What are the differences between a toggle switch 30A 125V SPDT and a toggle switch  15A 250V SPDT.  Plus which switch should I use with Lionel Fastrack? 

Original Post

Usually the 30A means absolute max of 30 amps of current.  This is typically a function of the size of the internal switch contacts and the process if how they mate.  The max voltage is part of the equation also.   I personally would divide the amps number by two and just plan for 15 maybe 20 amps of current going through the contacts.

The 125v is 125 volts and refers to the maximum voltage that is intended to be on the contacts when they engage at the specified current rating.  In the first example the manufacturer doesnt intend for more than 125volts with 30 amps of current to be waiting on the contacts when they close.  

The rating on a switch (of any kind) indicate the test conditions that were used under typical operation. It tells you that the contacts will operate properly under the given situation. Many will give both an AC and a DC specification. Again, just for typical usage. As long as you don't go over the rating, you're okay.

Although automotive switches are normally used with DC power, for the low voltage AC associated with model trains, they are well within specs.

I think the only issue would be if you are using DC 12v  toggle switches and they will be lighted, especially if you are running command control which requires a constant 18 volts. Over time, that would probably kill the lights.

The solution is to integrate a diode into the wiring for the switch to lower the voltage to the light.

I believe Chuck shows a 2N4003 diode wired into his switch in his schematic, shown above.

The diode is to protect the LED from the reverse waveform of the ac voltage. With 18vac and only half the cycle that the LED conducts for, the effective voltage on the LED circuit is only about 12 vrms. So you’re good on voltage. We wouldn’t need the diode if it weren’t for the peak reverse of the ac that can harm the LED.


Case in point.  So as the guys are saying, 125V AC or 12V DC are just data points.  You will be (very) hard-pressed to find a toggle switch specified with an Amperage capability at 18V AC (i.e., O-gauge command voltage).  There are so-called derating curves that effectively characterize how much current a 125V AC switch can carry at 12V DC (and vice versa)...accompanied by asterisks, disclaimers, at what not.  But we must be realistic - is anyone really going to stand around at Home Depot or Pep Boys and do the math? 

IF an illuminated toggle switch is of interest, for sure consider an automotive switch as you can't beat the price point.  The 30 Amp (12V DC) switch that cjack suggests will handle your Fastrack blocks no problem if you use the 5-cent diode as he suggests.  There have been OGR threads showing exactly how to do this if you choose this path.  I don't know about the price of the Calterm switch he suggests, but on eBay suitable (i.e., can handle the current for O-gauge block control) illuminated automotive switches can be had for about 50 cents each (free shipping from Asia) in a variety of LED colors and handsome looking to boot if I say so myself!

Otherwise, if a non-illuminated toggle switch is all you need...and Home Depot or the like is your store of choice, then expect to see the 125V or 250V ratings where anything, say, 10 Amps or more would be more than adequate for O-gauge 18V block control.

Separately.  For clubs or guys with massive layouts and dozens of blocks to control, there is the control panel issue.  You need dozens of toggle switches controlling power to blocks that can be up to 100 feet away!  This means the wiring to the control panel switches must carry the 10 Amps (or whatever) so you have thick, heavy, expensive cabling running the distance to the control panel.  In these cases, it can be advantageous to use relays located near the blocks.  Relays that can switch 10+ Amps go for about $1 per relay (no soldering required).  Then you use a much smaller control current (a fraction of an Amp) running from the control panel to the relay.  The wiring is smaller, lighter, less bulky, cheaper and you vastly increase you choice of toggle switches since the control panel switches only need to switch a fraction of an Amp. You will "make money" just on the savings from wiring cost!  If this is of interest there have been many how-to OGR threads which can be provided.




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