Issue with supply not turning off sometimes

[G-i-z-m-0]

Hi all,

I am hoping one of you kind intelligent people could help me with a circuit I have made which occasionally does not work properly.
The power switch on the right of the drawing enables the circuit to start with then the circuit is held on by the relay. I can now turn off the power and it remains on, which is what I want. Now the module side has a trigger wire to a microswitch and when enabled it kicks in the other relay which drops the first relay out and still puts a supply to the PSU. Once the timer has run down it then reverts the module relay back to the original position which then turns off the power, Sometimes. The only thing I can see is I get an arc across the relay sometimes which keeps the unit energised again. Not sure what component I need to add to this to create a slight delay to stop it pulling in the first relay again. Any help would be much appreciated from the clever people out there.Thanks

 

[Arouse1973]

Hello, can you just run through what this circuit is for. What does it do, what is it used for.
Adam

 

[Bluejets]

Not enough info but it sounds awfully like racing contacts.

 

[G-i-z-m-0]

Hi, I have just put another identical relay on it and it is working every time. I dont get the arcing now across the contacts. Could this be something to do with the gap between the contacts too close causing back emf?

This is a circuit to energise a 3d printer and when the component is complete the head goes back to a set position where my microswitch then turns on the delay circuit. The reason for the delay is to allow the cooling fan to let the head cool down before powering off. hope this helps. Do you think this relay will go the same way over time or do you think it is to do with the gap between contacts? would I be better getting a solid state relay or something with a larger contact gap?
cheers

 

[G-i-z-m-0]

Just had a couple of issues now with the new relay. Do you think a larger contact gap would cure this?

 

[Bluejets]

Do you know what racing contacts are and how it comes about.
May not be the problem as I do not see the complete circuit but it sounds like it may be.

 

[duke37]

It would help if you said which contacts are arcing.
A flywheel diode can be connected across the relay coil to reduce arcing.

 

[G-i-z-m-0]

Hi Bluejets, I have a rough idea but if you want to educate me that's fine.

I would prefer some suggestions to resolve the issue if that's ok
not sure what you mean about you don't see the complete circuit. That is the complete circuit in the picture apart from the timer modules circuit but all it does is switch the on board relay which is in the drawing where it says module.
Unless im missing something? please elaborate

 

[G-i-z-m-0]

Duke37, Ooops sorry mate, its the bottom left contact on the one labeled Relay

Also a flywheel diode would not work on an AC circuit which I did not explain
the coil is powered by 230v mains

 

[G-i-z-m-0]

Not sure if these are any help too. this just shows the circuit connections as an example from the site I bought it from

 

[duke37]

I have had another look. It seems that the relay switches the AC mains supply to the PSU (which must have two wires). This presumably has a transformer in it which is inductive and will stop the current dropping rapidly.
You could try a voltage suppressor across the power supply input. I do not know details of these.
You could use a much bigger relay.

Do not use a diode across an AC relay coil

 

[G-i-z-m-0]

Ah right ok so, yes it does have 2 wires to the Switch mode power supply but did not include Neutrals to keep circuit simple. So do you think that a double pole relay would cure the problem?

Would this fix the problem please?

http://www.maplin.co.uk/p/rc-contact-suppressor-rg22y

or this

http://www.maplin.co.uk/p/transient-suppressor-metal-oxide-varistor-cp76h

 

[Bluejets]

Am I correct in assuming the PSU is generating 240v ac from the 12v dc on the left of your diagram?
If this is the case then a word of warning about pumping 240v ac back into any supply system, dangerous and illegal practice.

 

[G-i-z-m-0]

No offense Bluejets but Duke37 has been very helpful with this and offered some constructive comments which I very much appreciate. I am an Electrical Engineer with Industrial and Commercial background NOT electronics and don't pretend to know much about electronics but do know my BS7671 and what's illegal, but that's another story. Duke37 understands my crude drawing so I will go with his suggestions that help me. Thanks to Duke he has steered me in the right path in resolving this with a simple suppressor I Hope.....Lol!!
Thanks again folks I will let you know the outcome

 

[G-i-z-m-0]

Update, The suppressor did not work unfortunately. I think I have found the problem though. Im guessing the back emf to the relay coil is pulling the coil back in and not the contact arc so I need to find a way of making the coil open circuit and staying open circuit when the module changes the relay over. bit like two relays in series I suppose.
Any comments would be grateful

 

[duke37]

I see that it is a switch mode power supply. These have a large input capacitor which take a massive current when first charging. Do the relay terminals weld together?
TVs which I have seen using a SMPS often have a series 4Ω resistor to reduce the inrush (to 60A)..
Also consider Bluejets racing contacts.

 

[G-i-z-m-0]

Hi Duke, Ive got rid of the suppressor and now used a second relay that breaks contact from the first relays coil and ta da it now works great every time as the emf cant re energise the first relays coil
Thanks for all the input you folks its been a pleasant challenge. I think I will stick to 200mm swa and BMS controls..........lol

 

[Bluejets]

You may well be an electrical engineer but you fail to answer my query.
The reason I bring up the concern is ...if your PSU output is 240V AC then, not only are you creating a dangerous and illegal situation, but your mains supply and your PSU supply can very rarely (if ever) be in phase with eachother.
Hence the sparking of the relay you mention ...lucky for you then this is the only problem you have encountered.

 

[G-i-z-m-0]

Bluejets, I think you need to have a lie down if you cant understand what Duke has got his head round and helped me resolve already. ITS NOT A STEP UP TRANSFORMER, INVERTER ITS A SWITCH MODE POWER SUPPLY DOWN TO 12V FROM A MAINS FEED. If you look at the drawing you will see that if it was a step up circuit the way your seeing it, Why would I be connecting the Neutral output from a relay coil ??!! Please tell me you understand now!!!!!!!. The sparking of the relay was caused by the back emf on the coil which was partly pulling in the contact. Its fixed no longer a spark.no longer a problem, move on its history, get over it. To be honest with you if you had bothered to read the previous posts you would of got that from Duke's observation and comment. Once more I would like to thank Duke37 for his help on this but I'm afraid you really did not help at all apart from getting on your high horse talking about illegal rubbish that I don't think you understand yourself. Its a shame people like you waste other peoples time and spoil these forums for the ones that genuinely want to help like Duke37. In fact post me your findings on what regs and legalities you so talk about. I'm looking forward to it ........LOL

Actually I don't think I want to read your comments anymore

 

[davenn]

Thanks GIZMO

pleased Duke was able to help you out

will close the thread to stop any other comments

Dave