I was talking with someone who’d rather remain anonymous regarding AC and DC power on train layouts. When using AC power, a common trick to reduce the amount of wire you need is to run a power wire to each loop of track, and feed that loop of track from a dedicated loop of wire. Then you can run a second loop that each of your loops of track use as a common ground. His question was, if you wired your layout like this, would it be possible to switch to DC power?
The answer is yes, with a possible caveat.
AC power reverses polarity 50 or 60 times per second depending on what part of the world you live in. DC power stays constant. A DC power supply is perfectly happy sharing its ground with the common wire from AC transformers. So you can run a loop of track on AC and another loop on DC with no conflicts.
You can also do what I do, running trains on AC, but running lights and accessories on DC. A second hand PC power supply that delivers 300 or 400 watts of DC power can cost as little as $15. Or if you fix PCs like I do, you may find a PC power supply is no longer stable enough to power a PC, but still works fine for powering the light bulbs on a train layout. So rather than dedicating a Lionel KW or ZW to lighting, I can use a cheap discarded PC power supply that delivers comparable wattage on its 12 volt rail.
The caveat
The caveat to powering your trains on DC with this type of wiring arrangement is that DC power supplies don’t like sharing a common ground if you reverse the polarity on a line of track to change the direction of a train. This isn’t a problem if you only run one loop of track on DC at a time and run the rest of your loops on AC. But if you run two or more loops on DC, the DC tracks need to always be on the same polarity.
This caveat is the reason why the wiring shortcut I describe has been in use for generations on three rail O gauge and S scale layouts, but never came into vogue in other scales that traditionally used DC power.
But if you only use DC on one loop at a time, or only use DC for powering accessories and never change the polarity like I do, you can mix AC and DC on the same circuit using a common wire across your lighting and all of your track loops.
This isn’t necessarily a huge limitation. Operators who want to use DC often have a mix of trains that run on AC only, DC only, or can run on either. In these instances, a common arrangement is to put a DC rectifier on each loop of track, with a switch that switches between AC and DC, and another switch that changes polarity. This makes it convenient to change from AC to DC at will and also change polarity.
If I’d had a DC rectifier on me at the time, I could have used this trick to run my friend’s Lionel LASER train from 1981 on his dad’s postwar-style layout last week.
David Farquhar is a computer security professional, entrepreneur, and author. He has written professionally about computers since 1991, so he was writing about retro computers when they were still new. He has been working in IT professionally since 1994 and has specialized in vulnerability management since 2013. He holds Security+ and CISSP certifications. Today he blogs five times a week, mostly about retro computers and retro gaming covering the time period from 1975 to 2000.