How to measure digital track voltage?

How to measure digital track voltage
with conventional circuit tester?

Christian asks me:

>The reason I tought it was something wrong with my 6021 is because I
>measured with a Volt-meter (alternate currant AC) at the outgoing reb/brown
>contacts. I tought the voltage should be constant (17.4V ) here since it is
>Digital, as in input yellow/brown. But it was not. I measured between 11.5
>and 18V depending on the speed-knob, and the functions active. I found this
>puzzeling, but I was told that this is the way it shall be...
>
>Christian
>Norway

My answer is:

Humm... If your AC Volt meter is conventional circuit tester, it is a normal reaction.
The wave form of digital system is far from pure sine curve. And your conventional
circuit tester is not measuring 'Peak to Peak' voltage. So the voltage of the
circuit vary widely depends on running signal / data pulse.
If you want to measure 'peak to peak' voltage (may be 18 to 20 V in marklin digital
track) pass the current to diode bridge rectifier and put small condenser in
DC stage of the circuit. And turn the range knob of circuit tester to DC V range
and plus about 0.7V, then you will find (almost)correct voltage.

NOTE:

This is not way to measure 'exact' voltage.
It is practical method for modeller doesn't have peak to peak circuit tester.

Thanks to Mr.Christian Kloumann.