dcc (again) tiny decoder.

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caitlin
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dcc (again) tiny decoder.

Post by caitlin »

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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dkightley
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Re: dcc (again) tiny decoder.

Post by dkightley »

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.
Doug Kightley
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martink
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Re: dcc (again) tiny decoder.

Post by martink »

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.

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

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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dkightley
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Re: dcc (again) tiny decoder.

Post by dkightley »

..... in double tractor unit cases

Or triple tractor units if you're looking at the HST!.
Doug Kightley
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mattd10
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Re: dcc (again) tiny decoder.

Post by mattd10 »

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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dkightley
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Re: dcc (again) tiny decoder.

Post by dkightley »

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.....
Doug Kightley
Webmaster here and volunteer at the National Tramway Museum http://www.tramway.co.uk

mattd10
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Re: dcc (again) tiny decoder.

Post by mattd10 »

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!

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

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

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 )

mattd10
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Re: dcc (again) tiny decoder.

Post by mattd10 »

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 18121 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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SmallCentral
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Re: dcc (again) tiny decoder.

Post by SmallCentral »

I do not have time for hobbies for the moment but I tried this decoder five months ago succesfully.
it's perfect for a 23m frame. I 'm working on a circuit board to add the lights. I forgot ... that's for a Budd RDC1.
DSCN1384_resized.JPG
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DSCN1396_resized.JPG
DSCN1396_resized.JPG (116.6 KiB) Viewed 13552 times
Last edited by SmallCentral on Mon Jul 06, 2015 9:19 pm, edited 5 times in total.

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

Whoops, I bought a tiny tiny train.

caitlin
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Joined: Sat Jul 04, 2015 4:58 pm

Re: dcc (again) tiny decoder.

Post by caitlin »

And thanks to the confidence SmallCentral has put in me I've grabbed a tiny tiny dcc decoder.
I didn't get it from digitrains in the end because of the friction they create by insisting on website registration and an implication that they'll only ship to a cardholders address.

It may yet prove a mistake, but I've ordered it from coastaldcc who took paypal and appear to be able to post things to my office with abandon.

If I can work out what track design software to use with osx and sort out a dcc encoder and booster to work with that java thing then I'm home free except for a lot of card, foam, flock and muttering :D

Thanks for such a warm welcome to this site and the awesome advice you guys have given me so far.

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SmallCentral
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Re: dcc (again) tiny decoder.

Post by SmallCentral »

Your welcome ;) It's possible , not easy , but possible 8-)

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

SmallCentral: What's the small component you have in series with the orange wire?
Is that anything to do with controlling the voltage to the motor?

I plan to run my rails at 12v and protect the motor either with voltage dividing or limiting the length of the pwm, assuming 10v would only ever reach the motorwires after losses, and hope that the effective emf the motor sees never really gets above 5v.

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SmallCentral
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Re: dcc (again) tiny decoder.

Post by SmallCentral »

It's just a resistor :!: With my Digitrax Central DCS50 , I have 14v out :shock:
Not friendly with the T gauge :mrgreen:

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

What value did you go for? Do you actually run that chassis much at 14v on the rails? I gathered before this thread that I was trailblazing, but knowing now that you've done some of the work already I'd dearly love to know your numbers to speed up my sciencing (and reduce expensive messed up motors!)

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

The dcc board is a pretty neat fit actually:

Image
Image

That would functionally go in there if the lighting board were ditched and the leads re-soldered to leave the board at 90 degrees rather than in the plane of it.
I think it can fit in a few other orientations also.

I don't know if i can replace the lighting board with the functions on the dcc controller or not but time may tell...

mattd10
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Re: dcc (again) tiny decoder.

Post by mattd10 »

Looks great!! I'm not famililar with the ICE units but is there any way of having the decoder in a trailing coach and run the wires through either via a micro socket or just having them permanently coupled together?

I look forward to hearing how you get on...could pave the way for DCC control in T!

caitlin
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Re: dcc (again) tiny decoder.

Post by caitlin »

I'm not convinced I yet understand how dcc works with multiheaded trains and I think that one of the carriages in my 8 car ICE train (which survived about 10 mins before being cracked open, poor thing) is a stealth tractor unit.

I'd either need to wire three motors together on a single dcc controller and never be able to unhook a train, or it is a £30 decoder per tractor unit and if they WILL fit in then they should because that's just neater. £90 to digitise one train is a bit "ugh" though.

I need to get some assistive tools to hold the chassis, a magnifier and a *much* smaller soldering tip for my iron and I think I can get this all rigged into that ICE unit.

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