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Old 16-02-2018, 08:44 PM   #1
JonMiles
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Default kickback diodes

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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Old 16-02-2018, 08:59 PM   #2
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Gulp.

Why not get an AD-S2sx? 1 or 2 point motors each side. Powered by DC or DCC. No need for relays, built in CDU.

Last edited by Cudworth; 16-02-2018 at 09:03 PM.
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Old 16-02-2018, 09:11 PM   #3
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Sorry, should have explained more.

Points and mimic panel are run via an Arduino board, so 5v signal from control panel triggers the relays that then change the points.

the CDU gives power to the commons on the relays and then with the trigger a 10ms pulse to the relays sends the current to the points, so two relays per point.

The reason I have gone this route is this all has to be modular so can be taken down when finished with, and the 5v trigger signal was the best bet rather than having to send CDU current through connectors.
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Old 16-02-2018, 09:21 PM   #4
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Yes I think that approach is wise. I don't think you need the diodes, but you'll have to wait for Flashbang to see this thread to get a definitive answer!
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Old 16-02-2018, 11:45 PM   #5
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Using a diode across relay coils in reverse mode, protects the operating contacts from possible arcing due to high reverse voltage generated by collapsing magnetic field - a process simlar to what was used in Crossbar telephone exchange equipment - that used a low value capacitor & series resistor.
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Old 17-02-2018, 07:03 AM   #6
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It is wise to use kickback or reverse biased diodes across the input/coil terminals of a relay as once the switch on current is removed the collapsing magnetic field can produce a back e.m.f. which can harm transistors and ICs.

Richard
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Old 17-02-2018, 08:02 AM   #7
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I operate my points manually via switches and a CDU and also by DCC using the acc decoder to momentarily pull in the coil of a relay which invokes the CDU side of the system. I have been errant in not fitting diodes to protect the acc decoders as the relay coil field collapses and so far I have not had a failure, but I really should retro fit diodes for peace of mind.

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Old 17-02-2018, 09:42 AM   #8
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this is the relay board I use:


and this is the simplified circuit I use
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Old 17-02-2018, 10:10 AM   #9
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Looking at the relay board there are two SMD components behind each relay. I would suggest one of them is probably a diode across the coil terminals. If the relays are only switching on bulbs, LEDs or motors than I would not concern myself too much about arcing across the relay contact points unless your are concerned about extended lifetime. However as with any switch the relay contacts will produce slight arcing and what is called switch bounce which if the relay is feeding an electronic circuit can produce spurious results and the usual cure is a small capacitor from output to ground.

Richard
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Old 17-02-2018, 10:55 AM   #10
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The relays are throwing the point motors using the power from the CDU
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