DCC on the cheap :)

[JVene]

Well, my son got hooked!

We visited what he now regards as the 'real' Thomas in Strasburg, Pa (a 1917 engine they've outfitted to look very much like the DVD Thomas).

While there he got Thomas, Annie and Claribelle.

Now he wants a Percy, and associated cars.

Two engines on one track means DCC.

I'm a software developer/engineer, so reading the specs on DCC made good sense to me. We have to economize, though, so while the engines and cars will be Bachman, I have to make the DCC power booster and digital controller myself.

To get his Thomas going, I made a power supply out of an old spare audio power amplifier on a chip I had in my parts box, with the obvious bipolar power transformer and associated components. The power amplifier runs the engine (in DC) with plenty of power to spare, plus I get auto shutoff on shorts, thermal and overcurrent protection, the works.

Since it's an audio power amplifier with bipolar output, and DCC runs at 50microseconds or longer transitions, it's the equivalent of about 9 Khz, well within the power band of the amplifier. The slew rate is also well above the DCC power timing requirements.

So, after studying the DCC control signal details, from the simple commands through extended and service modes, I've decided to write software that sends DCC control signals to the power amplifier from the audio card. It happens to fit just perfectly - an audio card from a computer handles the timing precisely for me, I just keep feeding it what 'looks' like audio, if square waves resembled audio, and let it keep the timing continuously precise (just like playing music).

The audio output from the computer will be +/- 1 volt, approximately (more for types that have a built in amplifier of their own), which will be fed into an operational amplifier to control the voltage swing to the power amp chip.

After that, the little power amp drives about 3.5 amps at +/- 16 volts, and works down to a load of 8 ohms, which seems like about 4 engines, depending on which ones.

The amp chip was about $3, the transformer was $15 (bipolar 18 volt, center tapped at 4 amps). Associated resistors, caps, diodes, breadboard was about another $8, with room for the 80 cent op amp and it's few components (another 25 cents). The computer is just assumed, and anything faster than an old Pentium 233 will do, I'm opting for an old spare P2 400Mhz we use for my son's room.

The same thing could be done for more power, either by choosing one of the higher powered amps (something around a 100watt device, if 8 ohms heaviest load planned), or there's a couple of devices sold as audio amps & servo controllers (a dual purpose device) that can handle 10 amps into a 2 ohm load - it's about $25 or $30 for that one chip, and needs a $10 heatsink.

This smaller one can be beefed up with a few power MOSFET transistors (on heatsinks) to do something similar, probably scaling up to stable performance on a 2 ohm load.

My guess, at around 40 ohms average per engine, that's up to about 10 (safely) or 15 engines, which I HOPE will take some time to get THAT large!

Plus, I'd like to wait until he's at least 9 or 10 before I pump that much current onto the track, even at +/- 16 volts.

Some small, simple, no frills decoders that can power Thomas are around for under $20, so I'm ordering two for experimentation/installation while I await Percy's delivery.

[mf5117]

what type of plc would you use to operate a dcc unit with 13 switches and 5 cars .i want to do this later down the line .

[alex wittman]

that's very interesting, i didn't know that you can do dcc with that kind of supply, but i am in to easy, electrofrog dc layouts, i will post the making on my youtube account.

[mf5117]

what i want a plc for is my lighting switches etc .to run them from my laptop.the dcc controller can be turned on and off from the plc but you would have to operate your loco's from the dcc controller.rather than a million micro spdt co momens it all at the pc .this will be in my future .i have a 4x8 ez track layout . im a maintenance engineer with alot more complexed electronics cnc's cranes boilers crome plating .thats where i get my dc exp from . but ho trains are a good challenge for me .   regards mf