UNFILTERED DIRECT CURRENT CAN BE HAZARDOUS TO YOUR ACCESSORIES.

Most model railroads use direct current to power the trains, and for years, that direct current came from a power pack that rectified house current drawn from the mains.  In recent years, various forms of command control with walkaround throttles became the state of the art.  Those power packs still have constant-voltage terminals for the accessories, but hanging onto them could be a false economy.

It’s a rare day when I see a layout that doesn’t have a cheap train set power pack powering some accessory somewhere in a corner.  It makes me shudder every time I see it, because if it’s powering any electronics, that’s just asking for trouble.  I understand it – most model railroaders aren’t electronics people, and we’re all a little cheap and like to get every possible decade of use out of something that we can.  But it’s a bad, bad idea, and I’m here today to explain why.

The problem is that these old packs were designed to do one thing – apply track power and spin a motor – and weren’t even terribly good at that.  The “DC” that they put out can only barely be called that.  It’s full-wave rectified AC from the transformer that’s probably going through a primitive rheostat and sent straight to the output.  There’s no filtering and no regulation at all.  At low load (such as a modern piece of electronics), this means you get DC that cycles between near 0 volts and around 26-30 volts at 120 times per second, even with the “throttle” barely turned on.  Often the electronics blow up from being pushed beyond their ratings, the customer doesn’t understand why, the manufacturer and product get blamed, and nobody’s happy.   So I’m here today to explain it and tell you: throw that power pack away!

Apparently, contemporary "wall wart" power supplies are designed with solid state electronics in mind, and they're more likely to deliver filtered direct current.  It's still useful to trust but verify.  "So, in summary, if you’re thinking about using that old power pack in the junk bin to power some new electronics on your layout, please reconsider. Leave the power pack in the junk box, or send it to power pack heaven (or the nearest garbage can or recycler) and buy a modern power supply."  That might be reason to be hesitant to buy one of those vintage power packs at a swap meet, even if it's priced to sell at a few bucks.