Need a 12v to 240v lighting circuit relay

[Greggorio]

I think....

I have a thermostat that is upto 24v but is powered by 12v (I think)

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I need it to switch on and off a 60W mains lightbulb. I do not wish toi get a different powerlightbulb or anything that is 12v etc.

I have been told I need a relay so I need a simple system whereby the switch in the thermostat tells the relay to switch on the light.

Maplins can't give me advice on what parts or things i need as it's 240v's and they aren't allowed to say. I have no clue whatsoever and really can't afford to break multiple components finding out. Please any help will be greatly appreciated

 

[Hero999]

The relay is rated to 125VAC@15A or 24VDC or 240VDC@10A so I doubt there would be a problem with 240VAC.

The only potential risk is that the clearances on the PCB might not be large enough to safely separate the DC control side from the mains relay contacts.

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[Greggorio]

is there any way to avoid this?

 

[Hero999]

There isn't a way to avoid it.

If you know what you're doing then you could look at the PCB and ensure that the distance between the relay contact pins (at mains potential) and the DC control side is at least 7mm; measure the distance from the edge of the pads. The distance between all tracks at mains potential and the DC control voltage should be at least 3.5mm

7mm is the minimum distance allowed between live mains conductors and save low voltage conductors or exposed metal parts which the user can touch. 3.5mm is the distance to satisfy for single insulation, the two layers of solder resist help to form another layer of insulation.

If you don't feel comfortable or understand any of this then play safe and don't connect the mains to this module.

 

[Greggorio]

solder resist?

Ok so is there anything you can coat the underside of the board with to avoid jumps of electricity from one track to another?

Is there a low amp heating method you could think of that is lower voltage?


Is there a way of restricting the flow so I only get 230v's and 0.25 amps? (I know that may be aq really stupid question but I had to ask)

If none of these are the case then what relay components would I need in between this connection and the mains voltage to switch on the lightbulb?


Thanks for helping so far by the way, I feel like a right prat for not knowing any of this but I suppose you have to learn somehow.

 

[Greggorio]

would one of these do it

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If yes I read somewhere that you need a diode on the 12v end so that the 230v doesn't feedback down that way. Though I don't even know what a diode is so haha


My only problem is I live in France and a friend is picking up the parts I've ordered from Maplins on Monday when the store opens so I only have until tomorrow afternoon to figure out what else I want to add to my order.

 

[Hero999]

solder resist?

Solder resist is the green coating on top of the tracks on the PCB.

Ok so is there anything you can coat the underside of the board with to avoid jumps of electricity from one track to another?

Arcing from one track to another isn't normally a problem because of the solder resist.

It's between exposed metal surfaces, such as the soldered pads when arcing can be a problem. Most of the time arcing won't be a problem anyway, the peak voltage of the mains is only 325V which isn't enough to arc, the problem is high voltage spikes occasionally occur (up to 2kV) which can arc a few mm and is why a 7mm clearance between the mains and 24VDC control side is required.

Is there a way of restricting the flow so I only get 230v's and 0.25 amps? (I know that may be aq really stupid question but I had to ask)

There is no need to limit the current, the load will just take as much current as it needs.

would one of these do it

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Yes, that will do the job, as long as you know the coil voltage is 12V, not 24V.

If yes I read somewhere that you need a diode on the 12v end so that the 230v doesn't feedback down that way.

A diode is normally only required when sensitive devices such as a transistor or IC is used to drive the relay.

The diode limits the large voltage produced when the current though the coil is interrupted and has nothing to do with the mains feeding back to the 12V end, you'd still need the diode if you used the relay to switch a 12V circuit.

Though I don't even know what a diode is so haha

A diode is just an electronic one way valve, it only allows current to flow in one direction only.

The high voltage pulse generated by the relay coil is always opposite to the voltage applied to the relay coil. The diode is wired in reverse parallel with the relay coil so that it short circuits the high voltage pulse generated when the coil is turned off.

 

[Greggorio]

ok so I think I vaguely understand. Don't need a diode. Possibly don't need this relay but it can't hurt so might as well. The relay needs to be mounted on a pcb board though doesn't it?
If so do I just buy the board and solder the wires onto it?

What's coil voltage and how do I check that it's 12v and not 24v?


I know it's so much to ask but Maplins only stocks like 15 relays or so. Feel free to say no but if you just say "get that one" I'll get it. If you say just attach the lightbulb to the thermostat then I'll do that.

I just have no clue what I'm doing so you literally do need to say "get that one, also get a pbc board, solder it on it and then you're done" or something like that. Otherwise I can't get my head round it. It's such an important part of what I'm doing but it's also the tiniest part. Everything else is mechanical or structural and is easy for me to get my head around.


Thanks anyway for your time so far

 

[Hero999]

The coil voltage is 12V. I was confused before, I thought your supply was 24V but it's 12V.


You can buy some stripboard if you like to mount the relay to a PCB but you can probably just solder the sores to the pins, although this isn't the best way of doing it.

If you're going to mount the relay to a piece of stripboard then you'll neeed to remove as much of the mains strips as possible ans ensure there's 7mm of clearance between the mains side and the 12VDC side.

How are you going to mount the PCB?

For safety reasons, you need to make sure the mains tracks on the PCB are at least 7mm away from the screws and any metal casing.

 

[leftfield95]

Hi Greggorio,

The safest solution for you would be to use a relay which isn't mounted to the kit. To do this safely you would require a different type of relay, one which mounts in a DIN socket. The socket will have screw terminals so you can safely terminate your mains wires, and simply use two small fly leads to connect the relay coils to the PCB in the kit.

Something like this ;

Min 5A DPDT 12Vdc N41AW £3.54

And its corresponding DIN socket is probably this ;

8Pin Rly Skt DIN N36AW £1.98


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I have that exact same kit mounted in a similar way and it works well, but I'm using heavier duty relays. The sockets I used are designed for DIN rail, but also have screw holes for mounting to a base board.


I hope this helps, any more questions, I’ll be glad to assist.

Regards,

Leftfield95