Reverse Contactor

[DamienGiles]

Hi all.

Thanks for picking the thread to read

I'm interested in finding a unit that works the same way as a contactor (able to switch a high load current using a low load current) BUT I’m interested in doing it in reverse.

So to try and explain, I have a switch (thermostat) that runs the contactor. The circuit is live until the thermostat reaches a certain temp at which the thermostat stops outputting voltage and in turn makes the contactor cut.

However I now want the circuit to do the same, but in reverse - i.e. the circuit stays live until the thermostat outputs a voltage and then the contactor cuts the output power.

The only way I can think about doing this is using an RCD where the output from the thermostat triggers the RCD - but this isn't ideal I understand.

Any ideas? Links?

Thanks

 

[Steve]

WHY? for a start.

For cooling applications, you use the other contact in the thermostat (theres a N/O and an N/C contact in most).

However, you can add on an auxilliary N/C contact that will do what you want. Not sure if they fit all contactors.

 

[DamienGiles]

WHY? for a start.

It's a safty switch. When the heat gets too hot, it'll cut offf the circuit.

For cooling applications, you use the other contact in the thermostat (theres a N/O and an N/C contact in most).

My stat does, but it doesn't have a high enough load to switch the circuit.

However, you can add on an auxilliary N/C contact that will do what you want. Not sure if they fit all contactors.

Could you expand on that? I just want to be 100% clear. Are you saying contactors come in two flavours - N/O and N/C? If so, I can't find a supplier that supplies both. TLC, Screwfix etc.

Thanks for the reply

 

[Steve]

What you want then is an additional stat, set much higher than the first would ever be, so in case the first fails, the second will cut out.

Wire the two stats in series.

The first stat is the safety stat, set at a high temp. This would cut power to the rest of the circuit, should the temp get too high. No need to be messing about with contactors.

 

[flameport]

It's a safty switch. When the heat gets too hot, it'll cut offf the circuit.

which is the same as

The circuit is live until the thermostat reaches a certain temp at which the thermostat stops outputting voltage and in turn makes the contactor cut.

If you are thinking of desiging a circuit where the safety switch only operates when a voltage is present, think again.

 

[DamienGiles]

The stat is only one of three sensors.

The remaining two are a firealarm and water sensor.

Using the stat on it's own with a contactor is easy.

However, trying to intergrate the firealarm and water sensor into it not so much so - hence my post (didn't want to bore everyone with the detail).

The firealarms are interconnectable switched via a 9v sensor wire.

The water sensor is still being designed but this could output anything upto 230v.

I mentioned contactor becuase I was trying to reverse it - it's probably not a contactor that I'm after - hence my post.

Can you suggest a way that I can use the input of all three sensors to be able to cut a large load curcuit?

Thanks again both for the replies

 

[Taylortwocities]

Surely use a 230V relay with a change over contact. You can then use the contact to be NO or NC to do whatever you need.

Something like http://www.maplin.co.uk/Module.aspx?ModuleNo=37495
or
http://www.maplin.co.uk/Module.aspx?ModuleNo=37519
( A DIN rail socket is also available for both of these)

 

[DamienGiles]

Surely use a 230V relay with a change over contact. You can then use the contact to be NO or NC to do whatever you need.

Something like http://www.maplin.co.uk/Module.aspx?ModuleNo=37495
or
http://www.maplin.co.uk/Module.aspx?ModuleNo=37519
( A DIN rail socket is also available for both of these)

Taylourtwocities - Thanks for the links.

How would I create some type of "reset switch" within the circuit?

I want a final layer of safety in this. If I go and buy a N/C relay, I want to have to "reset" the circuit. e.g. If the water sensor opened the relay thus stopping the circuit... I wouldn't want the circuit to re-power once the water sensor has stopped sensing water.

Is this possible?

 

[Taylortwocities]

Its too late in teh day to do an online consultancy design service.

Probably a latching relay - try this as a starting point (dont forget you're 230v, not 12v!)

http://www.enemythunder.com/latchrelay.html

PS I got this by typing in "latching relay circuit" into Google - try it yourself, its fun!!

 

[bernardgreen]

The water sensor is still being designed but this could output anything upto 230v.

Then it is possibly not too late in the process to choose a temperature sensor that goes OPEN circuit when the apparently UNSAFE condition of OVERHEAT occurs.

Basic rule of any safety circuit is UNSAFE = OPEN CIRCUIT sensor = NO POWER

If you select sensor that have voltage free contacts that are open circuit in the fault and un-safe condition then you connect them in series to supply power to the coil of a contactor. The contacts of the contactor supply power to the equipment. If you include a normally OPEN auxillary contact on the contactor in the series chain of sensors then the contactor will not be able to re-close ( energise ) until a push button is used to short out the auxillary contact.

 

[ban-all-sheds]

Damien - I don't know what the equipment is, or what the consequences of overheating would be, but it's clear that whatever other competencies you have, designing safety systems isn't one of them.

If this is a learning exercise, then fine, but if it's for a real application then please hand the job over to someone who already knows what he is doing.

 

[ColJack]

seriously, you want fail safe...
if the power goes off, with the latching relay the circuit would be reset to the "on" position, so that on restoration of the power it would start again..

you want it so that if the power fails it stays off when the power is restored..

 

[DamienGiles]

Its too late in teh day to do an online consultancy design service.

Probably a latching relay - try this as a starting point (dont forget you're 230v, not 12v!)

http://www.enemythunder.com/latchrelay.html[/QUOTE]

Appreciate that.

PS I got this by typing in "latching relay circuit" into Google - try it yourself, its fun!!

Don't appreciate that. Yes, I do use google but I wasn't aware of a latching relay. Your comments would have been deserved if I know I should be searching on a latching relay. See the OP...


bernardgreen - thanks for the post, noted.

ban-all-sheds - It's research and planning. I appreciate your concern. However, I am not one to build anything electric or otherwise without research, building, checking and testing.

The feedback you guys provide allows me to find a starting block and research - thankfully I have an IQ in atleast double digits
Just becauase I haven't been an electrician for my entire life doesn't make me incapable of doing most of the research and if nessasary, take on board your comments and get someone to check it over. Cost saving at it's best.

ColJack - Thanks. That's what I realised after reading the mapling page posted above and the specs from the manufacturers website.

Of the three inputs (firealarm, water sensor and stat), two of the three will output voltage up until it's activated. The firealarm works in the opposite method so that's where I am at the moment - trying to find the easiest way to get the firealarm intergrated.
This is my next bit of reading when I get in tonight.

Again, I appreciate everyones helpful posts.

 

[ban-all-sheds]

Good-oh.

It's just I kept thinking, Flixborough, Bhopal, Buncefield.....

 

[bernardgreen]

What type of fire alarm sensor are you using and how hot can the fire before the sensor must react ?

There are simple sensors that open a relay contact when fire is sensed or power to the sensor fails.

A very reliable fire sensor is a Woods Metal link that melts when heated by the heat from overheating equipment before a fire starts. It melts at about 70 degrees Centigrade, less than the temperature of boiling water.