Resistor hot in bathroom extract

[PcxP]

Just to add, it is a bit of a catch 22 to vent the electrical compartment, because let's say the thing still did catch fire for whatever reason, the fact the casing had been modded from original spec, likely invalidate my insurance. The fact I had to order the capacitor in a batch of 5, might just live with the fact I can easily replace again if/when the next one bulges (or any of the other components which are relatively cheap to replace versus a whole new fan or pcb).

 

[JohnW2]

Someone has asked me, off list, to explain the reason for the above - so, in case anyone else doesn't know, but would like to ...
.... I hope that may possibly afford a little help to at least some people!

I have been asked, again 'off-list' for further explanation/clarification.

To 'properly' address the maths, and literally consider the instantaneous voltage of an AC waveform "at every point in time" requires some understanding of calculus, which not everyone will have. However, what I can do is illustrate what happens if one consdiers 'many points in time', and then further illustrate what happens as one increases the number of those 'many points in time', and therfeore approaches what calculus would do.

The first table below considers a sine-wave waveform with a peak voltage (both positive and negative) of 339.41 V. It shows the instantaneous voltage at 1 mS intervals during one complete cycle. It also shows that instantaneous voltage 'squared', and hence calculates the mean (average) of those squared voltages over the whole cycle and, finally, the square root of that mean is taken to result in the RMS figure. It then shows exactly the same for a half-wave-rectified version of the same sine wave.

As can be seen below, that results in figures for the RMS of 234.22 V for the full waveform and 165.62 V for the half-wave one - figures which are a little lower than the theoretical ones (240.0V and 169.7V respectively).

It would obviously be tedious to post similar (very lengthy) tables in which the instantaneous voltage was considered more frequently than once every millisecond, but the second table (below) shows the answers. As one reduces the interval between one's 'look at the voltage' the calculated RMS gets gradually closer to the theoretical figures, which it actually hits (to 3 decimal places) when one looks at intervals of 0.1 ms or less. This is the same as what calculus would do, effectively considering an infinite number of infinitely small time intervals.

Again, I hope that might be of some help to some people.

Kind Regards, John

 

[JohnW2]

Just to add, it is a bit of a catch 22 to vent the electrical compartment, because let's say the thing still did catch fire for whatever reason, the fact the casing had been modded from original spec, likely invalidate my insurance.

I suppose so, theoretically, but if it resulted in a significant fire, I imagine that there would probably be no surviving 'evidence' of what you had done to the device - so (unless you told them ), the insurers wouldn't know

Kind Regards, John

 

[PcxP]

So I opted against drilling vents in the casing but there is enough headroom in the casing to locate the replacement 470uf 25v capacitor well away from the hot resistor, even though it is a bit bigger than the original 16v one (also went for Panasonic capacitor over cheapo ones). I also bent the 47uf resistor further away. Images below. Whilst I was up in the loft I also inspected my other mf100t which is the newer version. Again images attached. In this case a 47uf 25v capacitor was actually touching the resistor, great manufacturing!? I therefore again bent it away and added kapton over the chock-block which showed browning on the white electricsl tape covering it. Seems the manufacturer worked out for themselves that 25v capacitors are a better idea than 16v, maybe too many warranty returns/claims with the older version....

 

[Harry Bloomfield]

A similar issue caused our WM and those of several other owners to fail. Capacitor, mounted between two heatsinks. I used a similar fix, plus using a higher temperature rated capacitor.

 

[PcxP]

Last question on this topic from me hopefully...

With the isolator switch on, light is switched on and fan powers on, then light switched off, once the timing capacitor has discharged and the fan comes to a halt (10 mins later in my case) is the resistor still going to be constantly bringing the voltage down and generating heat?

In layman's terms, is the best approach after the fan has run for "x" time, to switch off the isolator switch?

If the circuit instead does not draw current (resistor generate heat) after the fan has stopped, then that would save me getting back out of bed 10 mins later after the fan has stopped to switch the isolator switch off.

 

[JohnW2]

With the isolator switch on, light is switched on and fan powers on, then light switched off, once the timing capacitor has discharged and the fan comes to a halt (10 mins later in my case) is the resistor still going to be constantly bringing the voltage down and generating heat?

Yes, if the fan is still getting electricity, the resistor will carry virtually the same current constantly (hence remaining at the time high temperature), regardless of whether the light and fan are on or off. There will be virtually no change in current through the resistor when the fan is running.

In layman's terms, is the best approach after the fan has run for "x" time, to switch off the isolator switch?

Well, in terms of reducing the amount of time the resistor is hot, thereby hopefully increasing the lifespan of the timer module, it would be by far the best approach. However, I can't believe that hardly anyone would do that - for a start, it would make a joke of having a timer module at all (you might just as well have a cheaper, non-timer fan and return to the room after 10-15 min and turning it off) but, worse, the fan would then not come on when you next switched on the lights (unless you first operated the isolator) - so, again, you might as well not have the fan 'light-operated' but just switch it on (as well as off) manually.

Virtually everyone simply leaves such a fan powered continuously, and they usually last a few years before any thermal damage results in the timer stopping working. I would suggest you do the same

If the circuit instead does not draw current (resistor generate heat) after the fan has stopped, then that would save me getting back out of bed 10 mins later after the fan has stopped to switch the isolator switch off.

Quite - as above, that would be pretty ridiculous, and you'd also have to remember to switch the isolator back on next time you entered the bathroom. As above, in effect you would have effectively anihilated the concepts of both 'light-operated' and 'timed run-on'

Kindest Regards, John

 

[ebee]

Yes you must decide.
1/ Do I want/need a manual fan
2/ Do I want/need/ must have an automatic fan.

3/ You seem to be thinking using an automatic fan as a manual fan which seems pointless and possibly harder work for yourself than using a manual fan in the first place.

When I`ve installed bathroom fans with timers I always wired them with the lightswitch (thereby fan starts) within normal reach. The 3 pole fan isolator switch at high level (often near the ceiling) purely to isolate for servicing etc but not within normal reach but certainly not in the loft which many ordinary folk would not be aware it is there and even if they did it would be awkward if they needed to isolate it urgently.

I do see quite a few mounted at normal height though so people can turn it off for use during the night to silence the fan - the sensible solution is to get a quieter fan.

 

[Murdochcat]

I would venture to suggest that this is the cause of a huge amount of cheap fan failures ........... see it a lot.

Buy cheap, buy multiple times ........

 

[PcxP]

Yes you must decide.
1/ Do I want/need a manual fan
2/ Do I want/need/ must have an automatic fan.

3/ You seem to be thinking using an automatic fan as a manual fan which seems pointless and possibly harder work for yourself than using a manual fan in the first place.

When I`ve installed bathroom fans with timers I always wired them with the lightswitch (thereby fan starts) within normal reach. The 3 pole fan isolator switch at high level (often near the ceiling) purely to isolate for servicing etc but not within normal reach but certainly not in the loft which many ordinary folk would not be aware it is there and even if they did it would be awkward if they needed to isolate it urgently.

I do see quite a few mounted at normal height though so people can turn it off for use during the night to silence the fan - the sensible solution is to get a quieter fan.

Thanks Ebee,

Isolator is already in easy reach and en-suite to the bedroom as below, so switching it on/off is no trouble. My main concern given the scorched PCB and popped capacitor, is longevity of the timer circuit and any (however small) fire risk in the loft from a constantly hot resistor, that is in an unvented compartment on the side of the fan.

I guess the small benefit I will get from the timer module is that for the 10 mins the fan runs on the timer, the bathroom lights will be off and therefore not using/wasting electricity.

 

[EFLImpudence]

I do see quite a few mounted at normal height though so people can turn it off for use during the night to silence the fan - the sensible solution is to get a quieter fan.

I don't follow that logic.

If people want the fan off during the night then the sensible solution is to turn it off.

 

[EFLImpudence]

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

 

[ebee]

I don't follow that logic.

If people want the fan off during the night then the sensible solution is to turn it off.

Yes but the isolator is there for servicing really, not for normal usage, that`s the whole point of it being automatic in normal operation if it is to comply with any building requirements to clear the air sufficiently in usage. putting the isolator there in easy reach for normal usage defeats the object really?

 

[EFLImpudence]

Yes but the isolator is there for servicing really, not for normal usage,

Not really. It is there because MI's call for one thinking it is necessary and people come up with daft reasons like being able to do repairs with the light on.

that`s the whole point of it being automatic in normal operation if it is to comply with any building requirements to clear the air sufficiently in usage.

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

putting the isolator there in easy reach for normal usage defeats the object really?

Not if the householder does not want it running during the night.

 

[RandomGrinch]

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

Because it's constantly dropping voltage, providing a 15V supply, being connected to the permanent live.