Methods for Measuring DCC Voltage at Track?

[Isambard]

What reliable methods are folks using to confirm adequate DCC voltage at the railhead on all parts of their layout, under full load operating conditions, or when trouble shooting balky or non responsive decoder/loco problems?

During yesterday's initial DCC test running on the club layout, having exerienced problems on some sections of the layout (and having determined that the problems were not due to the decoder/loco, wiring, dirty track, wheels or switch points) we used the quarter across the rails trick to determine whether power was getting through to troublesome sections.

However, getting a command station beep didn't appear to ensure that a lightly loaded loco would respond. Not getting a beep sometimes didn't necessarily mean the loco wouldn't respond, only that the quarter wasn't being properly used by the big hand from the sky.

We tried using Sperry multimeters, getting around 9 volt readings rather than around 15 volts, but we recognize that they are not designed/calibrated to handle high frequency square wave voltage.

Comments welcomed!

[:)]

[MisterBeasley]

Tony's Trains has a meter:

http://www.tonystrains.com/technews/rrampmeter.htm

For my purposes, it's a bit much, but for a club I think it might be a good thing. I'd wire multiple probes with a selector switch, and mount it at the main control panel.

I'm still a DCC diaper baby, so my probes are usually on the DC outputs of whatever locomotive I'm upgrading that evening.

[cacole]

You can use any Volt-Ohm-Milliammeter (VOM) set onto the 20 Volts AC range. You should get a reading of 14 to 16 Volts AC whether there is anything running on the track or not. Any reading lower than 14 Volts indicates that there is a wiring problem in the layout.

I've seen digital VOMs at discount houses for as little as $5, so your club shoudl invest in one.

[rrinker]

My cheapy $5 digital meter gives a reading of about 11 volts on the AC setting with a booster that is supposed to be putting 12.5 volts to the track. So your 9 volt reading is definitely low. Do you have adequate bus wires? What about feeders to the track? How long are the bus wires, and what size wire was used?
Stop at Radio Shack and get a bi-color LED and a 1K resistor. And a pair of clip leads. Solder one leg of the LED and resitor toegether, attach the clip leads to the free ends of the LED and resistor. Clip across the track - it should glow orangish. If your DCC system supports running an analog loco on address 00, select 00 on a throttle and turn it all the way up - the led should change to either red or green. Reverse direction - the LED should switch to the other color. If it goes one way but not the other, or if it glows green or red but not orange when no address is selected and all throttles are at stop, there is a problem with the booster.


--Randy

[Anonymous]

If you want to be nit-picky dead on accurate put an osciloscope across the rails. Measure the square wave on the screen and you have your voltage reading. If you don't have ready access to a 'scope and don't want to invest in one (THAT I understand) just deal with the approximation you get from a voltmeter. It's not accurate because the meter is designed to read a sine wave and give you the RMS voltage, but it will let you know if there is a signal on the rails and give you a relative feel for "how much" signal is on there.

Cheers,

Ed

[Blind Bruce]

Carefull with that 'scope reading. All of them are calibrated in peak to peak volts and will indicate about 2.8 times the RMS value. My VOM is only accurate to 20kHz. Isn't the square waves 40kHz?
BB

[Isambard]

QUOTE: Originally posted by Blind Bruce

Carefull with that 'scope reading. All of them are calibrated in peak to peak volts and will indicate about 2.8 times the RMS value. My VOM is only accurate to 20kHz. Isn't the square waves 40kHz?
BB

I don't have my copy of the Digitrax Red Book handy, but from my notes the book says that an NMRA standard command station sends out square wave pulses (ones and zeros) at 4500 to 9600 bits per second, depending on the command. A DCC one is specified to have a half cycle of 55 to 61 usec and a DCC zero a half cycle of 95 to 9500 usec.

As I understand it the loco decoder supplies the motor power at a much higher pulse rate-maybe your 40kHz? I'm sure the experts will leap to help us.

[:)]

[rrinker]

To get a truly accurate reading you need a True RMS meter, which isn't cheap. Or an integrator circuit. A circuit to allow a regular meter to make accurate measurements had been posted in various forums and message boards out there, it's pretty simple.
If like Digitrax your system has a ground point with respect to the rail signal, you can measure from one rail to ground and then fromt he other rail to ground, and add them together. They should be both the same - if one side reads higher than the other, you have a booster problem.
Getting within a volt or so is plenty accurate - this isn't the space shuttle we're trying to launch here! I think I did the calculations once, the error from a meter designed to read ordinary sine wave AC voltage to reading even square wave voltage is only about a volt at a nominal 15 volt track power level. If you REALLY must know exactly, find the circuit diagram for the integrator circuit, or get the Tony's RRAmpmeter. Otherwise, just use a regula DVM set on AC and call is close enough.

--Randy

[Isambard]

Bonjour y'all and thanks for the tips.
I found that the Digitrax DB150 gauge switch was set to N gauge rather than HO as it should have been. Checking voltage at the DB 150 output terminals with a Sperry digital voltmeter, I found 9.3 volts at N setting, 11.3 volts at HO and 16.3 volts at O.
We'll be doing some more test running this evening. Hopefully we should have fewer problems with the HO switch setting!
[:)]

[Anonymous]

Does this mean I have to quit sticking my tongue on the track?

QUOTE: Originally posted by cacole

You can use any Volt-Ohm-Milliammeter (VOM) set onto the 20 Volts AC range. You should get a reading of 14 to 16 Volts AC whether there is anything running on the track or not. Any reading lower than 14 Volts indicates that there is a wiring problem in the layout.

I've seen digital VOMs at discount houses for as little as $5, so your club shoudl invest in one.

[TomDiehl]

Yes Rob, it will corrode the track.

[pcarrell]

This might help;

http://www.wiringfordcc.com/track.htm#a4

Scroll down the page for your answer.