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This is a decidedly low-tech puzzle. But, here goes ...

I have installed a holder and an incandescent bulb at a point controlled by
an unused fan regulator (brand ROMA, the type that is pre-installed these
days in the switch board, (inductive?, electronic?). Have used the maximum
setting only (never used it for dimming the light).

The problem is, the bulbs keep getting fused after only a few weeks (even
only a couple of weeks, -- almost never lasts much longer than a month),
-- much less than the expected life of the bulbs. The failure mode is always
the same, -- the bulb fuses on turning on the switch, with a flash.

So, what is causing the problem, I'm just curious, -- any scientific reason
anyone can think of? What happens if I install fans on the same points, --
are they going to burn off also? Any electricians out there? This has
happened with two such regulator-controlled points, so it probably is not a
problem with a particular point. Thanks.

I don't know if it is the right forum for the topic. If not, mods please move it to the right one. Thanks.

May be there is a capacitor in the dimmer switch. So it might draw lot more current initially, there by burning the lamp. My elecronics knowledge is 20 years old, I could be wrong.

I am not fully certain on this, but tell me something. When you turn on the light how do you do it? Do you turn the regulator knob from 0 to 5 (low to full)? Or is it like you have a regular switch for on-off and the regulator which is connected to this circuit is kept at full all the time (this doubt arises because you say you never used the knob to dim the bulb)?

If you are using the knob to turn on the light (essentially using the knob as a dimmer switch) there can be a few things happening which you may want to check out,
* You are turning on the knob too fast and the cold filament which has a very low resistance for a few milliseconds fails to handle the huge onrush (there is a disconnect between the supplied load and what it can handle?) and burns out.

* Since the fan regulator is not a real dimmer switch it most probably also does not have a filter choke built into it. Now this will automatically cause an abrupt change in current. This in turn will mean sudden change in magnetism due to this sudden change in current flowing through the filaments. Now, I have a suspicion that this induction between the ultra-thin filament and those loops (what do you call them? filament holders?) through which the filament runs causes the filament to vibrate which will sooner than later cause the bulb to fuse. And of course it will happen when you turn the knob on a cold filament from low to high rather than when the knob is fully turned (essentially the full sine-wave is going through the filaments).

Again, it may not be the case here but definitely something to ponder about.

Thanks a lot guys, for your comments.

It's the latter, i.e. there is the "Fan" switch that I use as the light switch.
I never touch the regulator knob, which is always set at max (5).

I forgot to mention, but when the bulb gets fused (on turning on the
switch), the circuit breaker on that line also trips. i.e. there is apparently a
surge of current. I'm curious why this doesn't cause the bulb to fuse in the
first few weeks! Also, how come it is not supposed to harm a fan, --
because of the inductive coil in it ?

Samurai, I also was thinking something similar, but I was guessing the
regulator has an inductor in it. Of course, never tried opening up the
regulator in order to verify. :)

May be you shall try by putting a Fan :cool:

By the way why you are continuing with fusing the bulb always.
You simply can connect the input power wire going towards bulb to the output wire from switch ( which you are having to turn the dimmer thing On):D

I may be wrong, but here is my take on it.

the fan motor is an AC motor. in an A.C motor, the method of regulating the speed is by altering the input frequency. i guess this is what the fan regulator is supposed to do.

while on the other hand , in order to regulate the brightness of the bulb, what you need to alter is the input voltage and not its frequency.

so if you actually use a dimmer meant for the lights your bulbs many not fail so frequently.

A fan needs to overcome static inertia when switched on.

So I think that the initial power out will be very high. So that it can start rotating. Once the inertia is overcome the power being sent out reduces to the slow or high speed as per the setting.

This initial high power is blowing out your bulb.

If the fan regulator did not do that. Then the fan will remain stationery when switched on at low speed setting. This will heat up the coils inside and possibly cause a fire.

This is my understanding based on some googling.

small a.c motors running in a single phase circuit use a capacitor inoder to compensate for the inertia, for example in a fan this capacitor is found in the motor housing/ cover , it is not a part of the regulator.

I love team-bhp.

Totally forgot about that small cylindrical thing that costs about 15 odd bucks.
My electrician changed it once in front of me to get the fan working.

I did the same thing the next time a fan went bust. Worked.

Second time ended up blowing the fan wiring. :D
My mom had given up on me, now my wife has joined that club.

and in large motors there is dedicated starter mechanism which keeps the static coils in parallel at first and once it picks up the speed, changes them to series.

now, I totally don't remember what happens in a fan regulator. I think it's resistive control, but there can be a small controller choke/capacitor somewhere. It may have nothing to do with time, but chance when that surge happens.

Get a real light dimmer :p

Good question!

Firstly, it took me a while to realize that there is a problem.
Secondly, I'm using the bulb in a temporary configuration. So didn't want to
mess with the wiring.

Do the modern built-in regulators really alter the frequency?!
The old resistor-type (big) regulators don't (can't) control the speed this way.

if you are not using the regualtor switch, i would suggest by passing the regulator and connect the lamp directly to the switch.

whats the wattage of bulb your using?

The problem may be because of the Bulb wattage. Normally fan regulators made for controlling low wattage (75W). If you are using a 100W bulb it may burst please:

Try connecting a lower watt bulb 40w or 60 w and see.