# 120vac light bulb

Discussion in 'General Electronics Chat' started by superway, Feb 13, 2014.

1. ### superway Thread Starter Active Member

Oct 19, 2009
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0
Hello,

I have light bulb 120v 100W. If we calculate the resistance of this light bulb is about 150 Ohm ( I = 0.8Aac). When I used a fluke meter to measure the resistance at the bottom of light bulb is about 10 Ohm. So is the 10 Ohm not correct resistance for this light bulb compare to calculation?
Thanks

2. ### GopherT AAC Fanatic!

Nov 23, 2012
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You measured resistance at room temp. As the filament goes from room temp to white hot, the resistance changes significantly.

3. ### crutschow Expert

Mar 14, 2008
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That low resistance when cold is why an incandescent light bulb draws a large surge current (typically about 10 times the nominal operating current) when turned on.

4. ### superway Thread Starter Active Member

Oct 19, 2009
128
0
Yes, the resistance increases when the light bulb getting hot, but it is not 10 times as mentioned above. I left the light bulb on for 30 mins, then unplugged it and measured the resistance of light bulb by a fluke meter is read about 16 ohm when its hot compares 10 ohm when its cold.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Was it still glowing white hot when you eventually took the "hot" measurement?

6. ### superway Thread Starter Active Member

Oct 19, 2009
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The light was on for 30 mins, then disconnected it from the base, then measured while it is very hot. I can't take measurement when it is still glowing.

Thanks

7. ### takao21203 Distinguished Member

Apr 28, 2012
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463
When you measure the current, you can calculate the resistance with Ohms law.

Interestingly, you could record a current surge when the lamp is turned on. This makes lamps burn out actually- it will happen when they are turned on.

Reducing the current would prolong the life a lot- unfortunately, the lamp will be less efficient (not that this would really cut it in terms of efficiency).

So, here in Europe voltage was increased in small steps over some decades to burn out all the old equipment, and to shorten the life of new bulbs as well.

It was sold to customers with the explanation, advantage in cable materials allowed an increase in voltage while maintaining the current insulation thickness.

8. ### Metalmann Active Member

Dec 8, 2012
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"So, here in Europe voltage was increased in small steps over some decades to burn out all the old equipment, and to shorten the life of new bulbs as well. It was sold to customers with the explanation, advantage in cable materials allowed an increase in voltage while maintaining the current insulation thickness." Didn't hear anything about that....?

9. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Of course you can't if it's disconnected from the power - and therein lies your problem with trying to find the resistance when operating at working temperature. The Tungsten filament is operating around 3000 K when the lamp is operating at steady state. That will be well beyond what you consider as the hot state after you turned off the lamp and measured the resistance.

10. ### crutschow Expert

Mar 14, 2008
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Even if you measured the resistance right after the bulb was turned off, the filament likely still had cooled down to perhaps a few hundred degrees C, far below it's operating temperature of 3000°C. The only valid way to measure the resistance when operating is to measure the current through it and the voltage across it, and calculate the resistance using Ohm's law, as you did in your first post.

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11. ### The Electrician AAC Fanatic!

Oct 9, 2007
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So your mission, superway, should you accept it, is to devise a way to measure the resistance of the bulb when the filament is glowing white hot.

Jul 18, 2013
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I was involved in the UK Electrical industry many years ago and never heard that one either?

On a related note, early in my career I was involved in the conversion of an Historic building that originally had an old Dynamo (110v DC) system to power the building, when it was converted and hooked up to the 230v mains, the replacement of a couple of the older 110v light bulbs were overlooked.
When the 230 was switched on they ran fine and were still working when the project was done!
Max.

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13. ### BillB3857 Senior Member

Feb 28, 2009
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In my much younger years, residential power here in the US was 110V. Then it went to 115. Then it became 117. It is now 120. Supposedly, the utility can supply more wattage with the same wire size simply by raising the voltage.

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14. ### inwo Well-Known Member

Nov 7, 2013
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I've noticed 480 systems are always over 500 volts now.

15. ### #12 Expert

Nov 30, 2010
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Exactly. In 1970, 115 volts was proof of a good supply from the power company. I still have the same clothes dryer but it won't keep a heater element for more than a few weeks before it burns out at 125 volts. I replaced the heater with a Cal-rod because they can stand a lot more abuse than a nichrome coil. It works.

16. ### MrChips Moderator

Oct 2, 2009
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You have two simple solutions:

1) Put an AC current meter in series with the bulb. Measure both current and voltage.

2) Put a low value, high wattage resistance in series with the bulb. Measure voltage across the resistor and the bulb. Calculate current.

17. ### BobTPH Active Member

Jun 5, 2013
782
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Simple: Break the light bulb, heat the filament with an oxy acetylene torch, and measure the resistance.

Bob

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18. ### MrChips Moderator

Oct 2, 2009
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Another solution is to get your hands on an AC clamp meter.
That could be more difficult for me as I have never owned or used such a meter.

19. ### GopherT AAC Fanatic!

Nov 23, 2012
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You must get faster with your measurement, like superman. He is able to turn off his light switch and then get into bed before it gets dark.

You will have to measure the bulb resistance before it gets dark. Remember, the bulb emits white light because it is white hot. As soon as the filament cools, it no longer emits white light.

Last edited: Feb 14, 2014
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20. ### wayneh Expert

Sep 9, 2010
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The effect of temperature on the resistance of a tungsten filament is not exactly an unknown and there's no reason to horse around trying to measure it yourself. Just read the dang literature. It's been done.