# A Current Question?



## toddalin (Jan 4, 2008)

If an engine is running through one "block" and proceeds onto another "block" that has a lower voltage, but still fed from the same power pack (e.g., diode speed reduction over a span of tracks), the train will continue to proceed at speed produced by the higher voltage block until the last axle clears the prior block and all axles are onto the lower voltage block at which point the train will slow down.

OK, here's the $64,000 question. When the engine crosses over to the lower voltage block, with axles in each, do the axles in the lower voltage block provide *ANY* current to the engine? This of course assumes that the track and wheel pick-ups are all perfectly clean.


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## Pete Thornton (Jan 2, 2008)

toddalin said:


> OK, here's the $64,000 question. When the engine crosses over to the lower voltage block, with axles in each, do the axles in the lower voltage block provide *ANY* current to the engine?


I think that the minute the first pair of wheels contact the 'new' block, you essentially have a common electrical section as both blocks are now joined as on - eall the wheels are joined electrically via the track pickups. I've no idea what the power pack thinks of that! But I imagine the current flows to the motor as it always did - via the wheels with the higher voltage, which, due to the common pickup wiring, means all wheels.


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## Greg Elmassian (Jan 3, 2008)

No, unless the high voltage section cannot fulfill the current demands of the train


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## toddalin (Jan 4, 2008)

So then, an "in-rush supressor" in the lower voltage block will not/should not "manifest it's time delay/ramp up" so long as an axle is at the higher voltage. That's pretty much what I figured.


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## piercedandan (9 mo ago)

I am looking at this from a very different view. My RR has many cars with front and rear rear axles connected with power. When these cross the gap the power will be the higer one for both tracks. However when the end axle passes over the gap, only the lower power goes to the train on the lower powered track. Then comes the next car with dual power pickup and once again the lower powered track now has the high powered track supply. Can we call this the seesaw effect?


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## toddalin (Jan 4, 2008)

piercedandan said:


> I am looking at this from a very different view. My RR has many cars with front and rear rear axles connected with power. When these cross the gap the power will be the higer one for both tracks. However when the end axle passes over the gap, only the lower power goes to the train on the lower powered track. Then comes the next car with dual power pickup and once again the lower powered track now has the high powered track supply. Can we call this the seesaw effect?


There is a way around this and I do it on my observation cars. You put diodes in line between the axles and the bulb so that the power cannot go from one axle to the other but only to the bulb. Then you can even put a capacitor across the bulb and it will carry the lighting through dirty/dead track.

I have several blocks where trains stop for one reason or another and a lit car protruding into the prior block would keep the engines from stopping in the right places. This way the engines stop and the drumhead stays lit.


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## Greg Elmassian (Jan 3, 2008)

That's a good idea, have not had the issue in DCC, but the wisdom in your DC situation, I've never heard anyone ever think of that. I gather you set this up for forward motion?


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## toddalin (Jan 4, 2008)

Yes, the lighting becomes directional.


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## Andrew_au (Jul 27, 2021)

toddalin said:


> When the engine crosses over to the lower voltage block, with axles in each, do the axles in the lower voltage block provide *ANY* current to the engine? This of course assumes that the track and wheel pick-ups are all perfectly clean.


Looking at only one side, we can simplify the wiring to this:

Power supply
Path A - no load
Path B - small load (diode)
Motor input
The motor input is in series with both paths ("low" and "high" voltage). The paths are in parallel with each other.

Formula for parallel resistance is 1/R = 1/Ra + 1/Rb, or R = (Ra x Rb)/(Ra + Rb). We can approximate the resistance in the no-load path to zero, so the total resistance remains close to zero. Technically, the total resistance goes from tiny (no load path) to tiny minus a little bit (combined path).

Thus, while both paths exist:

voltage dissipated across the motor (vs the wires) _increases_ infinitesimally.
current is split proportionally across both paths, based on the actual resistance on each path.
(Caveat: I'm a little rusty on my circuit theory)


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## Andrew_au (Jul 27, 2021)

toddalin said:


> There is a way around this and I do it on my observation cars. You put diodes in line between the axles and the bulb so that the power cannot go from one axle to the other but only to the bulb. Then you can even put a capacitor across the bulb and it will carry the lighting through dirty/dead track.


Could you avoid the uni-directionality by putting a bridge rectifier on each side of the light?


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## Andrew_au (Jul 27, 2021)

Andrew_au said:


> Could you avoid the uni-directionality by putting a bridge rectifier on each side of the light?


OK, I don't think that works, as a path exists via 2 diodes from A to B.

However, I think I've come up with something even simpler that should behave correctly. Can someone with more skill in electrics verify this for me please?

Define our 4 pickup wheels as Front-L, Front-R, Rear-L and Rear-R.










I believe any path from any pickup to any other pickup must go through the load (the lamp). Lamp should light as long as some pickup is connected to source +ve and some other pickup to source -ve, with no short circuits or leakage between pickups.


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## Greg Elmassian (Jan 3, 2008)

basically a full wave bridge rectifier for each truck.... track inputs on the "~" inputs and plus and minus to lamp


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## Andrew_au (Jul 27, 2021)

Greg Elmassian said:


> basically a full wave bridge rectifier for each truck.... track inputs on the "~" inputs and plus and minus to lamp


Yes, thank you. It took me a while to realise that one could extend the standard 4-diode bridge rectifier to isolate an arbitrary number of inputs by adding extra "diode -> input -> diode" pairs in parallel.

Default wiring can take advantage of the lamp being non-polarised - it doesn't care which side is + or -. I was amused to realise that once you start trying to control current paths it's actually easier to polarise the path through the lamp, even though this isn't a requirement of the lamp itself.


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