Anyone else floored by the article on page 41 of the July 2011 MR Mag? There is a fatal flaw with using SPDT switches. Using Fig. 2 on pg 41, if you were to have Cab A's and Cab B's "direction" switch in different positions, then you'd short circuit the entire system! Keeping the "direction" switch the same on both power packs limits you to driving ALL trains in the SAME direction....
These were the reasons I was taught (as a boy) why you have to switch BOTH sides of the circuit using DPDT's.
Am I missing something or was this a massive mistake to print this article?
Dave Loman
My site: The Rusty Spike
"It's a penny for your thoughts, but you have to put your 2 cents in.... hey, someone's making a penny!"
Looking at it again, I see the whole 'floating DC Ground' aspect of it. I see, now, a *different* fatal flaw. While the "Direction" switches being different won't cause a short, it will put 24V differential across the insulated gaps on the track. Run over that with a loco and you just might damage something.
claymore1977 if you were to have Cab A's and Cab B's "direction" switch in different positions, then you'd short circuit the entire system!
if you were to have Cab A's and Cab B's "direction" switch in different positions, then you'd short circuit the entire system!
That's one thing I've never understood how to get around using common rail cab wiring. On my last layout when I redid the track and wiring, and again on my current layout, I've used DPDT switches for each block. With the center-off on them, I can easily completely isolate a block if needed.
Kevin
http://chatanuga.org/RailPage.html
http://chatanuga.org/WLMR.html
I've used common rail for years, and never have had a problem. Granted that adding 12V + 12V would seem to total 24v, what it actually is is two isolated 12V circuits with a common ground as long as the DC sources DON'T share a single transformer winding at the 120V-12V step-down transformer.
DPDT switching and a sectioned 'common' rail is a belt-and-suspenders approach - it's not a bad idea, it's just more expensive - UNLESS the two 12V circuits DO share a single secondary winding. Then it becomes essential.
Presumably, the power for the two-cab layout in MR would be two separate power packs. The first thing those circuits would have in common would be the ground/neutral side of the 120VAC house wiring.
Chuck (Modeling Central Japan in September, 1964 - analog DC, MZL system)
Chuck beat me to it. If you dig through older issues of MR and lookj at ads, particualr for MRC's power packs, back before transitorized packs became common, you will notice that they have multiple models that had two sets of controls. And if you read the fine print you will see that some will say that they are ok for common rail wiring, and others leave that out. And what Chuck said in his post is exactly why. The common rail ones had two compelte transformers in them, the other use a single transformer and dual rheostats.
The Atlas wiring and layout books using their components, which are common rail, always showed two compeltely seperate power packs for the dual cabs (and the sneaky one that had 3 cabs, still all with the Atlas components)
Even with just one cab when I wired a siding to cut power, I always gapped both rails and ran 2 wires. Never common rail wiring. Not sure why, back when I doubt I could have fully understood the nuances of common rail wiring, but that's just how I did it.
If converting a common rail DC layotu to DCC< you're fine as long as only 1 booster is used. As soon as you add a second one, you need to gap the common rail as well at the boundary between the two booster zones. There IS a common in DCC and a loco straddling the gap very well could see double voltage. There are also optoisolated boosters the break that common connection, but why pay for special boosters when all you need is a simple gap in the rail?
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I totally see how this 'works' now. Since the 12VDC output is ungrounded, the ground 'floats' in the center of the voltage. So if you took the + to ground, you'd see +6V and the - would show -6V.
In the senario where Cab A is in direction 'left' but Cab B is in direction 'right', the - of Cab A would be connected to the + of Cab B. This would be much akin to chaining two 12V batteries in series for a total of 24V. Now, instead of a floating ground, you take a tap inbetween the two Cabs and you get -12V to 0V for Cab A and then 0V to +12V for Cab B.
At a portion of the trackage where there's a gap and Cab A is on one side of the gab and Cab B is on the other, you'd have -12v on one side and +12 on the other, for a total of 24V.
This assumes that the power packs are not 'grounding' their - terminal. If the power pack's - terminal *was* grounded, then the short circuit i mentioned in my first post would happen!
Sounds like its time for a bit of theory to practice and set this up on my bench to verify my electrical theory. =D
I guess the question is: Is a DPDT really *that* much more expensive than a SPDT ? (i suppose on massive layouts, the answer is a resounding 'yes')
claymore1977 I guess the question is: Is a DPDT really *that* much more expensive than a SPDT ? (i suppose on massive layouts, the answer is a resounding 'yes')
Using Miniatronics as an example, the DPDT is nearly 50% more expensive than the SPDT.
A package of 8 SPDT cost $1.78 per switch.
A package of 8 DPDT cost $2.62 per switch.
Rich
Alton Junction
richhotrain claymore1977: I guess the question is: Is a DPDT really *that* much more expensive than a SPDT ? (i suppose on massive layouts, the answer is a resounding 'yes') Using Miniatronics as an example, the DPDT is nearly 50% more expensive than the SPDT. A package of 8 SPDT cost $1.78 per switch. A package of 8 DPDT cost $2.62 per switch. Rich
claymore1977: I guess the question is: Is a DPDT really *that* much more expensive than a SPDT ? (i suppose on massive layouts, the answer is a resounding 'yes')
It's not so much the extra expense of the DPDT switch. I usually install DPDT toggles anyway, although I use common rail wiring. Common rail wiring allows me to run a bus wire around the layout for the common rail. I just drop feeders where needed to the bus from the common rail. No home run wiring needed for the common.
For the switched leg, I run a sub-bus for each block. A heavier wire (typically 18 gauge) goes to a terminal block near the block location. From there, I run as many feeders as needed for the switched rail of the block. This helps keep the wiring orderly, and not a large cable coming into the control panels.
The terminal blocks and common bus also provide easy tie-ins for the frog polarity contacts on my turnouts.
And if you use as separate power supply for switch machines (highly recommended), you can use the common bus for the common wire to twin coil switch machines.
Finally, using common rail and DPDT toggles leaves a pole open for other circuits - could be signals, block occupancy lights, progressive or route cab control, or so on.
my thoughts, your choices
Fred W
They probably put it in the issue to show how things used to be done before DCC.
Springfield PA
Hamltnblue They probably put it in the issue to show how things used to be done before DCC.
And how things are still done on the vast majority of smaller and beginner layouts, and larger/more complex layouts owned and operated by people who have NOT drunk at the bottomless $$$ well of DCC.