using RRampmeter to multimeter won't matter most of the time for small devices like model trains. I can see that a RRampmeter is a tempting buy (I've been tempted myself). I'm not saying "don't buy one", just don't believe it's a necessity - if you have all the locos and other stuff you want and still have $50 burning a hole in your pocket ....
MIke: There's no point in trying to convince you that the RRampmeter is a useful tool in assessing track voltage and amperage consumed by the layout. I've used it for almost seven years and find it very useful in spot checking track voltage and amperage. Stay cool and run steam.....
I'm not saying it wouldn't be useful, just that for those that already have a multimeter I'm not sure it's $50 more useful. BTW, how do you use it to measure amps? I believe it has contacts to put on the track for voltage, but for current you must have to get it 'into' the supply lines. Or do you have it permanently wired in? (on a control panel or suchlike).
I have it semi-permanently wired in place for amps to the command station. Stay cool and run steam.....
Picked up a couple of extra banana jacks and made up two short adapters. Banana jacks on one end, Powerpoles on the other. I just connect the RRampmeter in series on the home layout or NTRAK layouts. I also have a 3 amp load equipped with banana jacks to test voltage drop. The battery equipped RRampmeter works quite well with DC as well. With Digitrax (and I assume others) set a DVM to DC volts. Measure from one rail output to ground (booster common). Do the same for the other rail output. Add the two readings together. Martin Myers
In comparing A RRamp meter and a standard multimeter to measure DCC voltage, you will of course get two different readings. The RRamp meter will (presumably) give you an accurate true RMS reading of the square-wavish (new technical term <G>) DCC waveform, the frequency of which can be as high as 8.3 kHz (a continuous string of "1" bits) (actually more, over 20 kHz, if you count the harmonics necessary to make the square wave square). A multi-meter will give you an inaccurate reading of a DCC signal as it is calibrated assuming a 60 Hz sinewave. Analog multi-meters will respond differently to the DCC waveform than will a digital multi-meter, but either one should be consistent in it's innacuracy, so if desired, you could come up with a "scale factor" to convert from your multimeter reading to an RRamp meter DCC reading. If you don't care about the calibration, and only want a "normal" reading is versus an "abnormal" reading, a multi-meter will do fine. It's kinda like kph vs. MPH vs. furlongs per fortnight. It's all what you're used to. Now if you want to compare readings with someone else, then a common reference is required. If that's your need, the RRamp meter might be worthwhile. Doug
