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dcc (again) tiny decoder.

Postby caitlin » Sun Jul 05, 2015 3:23 pm

I've seen around the forum this subject pop up once in a while, but revisiting isn't always the *worst* of all ideas.

I'm just contemplating a 3 train coffee table layout, and for lazy automation reasons dcc seems the easiest of programming things to use.
I've seen someone glue a picaxe to the roof of a t motorised chassis but I gather from their descriptions that they generated their own signals to instruct the train and it's far from nmra compliant.

The key bit of info which leads me to try and retread the topic here is this: http://www.digitrains.co.uk/ecommerce/s ... coder.aspx which I've not seen come up on any fora. It claims dimensions of 7.5 x 5 x 1.8mm. I don't yet posses any t-gauge rolling stock so I can't measure the insides of anything.

Does that sound like it would fit in any voids in either a motorised chassis or a trailing carriage?
I'm slightly in love with t-gauge and would be pretty sad to resort to clunky old z just to get dcc :/

Were this decoder physically able to fit into a chassis then I think the only thing to overcome is the voltage applied to the motor, I'm unsure if 10V on the rails and only using the first 64 bits of pwm speed control would protect the motor enough, or if some other voltage conversion would be needed to get it to 4.5V ish RMS.

Would anyone with experience of dcc at a larger scale have any advice for me?

Many thanks,
Caitlin
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Re: dcc (again) tiny decoder.

Postby dkightley » Sun Jul 05, 2015 3:50 pm

I've just done a few measurements.....

Standard rolling stock height from rail to top of roof varies from 10.5 to 11.5 mm. Height of floor of adjustable chassis is approx 3.6mm. with a roof thickness of say 1.5mm (to allow for curvature), we're left with an absolute maximum of 6.4mm. Very tight....but it may be possible!

Also note that current motors do run hot at 4.5v when run for extended periods....or when stalled for a short period!

Edit: I've just measured a clear shell as supplied with a loco and the vertical space when fitted to an adjustable chassis is approx 4.5 mm.
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Re: dcc (again) tiny decoder.

Postby martink » Sun Jul 05, 2015 6:38 pm

Using DC, I run my T gauge motors on 5.5V DC instead of 4.5 with a typical maximum duty cycle of about 70% with no ill effects. This also improves the low speed starts and stops. Being mechanical devices, motors tend to be reasonably tolerant, so 4.5V isn't an absolute limit.

You could probably get away with DCC and a 10V track voltage, but I would strongly recommend trying for lower if you possibly can. Assuming a 2V voltage drop in the circuitry to the motor, 8V at 50% would work but would be right at (or possibly just beyond) the safe limit. A track voltage of 8V would be perfect if the DCC decoders would still work at this voltage. If your power station cannot go that low, put two strings of paired opposed diodes into the feed to one rail - a string of three pairs of standard 1A power diodes would drop it by the needed 2V.
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Re: dcc (again) tiny decoder.

Postby caitlin » Sun Jul 05, 2015 7:13 pm

After space, the main issue is getting the decoder to power on stably. They apparently spike a bit of current at power on so that will pull the voltage down some.

I wondered about using a high rail voltage and using a resistive sink to shed some of the voltage before it reaches the motor.
If I've understood correctly, the dcc controller operates a FET to provide the duty cycle to the motor, I hope I can shove something in the wires to the motor to get the voltage drop.

Thanks for the body measurements, and advice I was really struggling to find the dimension information in particular.

Since I'm starting out, I was planning to get a Deutsche Bahn ICE starter set, since the units seem to be the longest.
I figured as a test case I could run a single tractor unit and carriage since as yet I have no idea how dcc works in double tractor unit cases :P
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Re: dcc (again) tiny decoder.

Postby dkightley » Sun Jul 05, 2015 7:17 pm

..... in double tractor unit cases

Or triple tractor units if you're looking at the HST!.
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Re: dcc (again) tiny decoder.

Postby mattd10 » Sun Jul 05, 2015 7:22 pm

If you angled a chip across the diagonal you may be able to squeeze one in a trailing coach that was permanently coupled. I'm not so hot on electronics so can't help otherwise but I'd love to see this develop!
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Re: dcc (again) tiny decoder.

Postby dkightley » Sun Jul 05, 2015 7:29 pm

That won't work, Matt. :?

It'll need a greater height if it is tipped over slightly..........it'll need tipping over more than about 40 degrees to require less height, and it'll then be too wide! :o

What's needed is a slimmer profile pcb.....
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Re: dcc (again) tiny decoder.

Postby mattd10 » Sun Jul 05, 2015 8:35 pm

Dang.....thought I was onto something! I'm sure there was a thread on micro dcc chips on the previous forum, but I'm sure that was the smallest around at the time. I'm sure you've searched to death, but ill try and dredge up my bookmarks and have a look!
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Re: dcc (again) tiny decoder.

Postby caitlin » Sun Jul 05, 2015 10:03 pm

Having dug out the best I'd found other than what I posted, it was you posting it!

http://www.talkingtgauge.net/viewtopic.php?f=10&t=42&p=272&hilit=dcc%2A#p274

The ones in my first post are a different shape :/ (I thought they were smaller, but maybe not! :/ just smaller in ONE direction )
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Re: dcc (again) tiny decoder.

Postby mattd10 » Mon Jul 06, 2015 8:48 am

Haha blimey my memory is terrible! Even though yours are smaller in one dimension, it's the right one as that is the most limiting in this scale.

As a proof of concept, here's a folded up bit of card that's close enough to 7.5x5x1.8 inside a body shell with the adjustable chassis underneath....
Matts.jpg
Matts.jpg (32.72 KiB) Viewed 1758 times

Obviously card has a bit more give but I'd say there's probably a bit of wriggle room in there. Definitely could work!
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