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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Do filament based light bulbs have any inductance and could they produce back emf if the bulb was pulsed DC via a PWM chip and MOSFET? Does it need a flyback diode or am I way off course?
Registered Member #2727
Joined: Tue Mar 09 2010, 02:39PM
Location: Montevideo - Uruguay
Posts: 33
Filament bulbs like any wire have some inductance, but the ratio X/R is very poor, so you don't need free wheel diode if your freq isn't too high and the wires to the bulb aren't too long. You can put a RC snubber in place of the diode, making slower the rise and fall time with good results. See you
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
A standard 120 V, 60 W light bulb has less than 1 uH of inductance.
Note that a cold light bulb filament can draw more than ten times the current of a hot filament. Make sure that your MOSFET is capable of handling the high inrush current that the cold filament requires, else you could damage the MOSFET.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Some types of metal filament light bulbs exhibited an effect that can be mistaken for reactance which mess up some instrumentation. It related to dR/dt thermal.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Thanks for the replys, sorry about getting back late as my internet has been terrible lately (Sky broadband and back-haul problems in my area).
Just one more question, are there any downsides to using a 12v bulb only rated for AC like this on 12v DC?
I mean if they are designed for AC then there will be a period of time where it "sees" 0v across it, which I am thinking might help reduce heating or life-cycle of the bulb. Not sure though.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The 12V AC is an RMS value, so the average voltage is 12V.
You could (I think) pulse more than 12V DC through it (subject to other points made above regarding resistance and temperature) as long as the total power doesn't exceed the rated power of the bulb (I think).
Maybe someone more knowledgable than I could clarify this?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That's right, RMS current (or voltage) is what you use to calculate average power dissipation in a resistive load. It's the "I" in I^2*R and the "V" in "V^2/R".
That is why it has to be RMS, because you're going to square it later.
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light bulb back emf?
