Track voltage in Z

Hello,

I need your expertise.

I begin in Z scale DCC.

I have a GP-35 MTL + Digitrax DZ123-M0 decoder.

I control it with a Roco 10764 amplifier, supplied with 12 V AC.

But when I measure the track voltage with a voltmeter on "AC" position, I have 16 V on the tracks.
Although my train is running, I suspect this voltage to be to high
Isn't it ?
How can I decrease this voltage ?

Jérôme

your meter is no good for the job
well, you could use it if you knew what you were doing (but than you wouldn't be asking here...)

you need something like this:
www.tonystrains.com/technews/rrampmeter.htm

if you are desperate to know why your meter is no good and why do you need special meter, I could try to explain ... but since it is not my forte I prefer to leave it to others better educated chaps

Chris

Jerome, running at 12v is fine, as long as you dont over do it. I been running dcc at 12v or over 2 years, and not burn out engine. I limit the speed of my engines by programming cv5, which is cv for top speed. Jeff is the expert in dcc voltage. Maybe he can chime in when he has time. There is additional information on the Zcentral archives under dcc forum. You will find it useful if you are into technical stuff.

Thank you.
But when you say that you "been running at 12v or over 2 years", you're talking about the power supply of the command station, not the tracks... Have you ever measured the voltage on your tracks ?

I know DCC is a square wave signal, and a classical voltmeter can't measure it (it is made for sinusoidal waves). So their is a sqrt(2) factor.
But when it's a square signal with a 50/50 ratio, the voltage measured by the voltmeter on AC position would be the max voltage. So in my case I suspect the signal to be a square wave +/-16 V, which is too much for my decoder...
If only I had an oscilloscope

Jérôme

if you had a scope, you would find something like this:
www.wiringfordcc.com/dcc_waveforms.htm
there was also very informative posting on tghis topic on Trainboard:
www.trainboard.com/grapevine/showthread.php?t=83897&highlight=waveform
have fun
Chris

Jérôme,

You'll be OK. Your 12VAC (very undefined voltage) would rectify in the amplifer (we call them Booster in the US) creating a maximum of 1.414x or 17VDC. Switched to the DCC protocal and likely also voltage regulated (many are) so likely track voltage may only be 12.75, 15, etc. HO and larger are usualy 15V.

Your decoders will be fine but it will be a bit harder on the Märklin Permanent Magnet motor's brushes and commutators. You may see shortened life of these motors. Coreless motors like MTL and AZl use won't have the problems.

If you cannot adjust the voltage down and you still feel uncomfortable, you can add the diode (rectifer) dropping string (back-to-back rectifers) that pass the "AC"-like DCC signal but loose about 0.6-1.4V per diode pair. See: www.members.optusnet.com.au/nswmn/excess_voltage.htm. Just make sure that you use high speed diodes (e.g. Schottky), not ordinary silicon types (e.g. 1n400x) and they need to be at least 3Amps or 5Amps, depending on booster current to allow for full short circuit protection.

Jeff
SF Bay Area Z

Thank you for your answer.

I will decrease the voltage all the same...

I will use this circuit :

Interesting that you come up with those circuits. I came up with similar circuits when addressing a different problem in analogue control.

I noted, when exhibiting in Inverness, that using Marklin controllers with Marklin trains they tended to get down to about 15mph and then stop dead. Similarly they would start up with a jerk. On checking the voltage supplied to the track, I noted that the minimum voltage registered as 2.85 (open circuit with Fluke 8074B meter). As a first iteration, I decided to put two diodes in series with the controller output to reduce the output voltage by a constant (well fairly) 1.4V. Worked like a dream. The first test was live at my Elgin exhibition. The result? Brilliant! My BR61 with its four coaches would drift along at a fast walking pace both on stopping and starting. Not quite so good with 3 pole but still better than nothing.

BTW, if you are using a moving coil voltmeter, there is a simple correction for taking AC square wave measurements. In reality, the meters measure the average (mean) voltage so have to be corrected to display the correct rms value when measuring normal sinusoidal ac. This must be removed again when measuring square wave. The adjustment is something like 1.1. Maybe someone else here can give more details.

All the best,
Chris.

I'm back from RailExpo, in France.
Their was a Z fan who used a self-made controler on his loco (Marklin decoder).
I ask him to measure his track voltage with my voltmeter.
He had :
- 12VAC to supply his booster
- 16V on his tracks (from rail 1 to rail 2) (voltmeter on AC position)
- 6 V in the other way (from rail 2 to rail 1).
He said me to do an average between those measure : 16 + 6 /2 = ~11 V ~DCC.
He used this method for a long time without damage.

I will measure on my tracks and I say it to you.

Jérôme