VOLTAGE ?????greenspun.com : LUSENET : Wiring for DCC : One Thread
HELLO, I HAVE JUST STARTED IN [HO] DCC SO THIS QUESTION MIGHT SOUND FOOLISH. I HAVE WIRED THE TRACK AND TESTED DIFFERENT ENGINES BOTH ANALOG AND DCC AND THE TRACK SEEMS OKAY. MY DCC ENGINES SEEM TO RUN REAL SLOW, SO MUCH SO, THEY DON'T SEEM TO HAVE AS MUCH PULLING POWER. I CHECKED THE VOLTAGE AND HAVE APP. 8VOLTS COMING OUT OF MY DIGITRAX TRACK TERMINALS. I ALSO HAVE APP 8 VOLTS AT DIFFERENT SPOTS AROUND THE TRACK. THE ENGINES ARE 10 TO 15 YEARS OLD, ATLAS AND ATHEARN, THEY HAVE DECODERS THAT ARE CORRECT.I WOULD APPRECIATE SOME INPUT. THANK YOU VERY MUCH, RAY
-- RAY CARIGNAN (RASACA@ADELPHIA.NET), January 31, 2003
If you only have 8V coming out of your Digitrax booster, do you have your booster set to N/Z? Make sure it is set on HO. (Note: Some people run their HO on N.)
Also, if you are using an old power pack to run your Digitrax booster, you may not be getting enough voltage out of it to provide full track voltage even if you do have your Digitrax booster (or command station) set to HO. Just for now, try using the AC accessory terminals on your power pack to see if you get a higher voltage out.
Lastly, you cannot use a typical meter to read the voltage out of your booser. Unless you have a relatively expensive "true reading RMS" AC digital voltmeter, you won't get the right reading. If you have such a meter, it will usually say "true reading RMS" on it. This helps make you feel better for how much you had to pay for it ! :)
There are other choices:
See my web page for a way to convert the track voltage to something you can read accurately. It's very inexpensive.
If you want a nice meter and not build anything, Tony's Train Exchange has recently introduced a product a product designed to measure DCC voltage and current and is reasonably priced.
-- Allan Gartner (bigboy@WiringForDCC.com), February 01, 2003.
Hi Ray, hi Alan,
I am an electronics engineer who is just scanning the net for a friend who wants to switch to DCC. This to put myself into the right perspective: I am interested only in technical issues and not in details of this hobby.
But something I can contribute here. It is about measuring the voltage of the DCC signal.
1.) If the multimeter used has an input imdedance (sometimes erroneosly called "resistance") in AC mode of one Megaohm or more it will most probably read the correct voltage within +/-10%. This is a rule of thumb, not a physical fact. But in practice most, even _very_ cheap, multimeters do it right. This lies in the nature of the DCC signal sent out from the booster: It is a square wave. And an ideal square wave has the same RMS value as a DC signal. So a square signal that switches (instantly in theory) from +18V to -18V or vice versa has an RMS value of 18V (RMS values are always positive, because they are absolute values). And a DC signal of either +18V or - 18V has the same RMS value: 18V.
2.) If you doubt the AC-performance of your multimeter you can use one diode and one capacitor in front of it and take the readings in DC mode, which usually has a much higher input impedance ("resistance"). The setup is simple: Take a 100nF capacitor (voltage rating above 25V) and short the input terminals of the multimeter with it, i.e. connect one pin of the capacitor to one input terminal and the other pin to the other input terminal. Then contact with one of the measurement leads one rail of the track. Grip the TIP of the other lead and one terminal of the diode (which one is unimportant), such that the terminal has a good contact to the measurement tip. The other terminal of the diode thus points into the air. With this free terminal you contact the other rail of the track and then take the reading of the multimeter. The displayed value is the (peak to peak) voltage on the rails without any correction factor and with an error in the low percentage range. NOTE: This works only in a pure DCC setup. If you have some additional power source connected to the rails it will _definitely_ display a wrong value even to the extent that the error is (almost) beyond any bounds.
Safety first note: Do NOT use the "shorthand mode" of grabbing the tip of a measurement lead (to contact the diode to the tip in this case) if you cannot BE SHURE, that the voltage to measure is less than 60 volts!!! Here, at the output of a DCC booster, you can rely on the fact that the NMRA standard protects you from higher voltages than 24V. But NEVER try this trick with an unknown object! You might electrocute yourself.
-- Uli Paul (email@example.com), March 11, 2004.