What Happens With Different Commons

The graphics included here are actual screen clips from analog simulation software using spice controls. I think they will do a good job of showing and explaining what happens and why.

First a little about the setup controls:

The boosters are user definable macros. I designed them for use when I was designing my detectors. They are running at 10 kHz. Here they are a basic pulse generator with an internal common at the bottom output, as I have yet to program a prom to represent a DCC typical modulated wave. But I would think for all practical uses here, they do fine. To do this I designed them using two function generators at the outputs with a the common connected between the generators. The second generator is running out of phase with the first by 1/2 the width. Both generators are capable of external triggers, once I program a prom, I can use it as the trigger and simulate a DCC typical signal.

All AC meters are reading True RMS.
The two AC meters in the center. The left meter is reading across the gaps from the N rail to the opposing S rail. The right meter is reading across the gap from the N rail to the N rail.
The two outside AC meters are showing the true RMS voltage across both rails of each booster.
The Lower DC meters are there for reference to booster phase/polarity.
The A with a green circle is the scope probe.
The Ref with a red oval is the reference point for the scope probe, the ground clip.

The scope is sweeping at 1 ms. This gives 10 pulses on the display with a 10 kHz square wave form.
The Y axis [vertical] divisions are in volts. The X axis [horizontal] divisions are in uS.

Common Rail

Opposing phases with the common switched across the gaps Common Rail Opposing Phases voltage doubling.

Notice the RMS AC voltage across the gaps from N rail to opposite S rail. This is the same as would be picked up by a Loco with off-set pickups. Also note the the P-P voltage on the scope across the same points. Scope probe [A] and the [Ref] points. This would prove not to be a good choice for any decoder..

Opposing phases with the common rail on the same side.

Common Rail Opposing Phases Voltage Doubling on same side.

Notice the RMS voltage from N rail across the gap to N Rail. And also the P-P voltage on the scope from the same as the meter. Scope probe [A] and [Ref]. Here the loco decoder would not see this voltage. But it is there and does present a voltage that the booster outputs would see as the loco wheels shorts across the gap. This would not be a choice either.

Opposing phases with common switched with the phase. Common Rail Opposing Phases No Voltage Doubling.

This as far as I can tell is the only safe way to handle reverse sections of opposite polarity with common rail. Here all voltages are within proper limits and no doubling. Note the P-P voltages on the scope. You will note that [Ref] is ground and not on the rails.

Direct Home Wiring 3 Wire system Proper phasing Direct Home 3 Wire System. No Possible Voltage Doubling.

This is a normal booster district to booster district. No opposite phase. No voltages over 14v RMS AC. P-P voltage on scope is across rails. Probe [A] to [Ref].

Opposite phase Direct Home 3 Wire System. No Possible Voltage Doubling.

This a typical reverse section with the opposite polarity across gaps. With the common off the rails this is the only possible way to wire it. Thus there is no possible way for added voltage doubling. There is only two possible voltages for a decoder to see 0v or 14v RMS AC.

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Last updated 09-05-98