Rotary switch ratings

[lionelsoni]

I got this by e-mail from Bob Anderson:

I bought some RadioShack 275-1386 2-pole 6-position rotary switches to create a mapping from terminals on my ZW to blocks of track. They say "contacts rated at 0.3A at 125VAC". I assume power is the relevant metric, and P=VI, so I assume they can handle about 3 amps at 12VAC. What I don't know is how many amps I'm drawing running a typical train (about 8 cars) on a 12'x10' layout. All are postwar non-electronic engines. Will these rotary switches handle the load? The little contacts look awfully puny for the 16-gauge wire I'm using for my main track power lines.

Thanks for any help on this...

Bob, I'm afraid that the .3-ampere rating still applies at low voltages. Unfortunately, the switch is way too small for powering trains.

Of course, the current that a train draws depends on a lot of variables; but I usually think of it as in the ballpark of 5 amperes for a postwar train.

You might consider using toggle switches instead of the rotary switches. A single-pole-double-throw (SPDT) and a double-pole-double-throw (DPDT) for each block can select among the four outputs that your ZW has. I recommend center-off switches so that a block can be completely shut off when needed.

[msacco]

Bob,
   A couple of years ago I purchased some rotaries from a gentleman on the OGR forum who was going to set up his layout conventionally using block control like I am. He told me the rating would be fine and that his club had used these.
   My question is this. I'm currently using these switches along with my modern ZW. I think based on the contacts that they could be rated as low as .03 but I'm not sure because the exact model number isn't on the switch. Only the brand Alpha is and MOuser does carry them and that's where this gentleman orginally purchased them from. I went to their site and cannot find anything rated at even an amp.
   I've been running trains for months now usually about an hour at a time or less. They never feel hot but I've experienced some funky speed changes with my mostly postwar stuff. It doesn't always happen. I'm wondering if underrated rotaries could have somehting to do with it or would the layout not work or blow breakers with underrated switches.
   Any insight would be greatly apprciated.

Mike Sacco

[willpick]

Mike, there are rotary switches that have high(12 amp +) ratings. Companies name is NKK, sold by Allied Electronics. BUT they are pretty expensive: cheapest I saw was about 30$. Some were upwards of 60$!  Better to use plain old toggle switches--

HTH!

[lionelsoni]

An under-rated switch should not trip a circuit breaker, but very well might reduce the track voltage, perhaps intermittently.

Toggle switches can be used to select from quite a few transformers.  As I mentioned earlier, and SPDT and a DPDT can handle 4.  An SPDT and two DPDTs can do 6.  And an SPDT and two 3PDTs, 8, although you would probably have to order the 3PDTs.