Resistor hot in bathroom extract

[PcxP]

Same board in a Manrose MF100t fan. See the heat mark under the resistor also. The resistor is more robust than the capacitors though, thanks to the Clive YouTube clip, I could see the larger 470uf 16v slightly bulged. Have bought a Panasonic 470uf 25v to replace the 16v one to get a bit more life out of it before having to replace this capacitor again. I might even stick a strip of Kapton tape across the two capacitors to at least protect them from some of the resistor heat. Considering these in-line fans are generally fitted in loft spaces or other spaces out of sight, you don't really want any form of heat making a fire risk...

 

[JohnW2]

A very strange ( or very inventive ) circuit design. A CD4001 is four logic gates which can each switch up to about 6.8mA maximum at 15 V supply

I'm sure that I must have posted these before. The CD4001 only 'drives' the triac. The dropper resistor (per this thread) always gets very hot, and after a year or three in-service the PCB in its vicinity is invariably appreciable 'scorched. ...

Kind Regards, John

 

[JohnW2]

Modern fans surely have some sort of digital timer?

A few might, but the vast majority are the same cheapo analogue circuit that's been used for decades.

... which is perfectly adequate for the job, even if 'cheapo'.

Kind Regards, John

 

[FrodoOne]

The resistor concerned appears to be a 1 Watt device with a resistance of 22,000 ohms.
Hence, there should not be more than 148 V AC across it.

It is in series with a half/wave rectifier (diode), so it is dropping about half the 240 V - 15 V waveform - according to the circuit in Post #17.
(240 - 15)/2 = 225/2 V = 112.5 V)
112.5 * 112.5 / 22000 = (about) 0.58 W

 

[bernardgreen]

Half wave rectification of a RMS supply voltage produces a DC voltage in a smoothing capacitor that is close to the PEAK voltage of the supply

RMS = Root Mean Square

PEAK = RMS x Root 2 = RMS x 1.414

240 x 1.414 = 339.36

 

[JohnW2]

Half wave rectification of a RMS supply voltage produces a DC voltage in a smoothing capacitor that is close to the PEAK voltage of the supply
RMS = Root Mean Square .... PEAK = RMS x Root 2 = RMS x 1.414 .... 240 x 1.414 = 339.36

Whilst you maths of the relationship between RMS and peak of the 240V supply is obviously correct, I don't think that's true (or, at least, relevant to this circuit), is it?

What the smoothing capacitor is smoothing is the 15V across it, not the entire have-wave-rectified 240V supply.

Relative to the neutral, the voltage at one end of the resistor is a fairly constant 15 V DC, and the voltage at the other end is the half-wave-rectified 240V (i.e. varying between 0V and, as you say, about 340V, for half of the time). The p.d. across the resistor will therefore be that half-wave-rectified waveform minus a constant ~15V - i.e. a slightly 'truncated' half-wave-rectified waveform, varying between zero and about 325V for about half of the time.

Turning to what interests us (the power dissipated in the resistor) that is entirely dependent upon the RMS value of the potential difference across it (i.e. the RMS value of that 'truncated half-wave-rectified waveform').

Is that not all correct?

Kind Regards, John

 

[JohnW2]

Turning to what interests us (the power dissipated in the resistor) that is entirely dependent upon the RMS value of the potential difference across it (i.e. the RMS value of that 'truncated half-wave-rectified waveform').

I didn't have time earlier to actually offer 'an answer'.

If it were not for the complication of the 15V of 'truncation' of the waveform, the RMS of a half-wave rectified 240V RMS (~340V peak) supply would be about 170V [ peak/2 or 'RMS of full sine wave' / sqrt(2) ]. That being the RMS of the pd across the 22k resistor, that would mean that the power dissipated in the resistor would be about 1.32 W (170²/22,000)

The 15V of truncation does not make a lot of difference, not the least because, at least in theory, current travels 'the other way' through the resistor for a very small proportion of the cycle, when the supply voltage falls to less than 15V. Furthermore, since RMS voltage depends upon the square of instantons voltages, a very brief period at 15V has very little impact in the context of peak figures around 340V. (for the pedants .... yes, I'm ignoring the voltage drop in the rectifier diode!).

If I've got my sums right, with that '15V factor' taken into account, the RMS of the pd across the resistor falls to about 160.2V, hence about 1.17 W dissipated in the resistor.

Hence, if it is a 1W-rated resistor, it's not surprising that they get pretty hot (which they do).

Kind Regards, John

 

[opps]

They are by design, designed to turn watts into heat, so yes in operation they can get warm, but they should not get too hot or there life will be limited. Is this unit perhaps marked as suitable for 230v, rather than a range of voltages including the 240v you are on?

Are you sure it's not a wax cartridge system, where the cartridge heats up, expands and that expansion opens the shutters?

My Icon extractor has a wax piston that opens the shutter, I have never noticed any discolouration.

 

[JohnW2]

My Icon extractor has a wax piston that opens the shutter, I have never noticed any discolouration.

Fair enough, but am I missing the 'relevance' of that (or am I perhaps missing an implied 'smilie' ?) ?

Kind Regards, John

 

[andy_nufc]

Hi JohnW2,
I'm interested in your discussions of the role of dimmers applied to those circuits, and it is good that the manufacturers appear to have thought of this possibility in their technical documentation, given that the usual consumer instruction manuals don't mention them at all.

I had a bathroom fan fail in much the same way as those here, with overheating resistor causing capacitor failure and various components to become loose from the board. I got around this by bypassing the timer circuit altogether and using the supply attached directly to the fan, hence using it in manul mode using the isolator to switch it on/off.

But what the technical docs you have posted seem to state is that it is perfectly "safe" to use a conventional dimmer on the SL part of the circuit, albeit that at low light conditions it will take a little longer to charge the timing capacitor and get the thing moving (this I can attest is what used to happen when it was working properly over several years).

This being the case, I would consider installing a new model with timer again into the current arrangement. (I rather like my bathroom dimmer. Don't want to be blasted awake in the middle of the night.) What do you think?

Cheers,
Andy

 

[JohnW2]

The resistor concerned appears to be a 1 Watt device with a resistance of 22,000 ohms.
Hence, there should not be more than 148 V AC across it.
It is in series with a half/wave rectifier (diode), so it is dropping about half the 240 V - 15 V waveform - according to the circuit in Post #17.
(240 - 15)/2 = 225/2 V = 112.5 V)
112.5 * 112.5 / 22000 = (about) 0.58 W

As I have explained in my earlier posts, your approach/calculation appears to be flawed.

You have effectively assumed that the RMS voltage of a half-wave-rectified sine wave is a half of the RMS voltage of the full waveform before rectification, which is not correct. ....

As I've said, if one half-wave-rectifies a sine wave supply, the RMS of the rectified voltage will be the original RMS divided by √2, not (as you seem to have assumed) divided by 2 - i.e. (forgetting the 15V offset, for convenience) about 170V (rather than 120V) in the case of a 240V supply. ... and, of course, it's that RMS figure which determines how much power will be dissipated in the resistor.

Kind Regards, John

 

[JohnW2]

The resistor is likely being used as a voltage dropper resistor, a very inefficient design - perhaps even designed for 220v, rather than the UK's 240v.

Yes, 'inefficient', and also undoubtedly one of the causes of eventual failure of these modules (due to thermal effects). However, it's also by far the simplest (hence, presumably, also cheapest) approach, and seemingly the approach taken in the timer modules of even the most expensive fans that I have looked at.

As for 'wastage of energy', if my calculation is correct in estimating something approaching 1.2W being dissipated in the resistor, that amounts to 10 kWh per year (maybe around £4 per year these days) - not zero, but also not a very big deal in relation to the 'big picture'.

Kind Regards, John

 

[JohnW2]

Hi JohnW2, I'm interested in your discussions of the role of dimmers applied to those circuits, and it is good that the manufacturers appear to have thought of this possibility in their technical documentation, given that the usual consumer instruction manuals don't mention them at all.

Do you mean that you would be interested to hear views about feeding the S/L of a fan timer from a dimmer - since it's not something that anyone has yet mentioned in this thread?

If that is what you mean, then I don't think that would be a problem - see below.

I had a bathroom fan fail in much the same way as those here, with overheating resistor causing capacitor failure and various components to become loose from the board. I got around this by bypassing the timer circuit altogether and using the supply attached directly to the fan, hence using it in manul mode using the isolator to switch it on/off.

I've done the same. It is possible to get replacement timer modules, but they generally don't cost much less than a whole new timer fan, so probably not 'very cost effective. However, the modules, which are very simple, can quite often be repaired.

But what the technical docs you have posted ....

If you're referring to what I included in post #17, they are not 'technical documents' but, rather, a description of what I discovered when I performed autopsies on a number of timer fan modules a decade or so ago.

.... seem to state is that it is perfectly "safe" to use a conventional dimmer on the SL part of the circuit, albeit that at low light conditions it will take a little longer to charge the timing capacitor and get the thing moving (this I can attest is what used to happen when it was working properly over several years).

Sure. I can't say I've tried, but since it takes very little to trigger the timer module, I would expect it that it probably would work with any degree of 'dimmed lighting' that resulted in a significant amount of light!

This being the case, I would consider installing a new model with timer again into the current arrangement. (I rather like my bathroom dimmer. Don't want to be blasted awake in the middle of the night.) What do you think?

As above, it sounds to me as if it would probably be OK.

Kind Regards, John

 

[Harry Bloomfield]

Yes, 'inefficient', and also undoubtedly one of the causes of eventual failure of these modules (due to thermal effects). However, it's also by far the simplest (hence, presumably, also cheapest) approach, and seemingly the approach taken in the timer modules of even the most expensive fans that I have looked at.

From memory, my fan uses a capacitor dropper, and so generates much less waste heat. Unfortunately the heated wax capsule louver opener, mains powered, failed due to over-heating. Fairly easily replaced.

 

[andy_nufc]

Do you mean that you would be interested to hear views about feeding the S/L of a fan timer from a dimmer - since it's not something that anyone has yet mentioned in this thread?

If that is what you mean, then I don't think that would be a problem - see below.

I've done the same. It is possible to get replacement timer modules, but they generally don't cost much less than a whole new timer fan, so probably not 'very cost effective. However, the modules, which are very simple, can quite often be repaired.

If you're referring to what I included in post #17, they are not 'technical documents' but, rather, a description of what I discovered when I performed autopsies on a number of timer fan modules a decade or so ago.

Sure. I can't say I've tried, but since it takes very little to trigger the timer module, I would expect it that it probably would work with any degree of 'dimmed lighting' that resulted in a significant amount of light!

As above, it sounds to me as if it would probably be OK.

Kind Regards, John

Thanks John, this is very helpful.
And yes, I was referring to the diagnosed autopsies that you put together, sorry for the confusion.
The house had a dimmer fitted to the bathroom when I bought it, and the fan worked for several years (and I think the fan has been there since new, so now getting towards 25 years old), but the timer started misbehaving a couple of years ago.

Cheers,
Andy