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Transistor to throw a CDU - Electrics - DC - Getting You Started. - Your Model Railway Club
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 Posted: Wed Sep 27th, 2017 05:54 am
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Jon Miles
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Hi All, 
I am still at the planning stage and am going to be using a cdu to throw the points.

However, as this will need to be a modular board that can be taken apart, the connectors to join the boards to the control board won't cope with the current with one cdu in the control board.

so my thinking was to use a CDU on each board (so 3) then a transistor at the points to act as a switch. 

The control board would only then need to run low amp current to the transistor to throw the points. So two transistors per point.

Has anyone tried this? and would it work?there is the discussion of the planned layout here

Many thanks

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 Posted: Wed Sep 27th, 2017 07:27 am
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Sol
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Jon, I know of such a circuit but the owner will not give it out to non-members of the club he is part of but I use this method
http://brian-lambert.co.uk/Electrical.html#One

basically a CDU per solenoid.

In the book Model Railroad Electronics by Peter J Thorne,he describes a circuit that uses SCR's per solenoid

https://www.amazon.com/Model-Railroad-Electronics-Concepts-Advanced/dp/0890241465



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 Posted: Wed Sep 27th, 2017 09:04 am
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The Q
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The Railway I have inherited, IIRC uses a 5V signal from the control panel to the local CDU board (up to 5 CDUs per board). It then uses  relays to fire the CDU and change over to fire the solonoid the other way.
 Using a transistor to switch a CDU would probably fry most Transistors, so using either a relay or SCR would be the way to go.



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 Posted: Wed Sep 27th, 2017 12:21 pm
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BCDR
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Hi Jon,

Seems complicated. The intermodule connectors I use handle 30 amps. That said, I think I'd go with Ron's suggestion and use one cdu per module where there are turnouts. Keeps the wiring simpler.

Nigel



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 Posted: Wed Sep 27th, 2017 08:31 pm
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Jon Miles
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Thanks all,
In no particular order

I was considering a 20amp transistor to avoid cooking it, but am in the theoretical realms here as I am still learning!

Relays I did look at but they seem overly pricey. As did a CDU on each point, although the diagram of a simplified system from Brian Lambert did seem a possibility.

I was wondering Nigel, what type of connector do you use?

And sorry, what is an SCR?

Many thanks and sorry for all the questions.

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 Posted: Wed Sep 27th, 2017 09:52 pm
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BCDR
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Hi Jon,

Anderson Powerpoles. There are other compatible ones. Now the de facto standard for module clubs here.

Nigel



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 Posted: Wed Sep 27th, 2017 09:59 pm
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MaxSouthOz
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Using a transistor seems like an overly complicated way of operating turnouts.

I'm still learning after all these years. 

Why not just a SPDT switch with a capacitor and a common return?

 Like the one in Sol's link.  I've made one as a demo and it worked a treat.

What am I missing?



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 Posted: Thu Sep 28th, 2017 06:01 am
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Jon Miles
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Thanks for the name, BCDR, do they come with multiple plugging and unplugging, it will probably only be put out for a couple of hours every few days?
I was trying to reduce the current through the connector MaxSouthOz, so I thought using a transistor as a relay.

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 Posted: Thu Sep 28th, 2017 06:36 am
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MaxSouthOz
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As I remember it, Richard the current was only for milliseconds.

I've graduated from solenoids and stall motors to servos, so my recollections may be flawed.  :roll:



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 Posted: Thu Sep 28th, 2017 05:45 pm
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emmess
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There's an interesting variation of the Brian Lambert approach in October 2017 Railway Modeller on page 826 that uses two capacitors and removes the diodes. It makes a nice compact package and approach if you want o use solenoids. It's definitely worth a read.



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 Posted: Thu Sep 28th, 2017 10:36 pm
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Sol
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Mike, as we here in Oz won't see the RM for another month or two, can you can scan that bit and post it in here or draw it out, take a photo and post it?

My friend has had a PCB made for the one capacitor version and this is the layout and wiring needed
the PCB is about 25 x 55mm



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 Posted: Mon Jan 15th, 2018 02:38 am
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Roy Low
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Try using a Darlington transistor. They have high gain, and work well. I use the "TIP" range, available in either polarity.

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 Posted: Fri Feb 16th, 2018 05:28 pm
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Jon Miles
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Thanks all for your help.

In the end i went with an arduino to control the points and mimic panell with relays for the points and isolations or sections of track

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