Free Lamp Bulbs

[powerdesal]

richard51.

Get a life / chill out / get a sense of humour.

[richard51]

My life is fine thank you.

Just so that miss information is not left unanswered I think that the reason that filaments will tend to blow at switch on is because of a rapid rise in current ( resistance is low at room temp.) which causes a rapid temperature rise, hence thermal shock and breaking of filament - especially if it is "worn".

As for back EMF you may be thinking of the instantaneous switching on of electric motors. This causes a back EMF which will momentarily reduce current and dim lights in the house.

[Chancer]

[quote user="Jonzjob"]Just as a matter if interest, did you know thta an incandescing light bulb is most likely to fail the instant you switch it on. That is because when it is first switched on the current is highest due to the fact that the tiny coils that make up the filiment have not had the time to produce a back EMF caused by the coil windings. This is due to the changing magnetic field caused by the AC, alternating current.

The back EMF produces a current in opposition to the AC power source and restricts the primary current. But that is only the case when the primary power is AC and is caused by the continous rise and fall of the voltage and therefore current provided. Lenz's Law come into that.

If it is a DC,  direct current, then there is no back EMF produced because there is no change in the current and therefore no change in the magnetic field in the coils.

A bit rusty I know, but I was in my teens when I learned that lot [:-))] and bow to any corrections [blink]

[/quote]

 

Some observations, dont now enough to say if they are corrections, the loose loops of an incandescent filament will not act as a coil except for in a theoretical sense, if you have ever tried winding a coil or electro-magnet then you will now that the coils have to be very closely wound, insulated (usually shellac) and hundreds of them.

 

Your comment on DC voltage would infer that bulbs would blow far more frequently on DC but this just is not the case, my grandfather was off grid for décades with a TVO generator and WW2 submarine batteries, everything was 240vdc (he used a rotary convertor for his one ac Appliance, a vacuum cleaner) and he never had a single incandescent light bulb blow in 25 years, his explanation and now i am old enough to question and think for myself I thin is the correct one, is that AC current causes the filaments to oscillate at 50hz which causes fatigue failure probably in combination with high temperature erosion, a noisy transformer winding will eventually fret through the shellac insulation and short out the winding.

 

 

[Chancer]

[quote user="richard51"] As for back EMF you may be thinking of the instantaneous switching on of electric motors. This causes a back EMF which will momentarily reduce current and dim lights in the house.[/quote]

 

Responding to possible misinformation.

I never really understood the AC theory I learned at college but I do recall something about inductance and/or magnetic flux meaning an AC motor is effectively a short circuit at start up hence the need for motor start capacitors and the dimming of lights, they dim because the current drawn by the motor is initially much more than the supply can give so the voltage drops momentarily, long enough to dimlights but not long enough to blow fuses or trip breakers, if the motor start capacitor fails then the fuses blow and the breaers trip.

 

Back emf is a voltage spike caused by the rapid collapse of an inductive field winding by switching off the power, it is why a condensor has to be fitted across the CB points on a vehicle ignition system and why relay coils have a diode in parallel, I do not know if back emf happens with alternating current but I think I have heard of it when switching AC motors, as I said the AC theory went over my head but I am fine with dc.

[richard51]

This may help

http://physics.bu.edu/~duffy/sc545_notes04/back_emf.html

Agree that it is the initial surge in current that actually causes the dimming.