Resistor hot in bathroom extract

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FrankieStaff

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Hey! Just a query really. Yesterday I noticed our extraction fan in the bathroom had yellowed on one edge. Touched it and it was hot, took the front panel off and the resistor was very hot and where it met the board it had blackened. Google led me to panic it could cause a fire, so we isolated it and today an electrician replaced the extraction fan.

Now, when iv gone up, this resistor is also hot. Is it normal to be hot? It doesn't seem as hot as the one in the previous extraction, I can hold my finger on it for a few seconds before it hurts. I read somewhere that resistors should be warm, not hot but not sure how true this is?
 
When a current flows through a resistor, the resistance causes power to be dissipated. This power is I*I*R and will cause the resistor to get warm. So, to answer your question, yes, it should get warm.

As to whether its "too warm", well a resistor has a power rating, 0.6W, 1W, 2W are common ones. Provided that the power dissipated is less than that rating all is well. Obviously I haven't seen your particular fan, but I hazard a guess that the resistor you are talking about is in series with a capacitor to form "dropper" for the front end of the fans power supply. In this configuration its common for this resistor to run rather hot.

Incidentally, if this resistor is doing the job I suspect, I would strongly advise against touching it with the power on - one of its legs may well be connected directly to the mains & it may bite you!
 
FrankieStaff said:
Hey! Just a query really. Yesterday I noticed our extraction fan in the bathroom had yellowed on one edge. Touched it and it was hot, took the front panel off and the resistor was very hot and where it met the board it had blackened. Google led me to panic it could cause a fire, so we isolated it and today an electrician replaced the extraction fan.

Now, when iv gone up, this resistor is also hot. Is it normal to be hot? It doesn't seem as hot as the one in the previous extraction, I can hold my finger on it for a few seconds before it hurts. I read somewhere that resistors should be warm, not hot but not sure how true this is?
Click to expand...

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?
 
It looks like it's a cheap fan to be honest, and it's not really extracting anything either. I'm quite disappointed, because I tend to stay away from super cheap electrical items on the basis it's cheap for a reason.

I have attached photos, it's the big grey thing I'm finding to be quite hot, like it feels warm through the plastic casing, is this normal would you think?
 

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A resistor that size: they are expecting it to get hot by design. Not great design in my opinion.
 
Pretty standard on these things, however as said above, not the best design but most of these fans are similar inside. The inccidents with these things going on fire tend to be more to do with stalled motors and the internal thermal protection in the windings has failed to operate, I've found them before where the motor was stalled, getting hot and had melted the cover fixings in place. The best thing you can do is to keep it free of dust and fluff and if the bearings start to fail (you'll hear it) switch it off before they go totally and seize up
 
This reminded me of the extractor fan in my dads shower room when I was a teenager. It was similar to your one and ended up having a browned resistor shaped mark on the casing, I guess they are much the same just a bad design
 
I remember being given an early fan timer years ago.

I worked by charging a large capacitor through a resistor, and turning off when the capacitor voltage was equal to incoming voltage.

A simple breadboard circuit.

Modern fans surely have some sort of digital timer?
 
Guitarguy said:
This reminded me of the extractor fan in my dads shower room when I was a teenager. It was similar to your one and ended up having a browned resistor shaped mark on the casing, I guess they are much the same just a bad design
Click to expand...

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.
 
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

cd4001 motor driver.jpg
 
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