Sometimes you want to know how many volts your train transformer is feeding your trains, in order to avoid damaging the motors. And it’s also helpful to know how many amps you’re pulling from your transformer, so you don’t damage the transformer.
Prewar Lionel transformers can deliver up to 24 volts to accommodate large Standard Gauge trains; postwar Lionel transformers intended for the slightly smaller O gauge trains top out closer to 19 volts. Marx and American Flyer trains are intended to run on no more than about 15 volts. You can power Marx and Flyer trains with a Lionel transformer, but you need to take care not to give them more than about 15 volts or you risk burning up the motor windings.
Yes, I say “about.” You get a little bit of wiggle room because you always lose a little bit of voltage between the transformer and the track. How much depends on how many feeder wires you run to the track and the condition of the track. You can measure the drop with a hand-held multimeter to get an idea of just how much wiggle room you have.
Similarly, you want to make sure you aren’t drawing any more amperage than your transformer can handle. You can approximate the amperage of your transformer using this calculator. Look up the model at Postwarlionel.com if you don’t know the wattage. Enter 18 or 19 for the voltage.
Every motor, every light, and every accessory you run increases your amperage requirements, so if you have anything more than a basic layout, it’s a good idea to get one large transformer for your trains, and use smaller, inexpensive transformers for lights and accessories. Or if you have a very large layout with a lot of lights, you may need a large transformer for lights too. Each miniature light bulb on your layout consumes about .2 amps. A cheap 40w starter-set transformer can power about 25 bulbs if you set them at 8 volts.
A 20v AC voltmeter suitable for most toy train applications costs less than $10 from Amazon. A 20a AC ammeter, more than suitable for measuring the output of a Lionel ZW, also costs less than $10. You need no more than one ammeter per transformer, but you’ll want one voltmeter per train that you run.
To wire the ammeter, run a wire from the A post of your transformer, or whichever post you’ve connected to the center rail of your track, to one post of the ammeter. Then run a wire from the other post of the ammeter to the center rail. You’re just splicing it in to your power lead.
If you have a transformer dedicated just to lights and accessories, splice the ammeter into the wire running from your A post.
To wire the voltmeter, run a wire from the center rail of your track to one post of the voltmeter. Then run another wire from the outer rail of your track to the other post of the voltmeter. Then you can measure the voltage that’s actually reaching your track.
You need one ammeter per transformer if you want to leave one connected all the time. You need one voltmeter per loop of track, which may mean you’re connecting more than one voltmeter to a transformer if you have something like a Lionel ZW or American Flyer 30B with multiple controls.