AIU cable length

Yes dear, I know that the BarryBook, 3rd Ed Rev 2, on page 68 covers this subject.

So, you do agree that...

This and a whole lot more is all in “The DCS Companion 3rd Edition!" This book is available from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at MTH's web store!

Mr. B, if I didn't think there was a "whole lot more" in the BarryBook, I wouldn't have bought each of the 3 editions.

Bob, I have a 35 foot run between AIU #1 and AIU #2.  I did one on a friends layout a few feet short of 50 foot.  Everything works perfectly.  On my own, the last switch on AIU #4 throws every time.  If anyone finds trouble with getting switches to throw using long runs, I have a circuit for back feeding the last AIU with 12VDC

Bob, your 25 foot run is no issue.  I go fron the TIU to AIU #1 with the factory cable.  From AIU #1 to AIU #2 is the 35 foot run.  From #2-#3-#4 I use the factory cable.

Interesting, Marty.  Seems like the problem with long AIU cables would be voltage drop in the power circuit, so that if one were to supplement the 2 inside cables with #18 wire, much longer lengths would be feasible???

Or use a buck AC>DC converter, like we use for LED lighting in passenger cars, feeding those terminals (watch polarity) through the Buck, right from the feed that supplies power to the AIU relay terminals.

What I posted works and I have used it.  Jason at MTH has also used this and can confirm.

I'm not questioning it, Marty.  Based on your experience, I'm thinking of getting a 4th AIU to place out near the far end of the layout. 

The fact that it does work had me ruminating about why there was the original advice that was floating around to limit AIU cable length.  And then thinking that the DC power supply you mention could be a buck converter powered right off the accessory feed; these run about $3 or less, compared to several times that for a conventional DC power supply.

RJR posted:

...The fact that it does work had me ruminating about why there was the original advice that was floating around to limit AIU cable length.  And then thinking that the DC power supply you mention could be a buck converter powered right off the accessory feed; these run about $3 or less, compared to several times that for a conventional DC power supply.

Note that most, if not all, AC-to-DC buck converter modules use a bridge-rectifier to convert the AC to DC.  In doing so this means you no longer have a common "ground" between the buck-converter's 12V DC output and the TIU/transformer common.  Bad things will happen.

A 12V DC output wall-wart which provides the necessary isolation can be had on eBay for $2-3 shipped.

This just bit me in the butt the other day!  I used one of my little power modules that I had a jumper to select for common ground or floating ground for a TMCC job.  I accidentally jumpered it for floating ground.  I was mystified why it didn't work until the penny dropped!  Moved the jumper to the proper place and all was well!

RJR posted:

...

The fact that it does work had me ruminating about why there was the original advice that was floating around to limit AIU cable length.

...

You asked for direct experience but IIRC the "original advice" was based on theoretical calculations.  That is, you have some number of feet of cable between TIU and AIU(s).  25 feet of #26 wire (if that's what you have) is about 1 ohm.  It's out-and-back so you effectively have a 2 ohm drop.

Numbers have been published on current draw in the AIU per active relay.  So someone did the math to estimate voltage drop (Ohm's law).  You need something like 9V to insure the 12V relays inside the AIU will trigger reliably.  Anyway, I believe that's the why.

In doing so this means you no longer have a common "ground" between the buck-converter's 12V DC output and the TIU/transformer common. 

Stan, as I think about it, does the cable into an AIU have transformer common, or a separate isolated output from a DC power supply in the TIU?

When Marty gives advice, it's usually more than theoretical:  I take it as gospel!

Now to locate 6-conductor phone cable and plugs.  My local Radio Shack is folding its tent.

RJR posted:

 

Stan, as I think about it, does the cable into an AIU have transformer common, or a separate isolated output from a DC power supply in the TIU?

A meter measures 0 Ohms between pin 3 in Marty's diagram above and the TIU's Black-banana power connectors.

I haven't really looked at all they have, but HD and Lowes have a bunch of stuff for phones and network cables. Just in case your local RS tent folds before you find your supplies.

Next trip to HD or Lowes I am going to look at what they have a little more closely myself.

Thanks for the reminder, rtr.  I had forgotten Lowes phone aisle.

Re-reading your comment, Stan, I realized you weren't talking about Marty's advice, but the original statements years ago.  Sorry for the misunderstanding.