Resistor hot in bathroom extract

[EFLImpudence]

Thank you. Why is that necessary?

 

[bernardgreen]

Why is it thought that the resistor is carrying current when the fan is off?

It is carrying current as long as the Live and Neutral are active irrespective of whether the fan is rotating or stationary

The components in the red box provide a permanent 15 Volt supply to the rest of the circuitry

 

[EFLImpudence]

But why is that necessary?

Does Clive say why?

 

[bernardgreen]

But why is that necessary?

Because if the logic IC was not powered then when the Switched Live from the light became Live the logic gate would not be able to react and turn on the fan,

When the light is turned OFF the logic has to be powered to allow it keep the fan running

 

[RandomGrinch]

But why is that necessary?

Good question

With this circuit in particular, there may be no guarantee of the state the NOR gates were in, when the power was turned on?

There would have to be an additional part of the circuit, to latch the fan to the permanent live for the timer duration.

Another circuit could be chosen, but this is probably the most economical for the manufacturers to produce.

EDIT, AHH not quick enough!

 

[EFLImpudence]

this is probably the most economical for the manufacturers to produce.

Cheap.

 

[Harry Bloomfield]

Cheap.

Yep, not much more expensive (just pennies more), would have been to use a capacitor dropper, to drop the voltage, instead of the resistor.

 

[ebee]

Is there such a building regulation?
Doesn't it merely state that there must be "adequate ventilation in the building".

If its because there isn't an openable window?
Actually some council specs do ask for an automatic fan which "must be capable of providing a 20 mins timed overun" . I do not think they usually check to see if it is actually set for 20 mins or more but I think by throwing the isolator you are giving the choice to make it not capable so defeating it by a normal user. Setting the isolator say 150mm below the ceiling discourages normal user slightly (If a timed transformer is involved for 12v SELV then a bit below that to allow airflow thru it to ventilate the transformer in order to avoid a dead airspace).

A normal person with a pool cue, long stick or small steps etc could reach it if they needed to but hopefully only for maintaince or if fan stuck/smoking/wet thru or somesuch.

 

[EFLImpudence]

If its because there isn't an openable window?

Nope, not even then.

Actually some council specs do ask for an automatic fan which "must be capable of providing a 20 mins timed overun" .

Ask?

I do not think they usually check to see if it is actually set for 20 mins or more

Of course they don't. It can be set to whatever the householder wants - whether good or bad.

but I think by throwing the isolator you are giving the choice to make it not capable so defeating it by a normal user.

Well, that should be against the law and punishable by hanging.

Setting the isolator say 150mm below the ceiling discourages normal user slightly (If a timed transformer is involved for 12v SELV then a bit below that to allow airflow thru it to ventilate the transformer in order to avoid a dead airspace).

Unless the householder doesn't want one.
As with the oven switch; there are no positional regulations for an item that is not required.

A normal person with a pool cue, long stick or small steps etc could reach it if they needed to but hopefully only for maintaince or if fan stuck/smoking/wet thru or somesuch.

For goodness' sake; it is their house. They can do what they want.

 

[ebee]

Nope, not even then.


Ask?


Of course they don't. It can be set to whatever the householder wants - whether good or bad.


Well, that should be against the law and punishable by hanging.


Unless the householder doesn't want one.
As with the oven switch; there are no positional regulations for an item that is not required.


For goodness' sake; it is their house. They can do what they want.

Oh no they can`t. An Englishman`s home is not his castle

 

[ebee]

ebee said:
Actually some council specs do ask for an automatic fan which "must be capable of providing a 20 mins timed overun"

Ok they insist

 

[JohnW2]

But why is that necessary?

As bernard has said, it's down to the powering of the 'electronics' which produce the run-on delay. If the supply for the electronics was derived from the S/L., rather than the permanent L, the fan would probably come on when the light was switched on, but the fan would then go off immediately the light was switched off, because the electronics sustaining the run-on would also have lost its supply.

To get around that would not be impossible, but would introduce more complexity, since a way would have to be devised to maintaining 'alternative' supply to the electronics after the light had been switched off - either with a battery (or very large capacitor) or by 'switching' to a supply derived from the perm L during that period.

Maybe I haven't thought this through well enough, but what I don't really understand is why they do not (as with cheap LED 'lamps') use a capacitive 'dropper', rather than a resistor, in which case the heat (and most of the 'energy wastage') would go away.

Kind Regards, John

 

[JohnW2]

Actually some council specs do ask for an automatic fan which "must be capable of providing a 20 mins timed overun" .

What is a "council spec" ?

Kind Regards, John

 

[EFLImpudence]

Oh no they can`t. An Englishman`s home is not his castle

I would agree that an Englishman's home is his castle only when it is a castle.

However, I should be interested to see the case where the Council took someone to court because they switched off their fan or even where they ordered someone to fit a fan.

 

[JohnW2]

However, I should be interested to see the case where the Council took someone to court because they switched off their fan or even where they ordered someone to fit a fan.

Indeed - or, if there were an 'openable window' taking someone to court for not opening it

Kind Regards, John