Hello all. I'm planning on using cab control (4 cabs in total) on the DC layout I'm building and it looks like the best way to go is to use rotary switches to assign power to my blocks. My question is what amperage rating should the rotaries I use have? I'm planning on running consists of (at most) 4 locomotives but I've read different rules of thumb for figuring amperage draw for N scale locos (ranging anywhere from less than a 1/2 A to 1 A). Thanks for any tips!
Rotary Switch Question
Rotary switches......
....can be a joy or a disaster.
As far a current handleing, I would err on the side of caution and figure on 1 Amp per locomotive. However, I highly recommentd that you install both an Ammeter and a Voltmeter to moniter the operation of each powerpack/throttle.
There are some other parameters you should consider.
Make-Before-Break (MBB) versus
Break-Before Make (BBM)
MBB means that as you move the switch from one position to the next the 'new' contacts connect (make) before the 'old' contacts disconnect (break). This can be hazardous when switching power sources.
The right choice would be BBM. This switch breaks the 'old' contact before it makes the 'new' contact as you move the switch from one position to the next.
I assume that you intend to use a rotary switch for each block. If this is true, I recomment that you consider switching both leads from each powerpack/throttle. Therefore you should get switches with two decks of contacts, one set of contacts for each of the two track power leads. The throttles should be wired to the outer ring of contacts and the leads from the rails go to the wiper (moveable) contacts.
Overall size is also a consideration. If the switches you select are too small they will be hard to wire up and solder. They may also be difficult for 1:1 scale fingers to manipulate in a 'crisis' situation.
Have fun.
Moose
Hey Moose. Thanks for the
Hey Moose. Thanks for the tips. I have a few more questions now. If I did happen to use an MMB rotary with say, for instance, 10 positions and I only wired every other position would that be sufficient to prevent any shorts? Your assumption is correct that I'll be using rotaries on each block. I was initially planning on using 2 pole rotaries so I was planning on switching power to both rails but I was recently considering common rail wiring so I may use single poles. It would appear to save me some money and create less of a wiring mess under the layout. What do you think about common rail wiring? Thanks again.
I'll jump in here........I
I'll jump in here........I wired my layout similar to what you are doing. You can't wire common, because "common" isn't Common in this instance. If you wire common, ALL your grounds are common, but all your blocks aren't being powered by the same pack, so if one block is running forward on pack #1, and one block is running reverse on pack #2, what happens to your common.....is it positive for pack #1 or negative for pack #2? It can't be both. If it helps, I used a 6 pole rotary from radio shack (part # 275-1386) it's rated for .3A @125V which translates into better than 2A at the 13 to 17 volts our trains run at. I've run up to 7 engines on a circuit with no problems. I use 5 poles, with position #1 empty as an "off" so I can shut power off completely to the block. It's just big enough to be able to wire all those connections without taking up too much space, and the price was $2.99 . Only problem I ran into was it isn't a big seller, so my local radio shack only stocked 2...hit the one in the next town for 2 more, and picked up 3 at the "BIG" radio shack 150 miles away....but I needed 8 so ended up ordering the last one.....all this took about 2 weeks.
....and there you have it.....
...in a nutshell.
If you use a Make Before Break rotary and skip a position between wired contacts the problem goes away. But the extra cost of the 8 or 10 position switch you need to do that doesn't make it reasonable.
Yes, common rail design will cause more problems than it is worth when you are using multiple powerpacks/throttles. That is why I recommend that your switches have two decks, each with a single wiper. One deck for each of the track wires.
Have fun
Moose
You can use common rail wiring
MRLdave wrote:
if one block is running forward on pack #1, and one block is running reverse on pack #2, what happens to your common.....is it positive for pack #1 or negative for pack #2? It can't be both.
Actually it can be both, that is the whole idea behind common rail wiring. When looking at one terminal of a power pack,it is neither positive nor negative no matter which direction the power pack is set, it only becomes positive or negative when it is checked in relation to the other terminal. Since the power packs are isolated through their transformers, you can connect one terminal of each power pack together. This common connection is neither positive nor negative, but may be positive relative to the other terminal on some power packs and negative relative to others.
kill switch tip with common rail wiring ...
Vin2112 wrote:
...I'm planning on running consists of (at most) 4 locomotives ...
To answer your question re amperage/loco - 1A is a conservative rule that you won't go wrong with - 3/4A is probably closer to the typical max.
However, my real tip has to do with common rail wiring. I recently wired up a two main dc layout using common rail and encountered something I hadn't considered that you may want to think about. On my layout I wanted to have a kill all toggle switch to cut power to the rails for the occasion when trains got out of control (think head on collision). With common rail, you may think (as I did) that just running the common wire to a toggle between power sources and rails would do the trick for a kill all. Unfortunately, I hadn't considered that once you put multiple trains on different mains (controlled by different power sources), each of the blocked power sources have a new path back to common through the locos on the other blocks (since the common rails are all wired together). This is not actually a problem under normal circumstances because the impedance through this circuit is higher than the normal common rail circuit - so you can run multiple trains just fine (as people all over do with this type of setup). However, if you use the kill all toggle to cut the common rail power - you will find that - especially for newer split frame locos - one loco can complete the circuit for another (this does not apply if the locos are all in one block) rendering the kill all switch ineffective. This typically manifests itself only when the locos are heading in the one direction or the other with respect to one another - what I mean is the kill all may work (may stop the trains) for one orientation of the trains headings but not the other. That's due to the fact that most locos are now using LEDs for lighting on split frame designs so current is only allowed to flow easily in one direction (true it can also flow through the motor but that's even higher impedance so not as noticeable).
The solution if you want a kill all with common rail wiring is to cut the blocked power sources not the common. With a two source setup - you can cut the common and just one blocked source as well and that will also work.
Hope this makes sense - if you're not going to use a kill all, you don't have to worry about any of this. Just thought I'd mention it because when I threw my kill all and trains kept running it took me a while to figure out what was going on.
Joined: 2005-02-01