Running N scale DC trains with a tablet using BlueRail

[peteski]

I'm not really seeing how running into the wrong block would damage the Bluerail board, if it has adequate short-circuit protection which it seems it does.  There's no damaging return path current I can see, as long as each blue-rail is connected to separate power supply.   

No current huh?  The block boundaries are gapped on both rails.  A loco crosses over the bloc boundary.   SInce all the locos wheels on each side are connected to each other electrically, the loco basically connects both blocks int a single electrical block. 

Imagine that each of those blocks is powered by a simple 9V battery. So they are totally separate electric power supplies.  If the polarity of those batteries is the same in each block and the batteries are in identical state of charge (have the same voltage across them) then no current will flow. But even with the slightest different of their voltages (imbalance) a current will start flowing from the higher voltage battery to the lower one.  If they are at opposite polarity then lots of current will flow.  While the are separate power supplies the current will still flow because both rails of the blocks are shorted creating a closed circuit, and each power source has electrical properties such as internal resistance.

Now add to the mix a modern PWM output stages or DCC booster power shorting both sides of those power sources creates even more complex "short".

[jagged ben]

No current huh?  The block boundaries are gapped on both rails.  A loco crosses over the bloc boundary.   SInce all the locos wheels on each side are connected to each other electrically, the loco basically connects both blocks int a single electrical block. 

Imagine that each of those blocks is powered by a simple 9V battery. So they are totally separate electric power supplies.  If the polarity of those batteries is the same in each block and the batteries are in identical state of charge (have the same voltage across them) then no current will flow. But even with the slightest different of their voltages (imbalance) a current will start flowing from the higher voltage battery to the lower one.  If they are at opposite polarity then lots of current will flow.  While the are separate power supplies the current will still flow because both rails of the blocks are shorted creating a closed circuit, and each power source has electrical properties such as internal resistance.

Now add to the mix a modern PWM output stages or DCC booster power shorting both sides of those power sources creates even more complex "short".

I didn't say no current, I said no damaging return current.  I don't see one Bluerail board serving as a low resistance path for another if they are connected to isolated power supplies.  And even if it did, robust OCP ought to shut them both down. Presumably it would kick in on both boards.

I think I'll just contact Bluerail and ask them what they think.

[peteski]

I didn't say no current, I said no damaging return current.  I don't see one Bluerail board serving as a low resistance path for another if they are connected to isolated power supplies.  And even if it did, robust OCP ought to shut them both down. Presumably it would kick in on both boards.

I guess you don't understand that there doesn't have to be a common "current return leg"  when 2 independent sources of voltage (which by definition also have internal resistance) are connected in a way that creates a current path (in our cases connecting on both tracks, the path over one track will supply the current while the other will be the current return path. How damaging it is depends on the voltage difference between those 2 sources of voltage and on the overall resistance of the circuit.  Complicating the matter is the fact that those sources of voltages (throttles) are more than simple batteries - they are semiconductor devices.

But maybe I'm just being too cautious - maybe you are right that using those on a block-control layout and accidentally shorting 2 outputs together or shorting the output to a DCC booster signal will be perfectly safe.

[jagged ben]

...

But maybe I'm just being too cautious - maybe you are right that using those on a block-control layout and accidentally shorting 2 outputs together or shorting the output to a DCC booster signal will be perfectly safe.

I have never seen it damage power supplies of any kind, which is my primary concern here.  (And I've seen plenty of opportunities if it were going to happen, small sample size is not an issue with that.)  And I put that down to all the power supplies being isolated and having adequate overcurrent protection to protect themselves.     Potential damage to locos is really beside the point: it happens, somewhat rarely, but the risk is a given if you use block control or power routing turnouts, and it can happen with a single power source just as easily as with multiple ones.   Also the Bluerail OCP might be an improvement over what we have now.

I think it's imprecise to talk about an isolated power supplies 'shorting' to another.  I know you understand electricity so I'm a bit baffled you're putting it that way.   Yes, depending on the internal resistances you could get a low resistance series circuit, but two 9V batteries is definitely not a good analogy for two Bluerail boards connected to separate DC wallwarts.  At a minimum, those wallwarts are each going to have a transformer and rectifier, and probably a voltage regulator and capacitors, and that could be way to much resistance to count as a 'short'.    But more important, it's all moot if the OCPD kicks in fast and reliably enough to protect the board and supply.

[peteski]

I have never seen it damage power supplies of any kind, which is my primary concern here.  (And I've seen plenty of opportunities if it were going to happen, small sample size is not an issue with that.)  And I put that down to all the power supplies being isolated and having adequate overcurrent protection to protect themselves.     Potential damage to locos is really beside the point: it happens, somewhat rarely, but the risk is a given if you use block control or power routing turnouts, and it can happen with a single power source just as easily as with multiple ones.   Also the Bluerail OCP might be an improvement over what we have now.

I think it's imprecise to talk about an isolated power supplies 'shorting' to another.  I know you understand electricity so I'm a bit baffled you're putting it that way.   Yes, depending on the internal resistances you could get a low resistance series circuit, but two 9V batteries is definitely not a good analogy for two Bluerail boards connected to separate DC wallwarts.  At a minimum, those wallwarts are each going to have a transformer and rectifier, and probably a voltage regulator and capacitors, and that could be way to much resistance to count as a 'short'.    But more important, it's all moot if the OCPD kicks in fast and reliably enough to protect the board and supply.

Yes I understand electricity and I see that you seem to be overthinking this. Think of the throttle circuits as black boxes which can have voltage presented and can supply current, and which have internal resistance at the 2 terminals visible to the outside world. What is inside of each does not matter. For all you know, those black boxes could simply contain 9V batteries inside with a rheostat in series with the battery. When you connect both terminals of those 2 black boxes together, depending on what is at those terminals current can flow.

I also never said "shorted".  That is too vague of a description of what would be taking place.  Short to me means close to zero ohms resistance placed across some device which is capable of producing current and supplying voltage.  No, what will be happening is not a short, but could still be damaging to what is inside those black boxes.

But this is all just theoretical speculations with no specific data.

[jagged ben]

Wait, I'm the one overthinking this?     
To my mind, you are the one introducing theoretical questions that aren't very relevant practical considerations, such as the internal resistance of power supplies.

To me the practical considerations are merely:
- Do the power supplies have adequate overcurrent protection?
- Are they isolated?

They need to be isolated to work as power sources for common rail block selection.  Multiple Bluerail boards connected to the same DC power source will probably not work, even if trains stay in their correct blocks.   Same with DCC decoders on the same booster with outputs connected to the rail.   So you can't talk about the DC sources as separate black boxes unless you know they are actually separate black boxes. 

If you're assured of overcurrent protection and isolation, the chances of anything going seriously wrong are close to nil. 

[peteski]

Wait, I'm the one overthinking this?     

If you're assured of overcurrent protection and isolation, the chances of anything going seriously wrong are close to nil.

You are absolutely right!  I don't know what I was thinking.  Sorry!  I have no friggin' ideal what got into me to produce such wild speculations.  I apologize. 

I must have been using some totally non-standard electrical theories - I deserve all those down-votes.  I have no clue about what I'm talking about.

[delamaize]

You're both over thinking this.

for this much effort, just go DCC.....

[jagged ben]

You're both over thinking this.

for this much effort, just go DCC.....

Please actually read the whole  thread before  raising points that have already been addressed.

[djconway]

Have either of you (jagged Ben & Peteski) actually touched this device?

[jagged ben]

Not me.  I put in a word to BlueRail and got a prompt but incomplete response.  If they are positive about the idea I may purchase one for the club to try out, or wait for the higher amp board.  We have some used OOP wireless throttles that are another possibility.

[peteski]

Not me. Why spoil a nice theoretical discussion in an online forum? 

[jagged ben]

Yeah, I got the 30 days old warning here, but I think I should update the thread...

I e-mailed Blue Rail and Dave over there said that he tried out a common rail setup with two boards and it worked fine.    That's the punchline.   

He did also say that when a locomotive ran into the wrong block it seemed to confuse the BEMF features on the board, and there was slowdown.  But then, if you run into the wrong block on a DC layout, you get what you get (and you don't get upset).