What am I doing wrong

[Dennis Alwon]

Ohms law says R=V2 / P so 120 volt circuit means 14400/100 watts equals 144 Ohms. Great... When I measure the ohms with 2 different EXTECH EX623 multimeter to measure the resistance of a 100 watt bulb I get 12 ohms. I even tried another cheapo multimeter and still got around 12 ohms. What gives here

 

[ggunn]

Ohms law says R=V2 / P so 120 volt circuit means 14400/100 watts equals 144 Ohms. Great... When I measure the ohms with 2 different EXTECH EX623 multimeter to measure the resistance of a 100 watt bulb I get 12 ohms. I even tried another cheapo multimeter and still got around 12 ohms. What gives here

The resistance of a light bulb filament increases radically when it comes up to temperature.

 

[Dennis Alwon]

I know but that much????

 

[Dennis Alwon]

I asked this question or similar one last year or so but I don't remember the ohms changing that much

 

[texie]

I asked this question or similar one last year or so but I don't remember the ohms changing that much

I would add that that's why you see tungsten ratings on some devices. Due to the low cold resistance the current inrush is probably 10 times the operating load.

 

[zbang]

I know but that much????

Yep, that much. One site gives the resistivity (μΩ·cm) of 5.65 at 300k and 99.54 at 3200k (http://physics.usask.ca/~bzulkosk/L...l-radiation-lab_Tungsten-Temp-Resistivity.pdf)

 

[roger]

I seem to remember rattus or gar showing some numbers and they were more than you would think.

Roger

 

[LarryFine]

That's why bulbs almost always burn out at turn-on.

 

[hbiss]

I know but that much????

From 12 ohms to 144 ohms obviously.

-Hal

 

[mikeames]

I do this in class as a brain teaser. Then discuss it and demo it. Measure the current and voltage to find the actual on resistance. I have even put it on a scope with a current probe a few times to capture it.

 

[Dennis Alwon]

I do this in class as a brain teaser. Then discuss it and demo it. Measure the current and voltage to find the actual on resistance. I have even put it on a scope with a current probe a few times to capture it.

That's what brought this up. I was in a class put 5 100 watt bulbs in a series but when we measured the resistance I was like -- what is going on.. I don't remember it being that low...

Thanks guys that helps..

 

[Dennis Alwon]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

 

[480sparky]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

Yep. And your meter is telling you the resistance when it's room temperature. Resistance causes heat, doesn't it?

 

[mikeames]

So I guess ohms law is giving the resistance when the bulb is hot? That doesn't make sense to me

If you use the power rating of the bulb yes since it makes sense that's the bulb rating is based on its running condition, but if you measure the cold resistance you basically get the instantaneous inrush current. The only way to see the inrush drop to running current is capture it on a scope. Or be really fast with a really fast meter.

 

[480sparky]

Say Goodbye to the Incandescent Lightbulb (as we know it)

One approach to getting full life out of a CFL bulb would be to leave it on longer, reducing the heater- filament-stressing on-off cycles. In my experience, described above, you could leave it on five times as long as an incandescent lamp and not use any more energy.

www.fluke.com

 

[zbang]

So I guess ohms law is giving the resistance when the bulb is hot?

Sure, the 100w rating is the design load at design voltage (120v). If you have a variac, put the light on that and measure volts/amps from barely glowing to full-on then calc the wattage from those.

Going by a chart I just found, at 24v the current is only .35 amp or ~65 ohm whereas at 48v we get i=.5 and r=96 (and v=96, i=.74, r=130).

Tungsten Lamps at Various Voltages

 

[mikeames]

Say Goodbye to the Incandescent Lightbulb (as we know it)

One approach to getting full life out of a CFL bulb would be to leave it on longer, reducing the heater- filament-stressing on-off cycles. In my experience, described above, you could leave it on five times as long as an incandescent lamp and not use any more energy.

www.fluke.com

Years ago on Myth busters show. There was a debate as to if it makes sense to turn the lights off or leave them on based on the inrush current and the normal running current. What was the break even point. I think it came out to 15 seconds or something close. If you were going to turn the light off and then turn it on in 15 seconds or less you were better off leaving it on. If you were going to leave the room for 15 seconds or longer turn them off.

Here's the episode https://www.vanderbilt.edu/sustaina...s-test-the-effects-of-turning-off-the-lights/

 

[don_resqcapt19]

The fact that an incandescent lamp is close to a short circuit when you first turn it on is why switches for incandescent lamps must be "T" rated.
From the UL Guide Information for "Snap Switches" (WJQR)

Snap switches suitable for the control of tungsten-filament lamp loads are marked with the letter "T" and are suitable for tungsten-filament lamp load ratings of 125 V or less.
Snap switches marked with the letter "T" are also suitable for the control of electronic ballast, self-ballasted lamp, compact fluorescent lamp, and LED driver lamp loads up to the marked amp rating of the switch at its rated voltage.

 

[tom baker]

I had a control panel that used candelabra based lamps as pilot lights. They had a resistor in series with the lamp filament so the lamp was on all the time but at a very low output. When the pilot light was supposed to turn on the resistor was put in parallel. the lamps lasted a very long time. I wish I had kept a circuit diagram.

 

[gar]

200206-2249 EST

Dennis:

I have referenced my plots and data many times on this forum. See http://beta-a2.com/EE-photos.html .

Variation of resistance with temperature is something you should have learned about in high school physics.

When you put several bulbs in series across the line you reduced the voltage on each bulb. Thus, a change of the resistance of a bulb vs with one bulb across the line.

There is much more to this discussion, but I won't go in to that.

A resistor in parallel with a bulb won't reduce the bulb inrush. It has to be in series.

.