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lightbulb's role in car amp


Matthew A.
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Inside my car audio amplifier I noticed a very tiny tungsten filament light bulb. I cannot for the life of me figure out what it's purpose is. The layout of the components on the PCB is extremely clean. Here's a link to the schematic I was able to download free after I visited one of the recommened german sites for finding schematics that I read on this site although a different forum. Anyhoot...here's my schematics showing the light bulb.

post-5670-14279141980166_thumb.jpg

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...further to ante's answer, the resistance of the bulb changes as the voltage changes. More voltage across the filament causes the bulb filament to heat up and the resistance across the filament goes down. Seems they wanted a variable resistance to ground in parallel with this capacitor. (As a bleed off, as ante has said)

I have used the same principal for automatic level control in audio circuits. By adding a bulb in the feedback loop of the op-amp, you can get the equivalent of an ALC (Automatic Level Control) circuit. In this method, a peak of voltage heats the filament and causes an immediate decrease in the resistance. Thus eliminating the volume transient by effecting the amplification resistor ratio.

Just wanted to add a little explanation about the usage of bulbs in this manner...

MP

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... More voltage across the filament causes the bulb filament to heat up and the resistance across the filament goes down.

... In this method, a peak of voltage heats the filament and causes an immediate decrease in the resistance.
MP


Hi MP, I'm sorry to correct you.
Actually, the resistance of a bulb's filiament increases when it heats up (like most metals). Its resistance when cold could be as low as 1/10th what it is when hot. That is why a small transistor would fail when driving a lightbulb that is within its current rating when hot, but draws 10 times that current when cold.

A light bulb is frequently used as a current-limiter when powering a project for the first time. With a suitable light bulb in series with the supply, if the project draws its normal low current, the bulb will have a low resistance and the project will work. If the project has a fault causing high current, the bulb will light, reducing the current to a safe level.
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You are correct. I explained this backwards....however, the point that I was making is the change in the resistance and not whether it goes up or down.
Yes, I did have to look up my old ALC design. Looks like I described my later design with LDRs.
Thanks for catching it, though.

MP

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Hi MP,
I'm glad that you realised your error in explaining the temperature affect on the resistance of a light bulb's filiament.

I just measured the resistance of some 100W, 120V bulbs. They should be 144 ohms, but measured only 9.8 ohms when cool. That is a 14.7 times difference!
If the bulb is turned-on at the moment when the 120V is at its 170V peak, it will draw 17.4A and dissipate 2,950W! What a huge surge.
No wonder they burn-out the moment you turn them on.

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Thank you MP and audioguru. From the detailed information you've each kindly given me, I don't feel quite as much in the dark as I was before.
Why use a light bulb instead of building a circuit that would do the same thing? Usually there's concern when using filament bulbs in areas subject to strong vibrations. Like a car enviroment.
Does a light bulb have certain non-cost related advantages over building a circuit of same purpose? Going by what you have told me and from what I can see looking at the schematics, the light bulb makes up part of the amps protection circuitry. Correct? Also, not that it's use has anything to do with it being used in a car amplifier, but what sort of resistance slope would a light bulb have? Linear taper or audio taper?

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Hi Matthew,

A circuit with the same characteristics as a cheap bulb would be complicated. Low voltage bulbs are not as sensitive to vibrations and shock as mains voltage bulbs (lamps). A workshop safety hand lamp with a 24Volt bulb can be quite violently handled before its filament burns off. The exact same wattage and size bulb for 230V will burn off with just a small knock if Murphy is near by. ;D

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intresting topic as for bulbs i used to sand little holes on the top of a torch globe and this exsposes the tungston element to the air ,if i light that bulb by conecting it to a 6 or nine volts battery the tungston ellement exsposed to the air will oxidise and burn out right away it glows orange then , now the short orange glow i got for a short time was all i needed to make home made 6-9 volts detonaters , by crushing up the matchhead stuff to a dust and pouring it into the globe without damagaeing the tungston ellement then sealing it. i did the same for a grain of wheat globe to put inside a pen,

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