DPDT Track Selector Switches -- what's the min power rating?

Discussion in 'N / Z Scale Model Trains' started by sabretooth47, Aug 24, 2006.

  1. sabretooth47

    sabretooth47 Member

    I want to make my own selector board using DPDT switches from radio shack like this one here:


    It's rated at 3 amps at 125VAC, but what is the minimum rating I can use in case I find something a little cheaper? Is this even good enough? Do I need a DC rated switch?

    When I get down to wiring the blocks, I want my schematic board to have the switches right on the blocks they operate, rather than just number them and have the blocks of switches (ie. Altas' Track Selectors).
  2. hooknlad

    hooknlad Member

    Hello Sabretooth47 and welcome to "The Gauge", The reasoning you want to use these DPDT switches is to throw the track switches , is this correct? The track Switch requires to DPDT switch to have a momentary switch. So you would need a DPDT center return momentary switch. Unless you plan on having the track switch snap and you physically placing the switch into the center position, you will be ok. If you leave the switch on, in either direction for any extended period of time, say goodbye to these snap switches. They cannot maintain being "ON" for long times - they are momentary only...
    AS far as the switches go, if you research Ebay, you will find brand new switches such as the ones from radio shack for considerably less. i believe i purchased 25 of them for about $12. Look around, $5 a pop is way high.. Good luck in your search, hopefully this helped...
  3. N Gauger

    N Gauger 1:20.3 Train Addict

    If you find something cheaper...... DON'T Buy it!!! these are great swtiches for the money, but even they (once in a while) break. The contacts inside will wear out over time (5 - 6 years).... I used these on all my HO and N layouts and have never had any troubles with current or voltage, but since I used them as reversing switches for blocks, a few (out of 30) wore out...

    They are great switches for what you want to do.... just be ready for the "most used ones" to go bad after a while....

    Also - I learned to put hinges at the top of my panels, that way all I have to do is pull them up to be able to work on the "contacts" of the switches - - makes it way too easy to solder connections....

    Now, in G - - I use industrial switches, so far only 1 of them has gone bad in 13 years.... not too bad at all!!!!!! I got them from a flea market for ham operators.. See my web site (in sig block) to see athe panels and switches I'm using now...

    Any questions, just PM me :) :)

    Good Luck!!!!!
  4. hooknlad

    hooknlad Member

    The ones i got online (EBAY) are brand spanking new - UL listed , still in the plastic,etc....I have not had a bad one yet. I guess its kinda like buying the no frills switches w/o all the great packaging... Radio Shack subs out all its components to different suppliers, so in fact, the no frills switches, just may indeed be the same ones that Radio Shack is selling as well.. Thats all im saying, ill be quiet on this subject any further. :rolleyes:
  5. N Gauger

    N Gauger 1:20.3 Train Addict

    And yes, Like hookinlad said - if you can find them in bulk, or on e-bay cheaper, that's good too :)
  6. ezdays

    ezdays Out AZ way

    Well, the one's I buy probably come from the same factory that makes the Radio Shack ones, but they are a fraction the cost. When we had our manufactuing company, I used these in many industrial and securty control systems without any problems. I buy them locally, but they have an 800# or you can use the web, and ship anywhere. Their web site is here.

    One tip when soldering to any switch, be sure that the switch is open for the contact that you are soldering. If the contact is closed, you can ruin the switch. With center-off contacts, that's not a problem, just keep it in the center position.
  7. sabretooth47

    sabretooth47 Member

  8. pgandw

    pgandw Active Member

    The toggle switches are normally going to be passing whatever current a train draws. Typically an N scale locomotive draws 0.3 amps or less. So even triple-headed, a 1 amp rating is sufficient. Only if you have a large layout running multi-engined lighted passenger car trains are you likely to exceed 1 amp in N. BUT

    The big gotcha is your circuit breaker or limiter or fuse for a short circuit. How muuch current will your power supply put out before it trips off line (due to internal circuit protection) or overheats. Unless otherwise spec'd I would use the output rating of your largest power pack. If you have an 18VA power pack, the max current will be 18/12 = 1.5 amps. If you have bigger than 18VA, divide by 12 to get the current. You want your track wiring and toggle switches NOT to be the fuses for the power pack - that's how fires get started. The wiring and toggles should be rated to handle the full load of a short circuit of the power supply, or 1.5 amps in the 18VA power pack example. Unless you insert lower rated current limiters, fuses, or circuit breakers, the short circuit current of your biggest power pack is the rating you should use for your toggle switches.

    Voltage rating is unimportant as long as it's at least 12V.

    Hope this helps.
  9. sabretooth47

    sabretooth47 Member

    That's exactly what I wanted to know.

    Basically, by your 18VA 1.5A example, what your saying is that if I use a 3.0A switch, my equipment (engine or throttle) will fry long before the switch fails?

    I'll have to check my powerpack for it's rating, but I'm sure it's average (it's a Railpower unit, but I can't recall the model number at the moment).
  10. pgandw

    pgandw Active Member

    You are correct. Since short circuits are rarely inside a locomotive - they are usually caused by metal across the rails - your power pack will be the give way point. Any decently designed power pack will have some form of protection built-in, but I wouldn't want to bet on all train set power packs.

    Remember to use your largest anticipated power pack. A 3 amp rating is safe, and a 2 amp will cover all but the largest HO/N power packs. 3 rail O is where this gets critical - a Lionel ZW tranformer will not pop its circuit breaker until 10 amps are flowing through your wires, toggles, and track - and that's getting into metal welding territory.

    The other solution is to put a 1 or 1.5 amp fuse or circuit breaker between each power pack and the control panel. Then you can use appropriately rated toggles and smaller wire. The nice thing about this solution is the the lower rated toggles can be physically smaller, cheaper, and may be a more attractive fit for your panel. I chose this route with a small HO layout - used 1 amp toggles with 1 amp circuit breakers on the inputs so I could use some nice looking minature toggles.

    Post a pic of your control panel when you get it built please.
  11. steamhead

    steamhead Active Member

    Just a note...(Not amp related). If using toggle switches to power turnout motors, there is no need to use a DP switch. A SP switch is all you need (and all you get if using "standard" push-button switches). The common pole can be routed ("daisy-chained") fom one of the poles of the power pack to the common contact of all the turnout motors. Use DP's only if: A) You like to do a lot of soldering, and B) you like to use a LOT more wire than you really need.

    Gus (LC&P).
  12. steamhead

    steamhead Active Member

    ...make that "from".

    Gus (LC&P).
  13. 60103

    60103 Pooh Bah

    Can I ask a question of the electrically knowledgeable? Is the amp rating independant of voltage? Is a 3 amp fuse safe for 3 amps at 12v and at 120V?
    If so, then a #amp 125V switch would be good for most model railroad applications up to HO gauge, unless you multiple unit a 4 or more locos with the old style motors.
  14. hooknlad

    hooknlad Member

    David - 3 amps is 3 amps no matter what voltage is applied - the voltage rating determines the design of item built. With voltage considerations - larger spacing is needed between conductors to prevent arcing between them. Insulation ratings need to come into play at higher voltages as well.... Now heres a lil unknown thought which should be asked, but it is trivial, whether the AC or DC rating matters - trivial for low current applications , model railroads = "NO", ac / dc ratings does not matter. When higher current applications are used about 5 amps or better it makes a difference... The construction of the switch, materials used are different, more costly.....
  15. ezdays

    ezdays Out AZ way


    I've seen switches and relays with different AC and DC ratings printed on the side. You are right, the size of the contact and the material used makes the difference in the rating. You could conceiveably run more current through a switch than it's rated for, as long as you don't switch the contacts while they're hot. When you do, you draw an arc and the contacts pit and warp, or worse, weld.

    There is also the case against running too little current through a switch. Using a 5 amp switch for a 10 milliamp circuit isn't that great either because it doesn't draw enough current to keep the contacts clean. Switches capable of switching low currents use expensive metals such as gold plating.
  16. hooknlad

    hooknlad Member

    Touche' Don - You Are my Mentor , Have I mentioned that lately? LOL :thumb: :thumb: :thumb:
  17. ezdays

    ezdays Out AZ way

    Thanks, you're not doing too shabby yourself. :thumb: I'm just drawing from a good 40 years of building custom control systems and learning a lot of these things the hard way.:rolleyes: (translation: Screwing up a lot until I got it right) :D stooges8 :D

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