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Hi all, I'm new to this forum, so the answer to this question is probably already tucked away somewhere I haven't stumbled across yet.

My query is: when wiring a CDU to point motors (plural) via a relay, is the discharge current high voltage, low amps or 12v - 15v, high amps? that is to say what amperage rating do the coil contacts have to be? The unit I have at the moment has a 4700 microfarad capacitor although I found an article on the web last night advocating the use of a 1000 microfarad capacitor for switching 4 to 8 points simultaneously.

I don't mind playing with the soldering iron but the grey cells won't cope with the calculus required to solve the equation I found on t'internet last night.

thanks for anyone able to help

Mike
 

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QUOTE (mikethetiler @ 7 Mar 2007, 21:40) <{POST_SNAPBACK}>Hi all, I'm new to this forum, so the answer to this question is probably already tucked away somewhere I haven't stumbled across yet.

My query is: when wiring a CDU to point motors (plural) via a relay, is the discharge current high voltage, low amps or 12v - 15v, high amps? that is to say what amperage rating do the coil contacts have to be? The unit I have at the moment has a 4700 microfarad capacitor although I found an article on the web last night advocating the use of a 1000 microfarad capacitor for switching 4 to 8 points simultaneously.

I don't mind playing with the soldering iron but the grey cells won't cope with the calculus required to solve the equation I found on t'internet last night.

thanks for anyone able to help

Mike

Mike the voltage will be somewhere around the supply voltage to the CDU but the current is likely to be much higher the that available from the supply source - one of the reasons for using a CDU. The solenoid of a point motor offers a relative short circuit to the supply and the more point motors you try to 'fire' the greater the current will be and the quicker the capacitor of the capacitor discharge unit (CDU) will discharge. A 1000 microfarad capacitor is fine for operating one maybe even two point motors but for 4 to 8 I would suggest you'll need at least a 4700microfard capacitor if not a higher value. It's not that the capacitor can't handle the current required it's more to do with how much energy the capacitor can store and can it operate all the point motors before it becomes discharged. The bigger the capacitor the more energy fit can store and the more point motors it will be able to operate at one go before it becomes discharged.
 

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The resistance of a point motor coil is a few ohms, so connected direct to 12v DC the current would be several amps, overloading most power supplies or causing the motor coil to overheat. That's why 'passing contact' switches are used so that only a short burst of current is delivered. The CDU often runs at a higher voltage so that the current pulse can deliver more energy, but when the capacitor is discharged, only a relatively small current is allowed to continue flowing through the coil, possibly around 0.25 amp. this is insufficient to overheat the motor coil.
So your relay contacts may have to take a surge current of some amps but this rapidly drops off and they only have to break the small residual current.

The alternative to relays (and a lot cheaper) is to use a 'Diode Matrix' to distribute the CDU output to two or three point motors - this lends itself to 'route switching' for example. The principles of this and CDUs can be found in "Wiring the layout" by Jeff Geary, KRB Publications ISBN 0 9542035 5 0. This book also comes with a useful CD ROM illustrating how to wire up a layout.

Regards,
John Webb
 

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Discussion Starter · #4 ·
QUOTE (Gofer @ 7 Mar 2007, 21:58) <{POST_SNAPBACK}>Mike the voltage will be somewhere around the supply voltage to the CDU but the current is likely to be much higher the that available from the supply source - one of the reasons for using a CDU. The solenoid of a point motor offers a relative short circuit to the supply and the more point motors you try to 'fire' the greater the current will be and the quicker the capacitor of the capacitor discharge unit (CDU) will discharge. A 1000 microfarad capacitor is fine for operating one maybe even two point motors but for 4 to 8 I would suggest you'll need at least a 4700microfard capacitor if not a higher value. It's not that the capacitor can't handle the current required it's more to do with how much energy the capacitor can store and can it operate all the point motors before it becomes discharged. The bigger the capacitor the more energy fit can store and the more point motors it will be able to operate at one go before it becomes discharged.
Sorry for the apparent confusion, I missed a nought - should have read 10000 microfarad, thanks for the help
Mike
 

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Discussion Starter · #5 ·
QUOTE (John Webb @ 7 Mar 2007, 23:25) <{POST_SNAPBACK}>The resistance of a point motor coil is a few ohms, so connected direct to 12v DC the current would be several amps, overloading most power supplies or causing the motor coil to overheat. That's why 'passing contact' switches are used so that only a short burst of current is delivered. The CDU often runs at a higher voltage so that the current pulse can deliver more energy, but when the capacitor is discharged, only a relatively small current is allowed to continue flowing through the coil, possibly around 0.25 amp. this is insufficient to overheat the motor coil.
So your relay contacts may have to take a surge current of some amps but this rapidly drops off and they only have to break the small residual current.

The alternative to relays (and a lot cheaper) is to use a 'Diode Matrix' to distribute the CDU output to two or three point motors - this lends itself to 'route switching' for example. The principles of this and CDUs can be found in "Wiring the layout" by Jeff Geary, KRB Publications ISBN 0 9542035 5 0. This book also comes with a useful CD ROM illustrating how to wire up a layout.

Regards,
John Webb
Seems I have some further research to do, thanks for the help
Mike
 

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Mike,
Have a look at the PDF file that can be downloaded from my web site for wiring diagrams of two different types of CDU.
The CDU I designed that uses normal switches and NOT passing contact type may be of interest to you.
This circuit uses transistors instead of relay contacts so the switching would be more reliable.
The design was inspired by the "Masterswitch" unit sold by Bromsgrove models and can easilly be addapted for use on DCC systems.

Here is the link to the page on my website. http://www.frankcollins.pwp.blueyonder.co.uk/documents.htm

Click on to "normal switch type CDU to open or save the Acrobat file.

Hope this helps
Kind regards
Frank
 
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