Cooker hood schematic, can anyone help?

[SUNRAY]

Cool thanks,

Any suggestions for a suitable enclosure for it? It is the only DIN type device I will have so I am not sure it justifies a case (Unless very cheap!), or whether I can just put it inside a plastic box with a screw-closed front.

MHRV timer - well, I don't think so but I called Vent Axia who said it does, but the advice they gave contradicts the manual!

If I was able to do it that way, can you suggest a suitable device? If it's only a few £ I could try that option first I suppose....

Thanks
Tim

I imagine any mains powered relay (230Vac) would do the job:
https://www.ebay.co.uk/itm/Europa-C...hash=item4b430c788b:m:mtRl4GVWr6zCi0mVyabPJlw

 

[SimonH2]

Maybe - depends on It's pull-in voltage. According to numbers given earlier, it could be asked to pull in at 163V - or lower if the mains is down a bit.
Many relays will pull in reliably at that voltage, but many won't pull in reliably or even at all.

 

[SUNRAY]

Maybe - depends on It's pull-in voltage. According to numbers given earlier, it could be asked to pull in at 163V - or lower if the mains is down a bit.
Many relays will pull in reliably at that voltage, but many won't pull in reliably or even at all.

You are quite right.
I have done a temp repair with a 230V repay in a panel but I had to go though a selection of s/h relays before I found one which reliably worked at 110V. I've recently used a 24Vac relay in a 12V dc situation and so far I have no regrets.

Just looked up the spec's for these 4 pole relays and the 3 makes I looked at quoted +/- 15 or 20% which will make it 184 to 276V or to use the MEDIUM winding.

 

[bernardgreen]

Given that the relay is being supplied from tappings on the motor winding ( if effect an auto-transformer ) some protection againgst back EMF pulses may be necessary. These pulses will occur when the extractor fan is switched OFF

Something like this but with a voltage suitable for the maximum voltage the relay coil can take without being damaged.

https://uk.rs-online.com/web/p/metal-oxide-varistors/7209960/

 

[JohnW2]

Given that the relay is being supplied from tappings on the motor winding ( if effect an auto-transformer ) some protection againgst back EMF pulses may be necessary. These pulses will occur when the extractor fan is switched OFF

To what extent is that an issue of practical importance, as well as being a theoretical one?

After all, virtually all relays with AC coils have their coils supplied from a transformer (usually one of the DNO's ones), the supply to which will sometimes be removed - so if protection such as you describe were required, would it not be required for almost all AC-coil relays?

Kind Regards, John

 

[bernardgreen]

To what extent is that an issue of practical importance,

If the relay is one with time delay or similar "intelligent" function then it will have some electronics in it.

 

[JohnW2]

If the relay is one with time delay or similar "intelligent" function then it will have some electronics in it.

I don't deny that but, as I said, virtually any relay with an AC coil will have its coil ultimately supplied by a transformer, which could create back EMFs if things were switched off (or its supply failed) - so are you suggesting that all relays (and, indeed, everything else supplied with AC that might be susceptible to 'voltage spikes') should be protected in the manner you suggest?

Kind Regards, John

 

[SUNRAY]

Given that the relay is being supplied from tappings on the motor winding ( if effect an auto-transformer ) some protection againgst back EMF pulses may be necessary. These pulses will occur when the extractor fan is switched OFF

Something like this but with a voltage suitable for the maximum voltage the relay coil can take without being damaged.

https://uk.rs-online.com/web/p/metal-oxide-varistors/7209960/

I've fitted 1000's of relays and I think it's far to say never fitted any such devices for AC relays. DC circuits are a different matter.
I appreciate the motor can step any back emf up but in this situation it will be limited by the motors inductance shorting the relay to some extent.

 

[SUNRAY]

If the relay is one with time delay or similar "intelligent" function then it will have some electronics in it.

In which case not a ploblem, the back emf will already be fitted within the electronics.

 

[bernardgreen]

virtually any relay with an AC coil will have its coil ultimately supplied by a transformer, which could create back EMFs if things were switched off

Each case has to considered separately. It isn't just damage to the coil that has to be considered, tracking between coil and contacts is possible when ther are spikes on the supply to either coil or contacts. The capacitance of long leads between EMF source and the coil can reduce the effect of the spike on the coil.

from the data sheet of a very common relay ""• 8 mm Creepage and Clearance, 6 kV (1.2/50μs) between coil and contact""

6 Kv is easy to create even with a small coil in the source

 

[JohnW2]

Each case has to considered separately. It isn't just damage to the coil that has to be considered, tracking between coil and contacts is possible when ther are spikes on the supply to either coil or contacts. The capacitance of long leads between EMF source and the coil can reduce the effect of the spike on the coil. .... from the data sheet of a very common relay ""• 8 mm Creepage and Clearance, 6 kV (1.2/50μs) between coil and contact"" .... 6 Kv is easy to create even with a small coil in the source

I don't really disagree with any of that, but I still don't really understand why you are seemingly singling out relays, nor exactly on what basis one should "consider each case separately". On the face of it, your concerns would apply to almost any electrical component.

I have to say that it would not have occurred to me to even consider providing the relay with the 'protection' you have recommended in this thread.

Kind Regards, John