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DC trains using DCC

Posted: Sun Jan 31, 2021 7:23 am
by dileTTante
Late 2019 I learned about the luxury cruise trains which run in Japan, and was fascinated by one of them, the Train Suite Shiki-Shima. I got a model from Tomix (N scale only) and set up a track around my bedroom wall to run it. I had bought a kit for a DC power pack which has PWM to take advantage of the constant lighting feature on Tomix models, but found the frequencies at low speed are rather rough for the train, and it is a stationary controller. Tomix made a wireless DC controller but it has been sold out since the start of 2020.

My solution was to run the layout from a DCC decoder. I tried it with a small Digitrax decoder shown in the first two pictures. Control is DCC but track is DC. The train runs well, better than with the DC controller. The circumference is about 36ft (9m) of track. I can use the Roco Z21 and wifi Multimaus which I already owned. Any DC locomotive can be run on the track from one decoder address which I labelled 'N Scale' on the Multimaus. Any accessories, such as turnouts, could be controlled from a function decoder connected to the command station.

I had other uses for the Digitrax decoder and wanted a bit more power handling, just in case, so I got a Zimo decoder for HO. I wired it in its box from Zimo, as shown in the last photo. It can be inserted between the command station and the track.

If we had train shows this arrangement could be used to run the old TT stuff on a display layout, such as HP or BTTB trains on a small oval, while controlling from throttles, phones, or tablets for the DCC of other displays. With a large decoder I probably could run my American Flyer trains in the same way.

Yes, only one train can be run at a time with this arrangement, but it's is no worse than with regular DC control, and this allows wireless control with equipment already owned.

Just for fun.

- Terry C

Re: DC trains using DCC

Posted: Sun Jan 31, 2021 11:39 am
by ConducTTor
This is interesting. Never occurred to me to run DC using DCC. This is useful for when I want to run a locomotive that I haven't decodered yet without having to change controllers :thumbup:

Re: DC trains using DCC

Posted: Sun Jan 31, 2021 11:41 am
by RodTT
So let me get this right - the decoder is connected to the command station and track and feeds DC current to the track?

Re: DC trains using DCC

Posted: Sun Jan 31, 2021 1:55 pm
by MacG
As I understand it, the decoder is connected with its track contacts (black, red) to the Command Station track out, and its motor outputs (grey, orange) to the track. As you can see in his last picture. However, this only works well if there are no chokes, coils, or capacitors connected to the motor in the DC locomotive.

Re: DC trains using DCC

Posted: Sun Jan 31, 2021 8:05 pm
by dileTTante
Instead of Command station DCC - Track - [Decoder DC] - Locomotive motor
it's Command station DCC - [Decoder DC] - Track - Locomotive motor

You keep locomotive capacitors, chokes, coils, whatever off the track so they don't interfere with DCC commands going to the decoder in the locomotive.

This arrangement feeds DC to the track just like a power pack so the gizmos in the locomotive don't matter.

I got the idea when I wanted to test BTTB locos on a DCC programming track and didn't have a power pack handy. So I inserted the decoder to convert DCC to DC. The locos ran fine (E94 and BR86).

The little Digitrax decoder ran N scale passenger train with lighted coaches or a loco with the Tomix track cleaner. No problem. The larger Zimo decoder < no problem.

As I mentioned, there's no more operational control than with the power pack, but the decoder runs the motors better and the throttle can be wireless.

- Terry C

Re: DC trains using DCC

Posted: Mon Feb 01, 2021 4:27 am
by MacG
I am still of the opinion that the gizmos in the locomotive do matter. Otherwise, you might leave them on the locomotive if you install a decoder. By the way, the decoder does not output Direct Current (DC) for the locomotive motor (here track) but uses Pulse Width Modulation (PWM).

Re: DC trains using DCC

Posted: Mon Feb 01, 2021 6:51 am
by dileTTante
Decoder output to motor is through rectifier diodes. it is pulsed DC.
In order for the motor to use the DCC power, the power must be converted to Direct Current. To do this, decoders use a Diode Matrix (see image). A diode matrix routes the digital signal to provide a DC signal to power the decoder. A diode matrix simply consists of 4 diodes (a device that lets electricity flow in only one direction) connected in a specific way.

I am not an authority on the subject but a lot of information indicates that modern DC power packs inject a pulse to DC output to help locomotives at low speeds. I think this must be similar to the output from a decoder

Typical information is like this --
Although I haven't used the Tomix power packs for their N scale trains, I have seen schematics. Tomix have a CL (Constant Lighting) feature which keeps lights on in locos and coaches while the train is idle. This is done by using pulses to DC, generating them from a 555 timer.

I got a throttle kit which provided pulses to DC in hopes of duplicating the lighting feature in my Tomix train. It works but the pulses are generated by an op amp from mains frequency and I found the results aren't great - the motor hums at lower power.

Some old locomotives like BTTB are not worth the trouble of installing decoders. When I install a decoder that's when I remove the capacitor and choke. I think these interfere with the digital commands from track to decoder, but I'm not controlling the decoder from the track.

So far, this Tomix train which is full of LEDs and probably filters of some kind runs better from the decoder than from my PWM power pack. Eventually when I try running old TT on a layout fed by a decoder it may turn out that you are right and the capacitors on the motor will have to be removed.

- Terry C

Re: DC trains using DCC

Posted: Mon Feb 01, 2021 5:35 pm
by Bigelov
Wow, that is a very intriguing idea.

I imagine that old DC loco's would run much smoother with the DCC motor control provided by the decoder.