Lighting circuit Power

[BJ Conner]

No prize award on nit-picking.

No prize award on nit-picking.

the answer of 172 VA is also incorrect

The current transient for energizing an incandescent lamp is an inverse starting usually 10 or 12 times the steady state value and reaching that value in 10- 20 ms. Bulbs usually fail in that time because the current is high.
Somewhere in inrush current transient the power could have had an instantious value of 1,000 va. There is no way to tell. Conditions before the lights went out are irrelavant and the prize award stands.
The purpose of the problem was when calculating something look over the results and apply the sanity test.

 

[mull982]

100802-1852 EST

charlie b:

It won't be 171 W. It will be less. Maybe 150 W. As I previously said I do not want to damage my one 75 W bulb. So to provide a closer estimate I connected a 100 W 120 bulb in series with a 60 W 130 V bulb. With 246 V input the 60 W had 197 V and the current was 0.57 A. Power to the 60 W bulb was 112 W, total power was 140 W.

.

Are you saying it will be less than the 171W because the resistance of the bulbs will change when not operated at rated values?

 

[mull982]

Turn on two of the switches, wait 5 minutes, and turn one back off.

Now, you'll have three states of bulb: on, off and cool, and off but warm.


Next?

So I understand, are you suggesting to operate these bulbs as you suggested and then take resistance measurments to determine which switch controls which bulb?

 

[gar]

100803-0749 EST

mull982:

Yes.

.

 

[LarryFine]

So I understand, are you suggesting to operate these bulbs as you suggested and then take resistance measurments to determine which switch controls which bulb?

No, I am suggesting just feeling the two unlit bulbs.

Let's say the switches are named 1, 2, and 3. Flip on switches 1 and 2 for 5 minutes, then turn 2 back off. Now, go look at the bulbs.

The bulb on switch 1 will be on, the bulb on switch 2 will be off but warm, and the bulb on switch 3 will be off and cool.

 

[Dennis Alwon]

Go to floor 1-- If the bulb is "on" then you know that is switch #1
If the bulb is "off" then touch the bulb--- if it is hot then it is switch #2 if it is cool then it's switch #3.

Do the same on floor #2.

 

[LarryFine]

Didn't I say that? :grin:

 

[iwire]

No, I am suggesting just feeling the two unlit bulbs.

Let's say the switches are named 1, 2, and 3. Flip on switches 1 and 2 for 5 minutes, then turn 2 back off. Now, go look at the bulbs.

The bulb on switch 1 will be on, the bulb on switch 2 will be off but warm, and the bulb on switch 3 will be off and cool.

Go to floor 1-- If the bulb is "on" then you know that is switch #1
If the bulb is "off" then touch the bulb--- if it is hot then it is switch #2 if it is cool then it's switch #3.

Do the same on floor #2.

You guys are all wrong, you need to lick the lamp and see how fresh the electricity tastes.

 

[Dennis Alwon]

Didn't I say that? :grin:

Yes you did but I thought I would try it another way to help explain it. You're still #1 Larry, don't worry but Bob's is inching closer with the tongue trick. :grin:

 

[LarryFine]

. . . Bob's is inching closer with the tongue trick. :grin:

That's just wrong.

 

[iwire]

That's just wrong.

You bet it is.:grin:

 

[gar]

100803-1305 EST

mull982:

I have now setup a 75 W bulb in series with a 100 W bulb with the series string connected to exactly 240 V. The 75 W is a GE 1085 lumens, 75 W, 1500 hour, 120 V bulb. Both bulbs were closer than +/- 1 W of their rating at 120 V. The voltage on the 75 W is 152 V, the current is 0.688 A, and total power to the two lamps is 165 W. Somewhat higher than I had guessed at.

This string in on a life test until I get tired of waiting for it to burn out. So far it has run for 40 minutes. Certainly time enough to find the eggs in the hen house.

.

 

[Dennis Alwon]

My dad , in a pinch, would take two pigtails and install 100 watt bulbs, in series, to test a 240 v circuit. He used to say that 2- 60 watts would burn out but the 100 watt bulbs would do fine.

Gar -- any truth to this????

 

[LarryFine]

Gar only?

 

[mull982]

100803-1305 EST

mull982:

I have now setup a 75 W bulb in series with a 100 W bulb with the series string connected to exactly 240 V. The 75 W is a GE 1085 lumens, 75 W, 1500 hour, 120 V bulb. Both bulbs were closer than +/- 1 W of their rating at 120 V. The voltage on the 75 W is 152 V, the current is 0.688 A, and total power to the two lamps is 165 W. Somewhat higher than I had guessed at.

This string in on a life test until I get tired of waiting for it to burn out. So far it has run for 40 minutes. Certainly time enough to find the eggs in the hen house.

.

So esentially the increased voltage and power on the 75W bulb will cause it to burn brighter and thus increase its resistance and thus the difference between the actual and theoretical calculations?

 

[Dennis Alwon]

Gar only?

I figured he would just change the bulbs in his experiment and find out right away. What's your guess or are you just feeling bad I didn't ask you.

 

[LarryFine]

What's your guess or are you just feeling bad I didn't ask you.

I was feeling so rejected, but I'm all happy again. :grin:

My 'guess' is that, electrically, he was wrong, but physically, the 100w filaments are hardier and may survive handling better.

 

[gar]

100803-1851 EST

Dennis:

I think Larry's explanation is probably the reason.


My 75 W at 152 V is still operating. What is it, about 6 hrs later. Could collect a lot of eggs in that time.

.

 

[Dennis Alwon]

Okay I am lost but I have never been good at this stuff.

If you have a 75 watt and a 100 watt bulb in series then to mathematically solve for voltage at the bulbs I would do this.....

R= V sq. / Watts
R= 120 sq. / 100 = 14400/100 = 144 ohms

R= 120 sq / 75 = 14400/75 = 192 ohms

In a series connection we would find the Total resistance by adding

R = 144 + 192 = 336 ohms

Total amps = 240/336 = .7

V= I R
240 = .7 x 144 = 100.8 volts at the 100 watt bulb
240 = .7 x 192 = 134 volts at the 75 watt bulb

Gar is getting 152 volts on the 75 watt bulb so what am I doing wrong?

BTW, I think my dad used the 100 watt bulbs because they burned brighter than the sixty so it was easier to tell if the voltage was close to 240 or not.

 

[gar]

100803-2314 EST

Dennis:

I had answered your question, but lost it. I will redo it tommrow.

.