Is a CDU pulse only ???

[collectors]

Hi, Is a CDU a single pulse only ??? or will it carry on trying to send voltage as long as a switch is engaged. The only reason i ask, is that the points are being operated via relays & have had the point contact weld together.

[Flashbang]

Hi
A CDU should be designed so as its a single pulse for only a few milliseconds. Once discharged it cannot recharge until all point operating levers or switches or contacts are in their Off / Open positions.

Suspect your relay contacts are not of sufficient current carrying capacity, that is why they are welding probably!

[Bufferstop]

The air gap in small plug-in relays isn't big enough to quench an arcover. If there are spare contacts on the relay try putting two pairs in series, that will give you twice the air gap and may be enough to prevent welding. Load sharing by putting contacts in series sounds impractical, but it works! Think about it.

[collectors]

Hi Flashbang, I have only had this problem when there was a fault at the load end when using relays, but this was on mains voltage. So i wasn't sure with these. They are rated at 24v 16amp so should be fine.

Bufferstop: These are are only 2 contact single pole switches x 12 on a pc board. with latch/pulse switching using either V+ or V- to switch.

[Flashbang]

First, I assume you are not switching mains 240 volts on these relays? You were just saying that in the past the contacts welded when the relay was used for another purpose? On no account allow 240v to get on the layout.

Usually welding of switch or relay contacts occurs when the contact opens, as the load is present and as the contacts part the load tries to maintain the current flow until the contact gap becomes too large and an arc is created and the heat from that melts (welds) the contacts together. Of course if the contacts do not close correctly then again arcing will occur under a load and melt them together!

When using a CDU there is no load current when the switching contacts open, as the CDU has already discharged all its power into the solinoid coil and if the CDU is designed correctly only a very small amount of milliamps flow into the point motors coil. The CDU should not be able to recharge and then become ready for another pulse until all point operating contacts are open.

[Suzie]

You need to put diodes (1N4148 for example) across your solenoids to shunt the back EMF when the relays let go. There will be considerably more than 24V across the contacts of the relays otherwise and this high voltage arcing will be damaging the relay contacts rendering them liable to weld up.

[collectors]

Hi, no 240 volt, that was just past experience working on relays with mains electric, "not to do with trains". This will be just 15v dc into the CDU & off to the points via an a relay rated at 24v x 15amp.

Suzie: am i correct that best to put 2 x 1N4148 diodes on each set of points. one inline for each + on the coils" ?? Thanks

[Flashbang]

OK so your first post was based on previous contact arcing noted when using mains or other high powered items switched by relays. Not when used on a model railway solinoid motor.

This link explains how the Bemf and diodes work but they do not show of course a CDU in circuit. https://progeny.co.uk/back-emf-suppression/

As I said previously, when a CDU discharges in millisecond's into the solenoid coil it cannot recharge again until all CDU feed relays or switches etc are in their Off position. Therefore there is virtually no current flow of any concern when the CDU has discharged with the relay contact closed or when its breaking contact to open.

TBH.... 15v DC is a bit low to feed into a CDU consider if possible using a DC supply of 19 to 22 volts or alternatively an AC supply of 16volts.

[Suzie]

...This will be just 15v dc into the CDU & off to the points via an a relay rated at 24v x 15amp.

Suzie: am i correct that best to put 2 x 1N4148 diodes on each set of points. one inline for each + on the coils" ?? Thanks

No, you need to put the diodes across the coils, one diode for each coil.

15V DC will be OK if it is rectified from a transformer, but will be a bit low if from a regulated DC power supply, a 24V regulated DC supply will be better.

How long do you operate the relays for?

[collectors]

Suzie:
How long do you operate the relays for? I have a 16 way relay board that i can set for latch or pulse. So pulse for a split second using an RF transmitter for the eight points i have.
Is the points diode image correct ?? Thanks

[RAF96]

My relays shunt the CDU output to solenoids, but only for as long as the pulse from my DCC acc decoders pull the relay coil in for. I don’t have protective diodes fitted and they have been in use for a decade or more without failire. They are however standard Lucas/Bosch 12v 10A hefty car relays not the tiny PCB mount type.
In answer to the original question the CDU will provide output for as long as the switch is held but only until the capacitors are discharged.

[Suzie]

Suzie:
How long do you operate the relays for? I have a 16 way relay board that i can set for latch or pulse. So pulse for a split second using an RF transmitter for the eight points i have.
Is the points diode image correct ?? Thanks

The diodes are the wrong way round - the cathodes should go to the positive, but apart from that the diagram is correct.

I normally fire the solenoids for 100mS and this appears to be adequate if they are well lubricated and free moving. I don't use relays so your timing might need to be adjusted to take account of the time it takes the relays to activate. Don't set them to latch or the CDU will never charge!

[collectors]

Thanks for your help everyone Corrected image