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Old 26-09-2017, 11:37 PM   #1
JonMiles
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Default Transistor to throw a CDU

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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Old 27-09-2017, 10:34 AM   #2
whizzo
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HI just an idea had you thought of 2 diodes one switch run on 16volts ac - it is how g scale ,lgb track switches points , hope it might be of help , regards Dave
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Old 27-09-2017, 10:55 AM   #3
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What type of connectors are you using? When using a D-sub type I used two pins for the point wires to effectively double the gauge of the wire.
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Old 27-09-2017, 12:03 PM   #4
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I wonder if the Transistor idea may suffer from interference in the transistors base feed line? Causing point motor operation when not needed?
The idea is good and using a suitable MOSFET per motor coil would provide the required current switching. Of course you would need a suitable power supply to feed the CDU at the location to allow solenoid point motor operation.

Personally I haven't had issues with using multipin D connectors as the cross board plug and socket with solenoid motors fed from one panel mounted CDU. So long as you use a suitable wire size for all the feed wires (16/0.2mm is about the minimum) and if necessary double up the pins and wires then you shouldn't have issues.

Another other option would be to use a basic two wire DCC principle to operate all the accessories? One such that may fit the bill is the self build MERG kits, where a four port solenoid accessory decoder Kit No 52 and Kit 51 panel encoder are used over a pair of wires plus a power supply and each Kit 52 has a built in CDU. Adding more kit 52s as needed for additional point motors.
Another is from MegaPoints which is a ready built control system.
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Old 27-09-2017, 12:39 PM   #5
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I think you will find that a feed from the CDU to each board will be OK. You will be only throwing one point at a time. How do you intend to operate the points, by biased off switches.

Mike
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Old 27-09-2017, 01:54 PM   #6
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Quote:
Originally Posted by Flashbang View Post
Another other option would be to use a basic two wire DCC principle to operate all the accessories? One such that may fit the bill is the self build MERG kits, where a four port solenoid accessory decoder Kit No 52 and Kit 51 panel encoder are used over a pair of wires plus a power supply and each Kit 52 has a built in CDU. Adding more kit 52s as needed for additional point motors.
There are a number of other CDU based MERG alternatives, such as the PD3 and PMR1 designs which are designed to be mounted locally to the point and controlled via low current wires. Much better to keep the very high currents as close to the coils as possible, for various reasons.
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Old 27-09-2017, 10:32 PM   #7
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A good old 2N3055 transistor will do the trick mounted on a heatshink. I have such a CDU in metal box with a pair of transformers - one giving two 16v ac outputs and the other a 20v ac output which I have the CDU connected to. Bought the CDU decades ago as a kit. A quite google throws up this circuit which is similar
https://www.rebelhosts.com/tt/te/p16.htm

A single lead to the baseboards and then linking wires to the other baseboards.
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Old 27-09-2017, 10:51 PM   #8
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Many thanks for all your replies.

to answer questions:

I was intending to use On-Off-On momentary switches for points and semaphore signals.

I was looking at 25pin d-sub plugs and sockets to join to the boards. either daisy chaining, or more likely now 3 outputs from a control panel one to each of the boards.

I had not heard of the MOSFET transistor before (still very much learning) but that I will investigate.

I was planning on using 55/0.1 cable as it seemed cheaper (4 for 25m) and was rated be 6amp continuous, but for price, I may have been looking in the wrong places

Then I was lost at the MERK kits and PM1 etc, also the two diode one switch sorry.
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Old 28-09-2017, 11:19 AM   #9
Tarifa Mike
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Jon,

You need to be a member of MERG to obtain the kits.

Using the biased off single pole switch makes sense. Each point motor needs a 3 wire feed but the common can be a single feed per board thus reducing the number of wires through your "D" connector.

I find a good source of electronic components is Rapid On Line and Farrnell. Much less expensive than the likes of Maplins.

Mike

Last edited by Tarifa Mike; 28-09-2017 at 11:21 AM. Reason: Spelling
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Old 28-09-2017, 01:49 PM   #10
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Quote:
Originally Posted by JonMiles View Post
I was planning on using 55/0.1 cable as it seemed cheaper (4 for 25m) and was rated be 6amp continuous, but for price, I may have been looking in the wrong places
55/0.1 wire is typically used for test leads on DMMs and the like, as its high number of small strands makes it very flexible.
16/0.2, 24/0.2 or 32/0.2 are more common sizes for equipment power wiring.
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