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Offline tiono  
#1 Posted : 19 March 2020 04:02:30(UTC)
tiono

United States   
Joined: 09/02/2010(UTC)
Posts: 234
Recently there was a heated debate whether digital signal in 3-rail is AC or DC. Some people insisted that it is DC (really, when did Marklin change from AC to DC?) while some others said that it still AC.
Therefore herewith I will try to explain.
Years ago, when Marklin went to digital, the power to controller was supplied by a transformer, and they use conventional method to generate the digital signal (Diagram on figure 1 below). Inside the controller there are two power regulators supplying positive and negative voltages, which passing thru solid-state switches before being fed into the track. When measured against the power ground, the voltage is +/-18V (36 volt peak-to-peak) AC signal. The outer rail is connected to ground, therefore the decoder see the same voltage as what we measured against the power ground.
Everybody happy, and proudly said that it is AC signal.
Then several years later, the transformer gave way to the more efficient switched power adapter. Although a switched power adapter can be designed to provide positive and negative voltages, but this will be more expensive, while the single positive voltage adapter is widely available and cheap. (Furthermore many laptops are using +19V power adapter as well). Thus Marklin decided to use single output adapter combined with solid-state switch called "H-bridge" to generate compatible output.
With this method, there is no negative voltage inside the controller. But the decoder had been designed to work with AC signal and they have to be compatible, how to accomodate this? and there are devices which had been equipped to use the negative voltage from the track, for example: the Rheingold coaches, such as Marklin #4228, get the current to light up the interior LEDs from the negative voltage on the track.
They solved this by "floating" the track (disconnecting from power supply ground) combined with "H-bridge". (Diagram on figure 2 below) The "H-bridge" alternates the polarity supplied to the track; when the center rail get positive voltage, the outer rail will get the ground. And then when the center rail get the ground, the outer rail get positive voltage. This alternate switching is done by the "H-bridge" switch and controlled by the firmware, based on the modulation protocol.
When somebody measure the center rail against power ground, they will see positive voltage, no negative. It is a modulated DC signal. The same happened when he measured the outer rail, which is also positive voltage, a modulated DC signal. Thus he proudly proclaimed that the "H-bridge" controller is now generating DC signal instead of AC. This is when things got ugly. Of course he was right that the controller generating DC signal (if measured against the power ground), but he forgot that the purpose of a controller is to provide power (and signalling) to the decoder, therefore the voltage must be measured as in the decoder (as how the decoder will get).
The decoder can only connect to the track; the center rail and outer rail. While the outer rail is now "floating", not the power ground anymore, therefore the decoder will never see the power supply ground as the controller does. If we measure the voltage between the rails, as what the decoder get, then it is AC signal, its polarity alternates, +/-18V (36 volt peak-to-peak). So the decoder does not see any difference in electrical parameter. It still get the AC signal as what it was designed to. While the controller does not need to have a negative power supply (and there is no negative voltage on the rails too, if measured against power ground). All decoders whether in Marklin digital or DCC, need a rectifier to get DC supply for their on board chips.
At the end, everybody happy, the new controller is compatible with the existing decoders and devices (the Rheingold coaches still lit up beautifully). The only catch; don't connect the outer rail to power supply (adapter) ground.

So is it DC or AC? well, depending on your frame of reference. If your reference is the power supply ground, then it is DC (no negative voltage across the system). But if your reference is the track (voltage between rails, as what the decoder sees) then it is AC.

Figure 1.


Figure 2.


Note: DCC use the same method albeit different voltage (+/-14V or +/-12V). The DCC rails are DC signals if measured against power ground, but the decoder will get AC signal.
For more detail about DCC power, go to this URL: https://dccwiki.com/DCC_Power
thanks 7 users liked this useful post by tiono
Offline Bigdaddynz  
#2 Posted : 19 March 2020 04:54:02(UTC)
Bigdaddynz

New Zealand   
Joined: 17/09/2006(UTC)
Posts: 18,661
Location: New Zealand
I locked the previous topic for a reason, please do not try to circumvent that by opening another thread about the same subject.

Edited by user 19 March 2020 10:51:19(UTC)  | Reason: Clarification

thanks 7 users liked this useful post by Bigdaddynz
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