More on my "Ultimate TTRAK control box"

[Maletrain]

Peteski, I was starting from the point of having 2 DC power packs controlling the voltage on the 2 different loops such that polarity is matched and voltages are as close to matched as is practicable with knobs and no voltage readouts.  So, assume a slight voltage mismatch but matched polarity as a DC (no decoder) loco crosses over.

From a simple physics standpoint, this seems like it would not be much of an issue, since the circuits are isolated except for the crossing loco bridging them.  Sort of like putting 2 batteries in parallel to feed a piece of track instead on just one. 

On the other hand, modern DC controllers are not as simple as batteries.  They may have pulse width modulation instead of steady voltage application, and may have feedback designed into the circuits to smooth out the motor performance.  SO, I am not confident about the battery analogy.

I was hoping that somebody with practical experience with such a situation would share their knowledge here.

[Jbub]

Peteski, I was starting from the point of having 2 DC power packs controlling the voltage on the 2 different loops such that polarity is matched and voltages are as close to matched as is practicable with knobs and no voltage readouts.  So, assume a slight voltage mismatch but matched polarity as a DC (no decoder) loco crosses over.

From a simple physics standpoint, this seems like it would not be much of an issue, since the circuits are isolated except for the crossing loco bridging them.  Sort of like putting 2 batteries in parallel to feed a piece of track instead on just one. 

On the other hand, modern DC controllers are not as simple as batteries.  They may have pulse width modulation instead of steady voltage application, and may have feedback designed into the circuits to smooth out the motor performance.  SO, I am not confident about the battery analogy.

I was hoping that somebody with practical experience with such a situation would share their knowledge here.

If you're connecting two power sources in parallel that are the same voltage then you are increasing the AVAILABLE amperage to be used. If both sources can produce 9 amps at 12 volts, when connected in parallel you will have 12 volts and 18 amps available. Of course the wire or track will have to be large enough to handle that current flow other wise things get hot and melt. We do the same thing when dry camping with RV's, we use two low amperage 12 volt batteries in parallel to increase the amp hours available or two high amperage 6 volt batteries in series to get the 12 volts.
Edit: I see what you mean about the PWM. Honda makes linkable generator to increase wattage (amps x volts) and to do this they have to be phased correctly. They make a cable that connects the two generators together in phase.  So yes, if it's not straight DC, the two sources will have to have the same phase.

[peteski]

Yes, if you connect 2 DCC boosters out of phase, you have a big problem and likely an expensive repair.  Even if the boosters are totally independent.

Steve, if the DC throttles are at the same polarity with slightly mismatched voltages., that should not cause much damage (especially since throttles used in N scale don't supply more than 1A of current.  Actually my earlier warning was basically to bring some fear into someone looking to run into those situation.

Back in the day of block control DC operations there have been many occasions where someone ran over a block which had a live throttle voltage (or the same or even opposite polarity, and no damage occurred (except for the engineer driving the train having a tense moment).  I have both witnessed and also caused such events.

[Ed Kapuscinski]

I haven't seen any comment on my question about whether crossovers between the 2 loops would be a problem if the 2 loops are both DC, but powered by different transformer power packs. 

Sorry!

I wouldn't worry about it.  Each DC throttle already has that compatibility button: it's the direction switch!
If anything, you might want to ensure that you've got some good circuit breaking in there. It's handy for situations where something with metal wheels picks a switch and, more importantly, for this very situation.

As Pete points out, we've been dealing with that exact problem for ages on regular NTRAK layouts. Circuit breakers with audible warnings, aka screamers) have generally solved it perfectly fine (which is great, because one of my most treasured models got melted on the layout at the 1992 Birmingham NMRA Convention and I'm still not over it).

[brill27mcb]

With two DC pulse width modulation (PWM) power packs, having the same polarity when a loco crosses from one's block over to the other's block, there will likely be a momentary jump in the loco speed while its trucks bridge the gap.

If, by sheer luck, the pulses of the two supplies happen to be perfectly in sync, the average voltage seen by the gap-bridging loco will be unchanged and you will see no momentary speed increase. (Just as if if you had two pure DC supplies set to the same polarity and the same exact voltage.)

However, if their pulses are totally out of sync, then the average voltage (duty cycle) that the bridging loco sees will double and the loco speed will momentarily increase (even if the two voltages are set to the same level).

If the pulses overlap (which they must do, if both are longer than 50% duty cycle), then the voltage increase the bridging loco briefly experiences will be somewhere in between these two extremes.

In other words, to cross over safely and have no momentary jump in speed with PWM, not only do you have to have the polarity and voltage settings match, but the pulses also have to be in sync.

I imagine that there might be possible issues if one supply backfeeds power into the other supply, depending on how they are designed.

Rich K.

[peteski]

Yes, my description assumed old-fashion analog variable-voltage DC power suplies.  PWM type of DC throttles change that scenario to what Rich describes.