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Kickback diodes - Electrics - DC - Getting You Started. - Your Model Railway Club
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 Posted: Fri Feb 16th, 2018 08:46 pm
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Jon Miles
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Hi all,

I have read different opinions on this and just wondered what other people thought.

I am using 30V 10A (DC) relays to control the seep points using a CDU.

I am reading conflicting advice as to whether diodes are needed to protect the switch (in my case the relay) from the reflected current.

Have people used diodes for this? And does it make any difference?

Many thanks for your help.

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 Posted: Fri Feb 16th, 2018 09:28 pm
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Sol
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Yes, I use them on all off my relays as it protects the switch that operates the relay.
This concept of protecting contacts was used a lot in LM Ericcson Crossbar telephone exchanges using a small value capacitor & resistor.



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 Posted: Fri Feb 16th, 2018 09:36 pm
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Jon Miles
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Thank you Sol,

What rating diodes do you use?


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 Posted: Fri Feb 16th, 2018 10:08 pm
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Sol
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Jon, common 1N4004 are sufficient. The cathode/bar/line on the diode is connected to the negative power side of the relay coil - that is the diode is reverse wired to allow the current created by the collapsing magnetic field to dissipate. In practice, the relay stays operated for an extra microsecond.



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 Posted: Fri Feb 16th, 2018 10:12 pm
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Jon Miles
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Ah ok, so its on the switch side of the relay not the CDU side?

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 Posted: Fri Feb 16th, 2018 11:35 pm
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Sol
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A good CDU, http://www.talkingelectronics.com/projects/CDU%20-%20overview/CDU%20-%20Overview.html

has a transistor in the operate path , once the connection between CDU & solenoid is made, keeps the input voltage to the CDU switched off so that when the solenoid operate switch is released, there is only a very small current flowing flowing through the switch so it doesn't burnout under high current. Then the CDU recharges ASAP if not faster.

I am not sure how you intend to use the relay to operate the CDU output though - have you a circuit you can post?



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 Posted: Sat Feb 17th, 2018 09:29 am
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Jon Miles
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this is the simplied diagram with mot exact example of the switched but they are of the correct type.

This is the relay board i use:

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 Posted: Sat Feb 17th, 2018 10:11 am
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Sol
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What units are they ? When I said a circuit, I was refering to something like this
http://www.azatrax.com/image/mrd2-loop-control750.png

or

http://no1pdfplans.de.vu/diytrainlayouts/wp-content/upload/2014/03/train/dcc-wiring-diagram.gif



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 Posted: Sat Feb 17th, 2018 10:47 am
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Jon Miles
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Damn, ok that may take a little longer to create lol.

Basically, the track is the Hornby track pack layout:


There are point motors at each of the points.  So 6 in total (two points throw in tandem)

The relay also controls power to the 6 sidings so they can be isolated.

when the switch is thrown it sends a signal to the Arduino board that then sends a signal to the relay via a 5v trigger.  this is on a different board connected via a 22way cable.

The Arduino closes the relay for 10ms and then reopens it, which is long enough to throw the point.

The  +ve from the CDU is connected to twelve of the commons on the relay and the -ve is connected to all the common returns of the point motors.

The relay is then connected to each side of the point motors, so one point motor needs 2 relays, one for each direction of the point.  The NO output from the relay connects to one side of the point motor. closing the relay completes the CDU circuit to throw the point.

The relay board can be found here


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