bulb filament

gar

Senior Member
120414-1935 EDT

Most incandescent lamps have a tungsten filament. Radio vacuum tube heaters or filaments are also tungsten. These have a positive temperature coefficient of resistance. Thus, large initial current. In contrast the Edison lamp had a carbon filament and this has a negative temperature coefficient of resistance.

You can see an Edison lamp in photo P13 at my web site http://beta-a2.com/EE-photos.html
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kwired

Electron manager
Location
NE Nebraska
120414-1935 EDT

Most incandescent lamps have a tungsten filament. Radio vacuum tube heaters or filaments are also tungsten. These have a positive temperature coefficient of resistance. Thus, large initial current. In contrast the Edison lamp had a carbon filament and this has a negative temperature coefficient of resistance.

You can see an Edison lamp in photo P13 at my web site http://beta-a2.com/EE-photos.html
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You do realize certain lamp types do not have a filament at all - such as fluorescent lamps, and HID lamps.

The filament at the ends of certain fluorescent lamps is only for lamp starting and not for creating the light.
 

edward

Senior Member
Thank you both for the replies.
So if most incandescent bulbs including the Edison base have tungsten filament, then i suppose the switches that are installed to control these bulbs must have the "T" rating. But almost non of the switches are "T" rated unless you ask for that specific switch, and non of the dimmers (at least i don't know of any) are "T" rated. Further more i haven't had any inspector ask me if the installed switch is "T" rated.

i am confused. :?

i looked up the white book and it said if a switch is to control a tungsten load then the switch must be "T" rated.
 
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gar

Senior Member
120414-2331 EDT

edward:

A standard normal life 120 V 100 W tungsten filament lamp has a room temperature resistance of about 10 ohms (I just measured one). Its operating resistance at rated voltage should be close to 120[SUP]2[/SUP] / 100 = 14400 /100 = 144 ohms.

For said 100 W bulb the RMS current at 120 V is about 0.833 A once at steady state operating temperature. Peak turn on current for a room temperature bulb at a voltage peak is about 170 / 10 = 17 A. See my P1 waveform at http://beta-a2.com/EE-photos.html .

If you had 18 of these bulbs on one switch your peak current some of the time might approach 18 * 17 = 306 A.

In halogen bulbs the filament is run somewhat hotter so the hot to cold resistance ratio maybe somewhat more than 14.4 . I do not have one to measure.

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edward

Senior Member
120414-2331 EDT

edward:

A standard normal life 120 V 100 W tungsten filament lamp has a room temperature resistance of about 10 ohms (I just measured one). Its operating resistance at rated voltage should be close to 120[SUP]2[/SUP] / 100 = 14400 /100 = 144 ohms.

For said 100 W bulb the RMS current at 120 V is about 0.833 A once at steady state operating temperature. Peak turn on current for a room temperature bulb at a voltage peak is about 170 / 10 = 17 A. See my P1 waveform at http://beta-a2.com/EE-photos.html .

If you had 18 of these bulbs on one switch your peak current some of the time might approach 18 * 17 = 306 A.

In halogen bulbs the filament is run somewhat hotter so the hot to cold resistance ratio maybe somewhat more than 14.4 . I do not have one to measure.

.
Thank you for the explanation and i do agree with it. But doesn't the switch have to have the "T" rating on it?
 
Thank you both for the replies.
So if most incandescent bulbs including the Edison base have tungsten filament, then i suppose the switches that are installed to control these bulbs must have the "T" rating. But almost non of the switches are "T" rated unless you ask for that specific switch, and non of the dimmers (at least i don't know of any) are "T" rated. Further more i haven't had any inspector ask me if the installed switch is "T" rated.

i am confused. :?

i looked up the white book and it said if a switch is to control a tungsten load then the switch must be "T" rated.
T rating is required for DC. AC only you don't need it.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
T rating is required for DC. AC only you don't need it.
Thanks for making the correction. I did not read the UL Guide Information for Snap Switches (WJQR) far enough.
If the switch is an AC-DC general use switch, the "T" marking is required.
If the switch is an AC general use switch, the "T" marking is not required.
 

Electric-Light

Senior Member
Which bulbs have a tungsten filament? All bulbs or just halogens?

Thanks.
Just about every incandescent and fluorescent lamps.
Exceptions are cold cathode lamps and novelty carbon filament fluorescent lamps. The filaments in fluorescent lamps are for not just for pre-heating. They allow the cathode coating to operate at thermionic temperatures.

Even though they're not energized, high temperature cathodes are crucial for long life and performance. When they're dimmed to the point of not being able to sustain optimal temperature, they have to be heated by energizing them or the lamp will sputter and fail rapidly.

As for across the line type bulbs, they have a considerable inrush current. When a 500W work light is plugged in, there is a noticeable voltage drip that is enough to dim other lights on the same branch. As such, many plug-in consumer switch/control devices have three ratings, motor, resistive and tungsten. The tungsten wattage rating is lower than resistive load.
 
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