Wayne Roderick, 3rd Division, PNR, NMRA (life)

01/09/98 rev 01/16/07

Frequently, we have to re-invent the wheel
This time, it's High Frequency Lighting (HFL). The Digital Command Control (DCC) folks are touting how nice it is to have constant power on the track for lighting, smoke generators and any number of other gadgets that are on board our trains. It's nothing new on the TETON SHORT LINE (TSL). One of the oldest schematic diagrams in the TSL files is dated 11/27/67 and it documents our first (and current) HFL unit. The last thing I saw in print on this subject was in MR July '62 and the article talked about 600 volt power supplies and antique glass bottle like devices called "vaccuum tubes".

While the TSL has finally abandoned this device with the advent of DCC I'm sure there are still a lot of folks that haven't made the conversion, so here it is.

The whole idea is to put power on the tracks that the loco motors will ignore, but lights and other gadgets can use.

The TSL built a HFL device using primitive germanium power transistors in 1967. I believe the transistors were 2N174, if you can imagine such a low number. You had to keep them cool to avoid thermal runaway, so we had some feedback windings to make them switch fast and run cool. It worked so well that we blanketed the AM broadcast band in the near neighborhood and had to back off our feedback, letting them run a bit hotter. This device put 8 volts at 20 kHz with a capacity of 1 amp on all the tracks of the TSL. A typical DCC power unit puts about 14 volts at 8kHz on the tracks. The loco motors ignore both frequencies until it is rectified back to Direct Current (DC)

Among things that we do with it:

How do we build it. Lynn Westcott of Model Railroader asked me to write an article on it back in '72 or '73. Our gadget was built from the junkbox and I said "first, get a flyback transformer from a 1959 Crosley TV--". It ended there, until the local module club decided they wanted one and I had to rummage in the junk box again and put together another one. Don't let that 1962 MR article scare you- it's much simpler than that to build and to install.

. Schematic, High freq' generator

Unlike the original on the TSL, this one uses a convenient regulated DC power supply from our junk box. We sample the output and feed it back to the sense terminals to improve voltage regulation. This is not essential! The supply voltage is not critical; use whats handy, but it must be filtered and reasonably stable. You control the 20kHz voltage output by the number of secondary turns on the transformer. There's lots of room for big wire; we used common plastic insulated hook-up wire. We couldn't find that Crosley TV, so settled for the flyback from an old color set. The trick here is to find one old enough, so that the flyback transformer is not potted. All we use is the ferrite core, stripping off all the old wire. I suppose you could get fancy and buy a toroid core, but what's the fun doing that way? The 2N3055 transistors need a few square inchs of heat sink. The driver transistors were from the junk box too. Many others will work, so I leave it up to you to dig into your own junk box or possibly try your nearby Radio Shack for something suitable.

Integrating it into the layout is simple. If you use common rail, just bring the single common thru the transformer secondary winding before continuing on to the common power pack wire. The capacitor that I show on the output is to bypass the HF around the power pack(s). It doesn't have to be here as long as it's non-polarized and across the power pack terminals. You can even use a polarized electrolytic if you put it ahead of your direction switch. I show two secondary windings that we use for the module clubs double mainline, because they are not common rail. The TSL also used two windings to provide for turning sections. You can use more outputs, if you find a need.

If you go through cab switchs that can turn the track off, rather than just selecting a cab, then you may want to tack a capacitor across them to let the high frequency pass.

I'm not going into the details of hooking up the various gadgets yet, but If you you're enough of a techi' to build this, then you can work out the applications. Remember- You can limit the current into a load without heat or power loss with a capacitor. Then, you can use a bridge rectifier to recover DC. That's how we run the snow blower motor at constant speed using an old tape recorder motor.

Some things to be aware of:

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