DC cab control question

This probably belongs on the DCC/Electronics forum, but I didn't see any non-DCC questions there and I presume that instead of a direct answer I'll get lots of advice to switch to DCC. So here goes...

I'm building a rather large layout with walkaround DC throttles (Innovator 3000). I have three control panels so far with two cab switches and a couple of local (switching cabs). I constructed it with a common ground bus. All the wiring manuals I've seen show that it is okay to use a common bus for multiple cab operation. The problem is that when the throttles are in the opposite polarity, (one forward, one reverse) they short out. This seems logical to me but hey, I'm just a mechanical engineer. I've isolated nearly every circuit to be sure I don't have a short but it didn't matter. Finally, I removed both feeder connections and wired them to a terminal block that was wired to ground and tried again. Still the same result. I also checked the throttles by putting a meter on them and operating them with no ground or feeders and they operate just fine with no problems when changing their polarity.

I'm figuring it has something to do with a dual current path in the common return bus, so I probably need to add a separate ground for each throttle, connected via DPDT switches. That will be a lot of work that I'd love to avoid.

I seem to recall that I operated each cab throttle successfully before changing some of the wiring, but I also don't remember if I tried running them simultaneously, and especially not with opposite polarities.

Any suggestions other than 'switch to DCC' are appreciated.

I'm not adverse to DCC, but for one thing, I have over 40 locos to convert, and the main reason is that I am lucky to spend 4 hours a week on the layout at the present time.

Thanks in advance for any replies.

 

Common negative connections are fine provided the circuits are totally isolated from each other. If not, once you start reversing the polarity of any connected supply you have the problems you are experiencing.

Unfortunately, the solution is to go to a non-common rail set-up. It means running additional wiring, cutting and isolating the common rail at the block ends and adding DPDT switches but you will avoid any further problems. It will take a little time to run out the extra wiring and cost a bit more for the replacement switches.

Fortunately, it appears you haven't got so far in your layout building to make it nigh impossible to change it.

Hope this is of help.

 

My old DC control system was wired w/ a common bus. My power blocks or districts were double-gapped. I had no problem with loco's running in different directions in different blocks. The selectors I used were double pole, 4 position rotary switches I got from Jameco Electronics.

My old system was set up w/ 3 cabs.. a red, green, and blue cab. The yellow position was "off". This control panel has long been removed and "junked" in favor of a simpler panel for DCC.

There is a way to do what you want, but it would take a session w/ some wiring diagrams and nitty-gritty specifics for your layout.

 

Gary and Paul, you are both right.

What seems like a complex problem generally has the simplest solution. I was feeding both cabs from the same power supply with multiple terminals. The power supply was 16VAC, the transformer to variable DC is built into the walk-around controller. I didn't even give a thought to the possibility of a ground loop through the AC circuit. D'oh! I hooked up the second cab to another power supply and both cabs now work fine.

I'll think of the time spent Sunday analyzing the problem as 'lab period', because it did give me the opportunity to really learn and document the layout's circuitry. And I'm most gratified to know that I won't have to add separate ground circuits for at least 35 DPDT switches!

With the time saved, maybe this weekend I can get some more scenery done and update pics in my Railimages album.

Thanks again for the advice.