Light Bulb Resistance

[lightbulb]

Hello folks. I have a question about a common household light bulb. From Ohms Law, we know that V = I * R, and from this we can derive that Power = V^2 / R. Solving for resistance, R = V^2 / P. For a 60 W light bulb operating at 115 V AC, the resistance should be 115^2 / 60 = 220 ohms. No problem so far.

I measured the resistance of a cold light bulb by measuring across the + and ground points on the bulb. It measured about 220 ohms, so all is well...so I thought. I broke the light bulb open and measured the resistance across the filament, and it measured only about 5 ohms. I was curious why the resistance changed, so I searched the Internet for information about how light bulbs work. I came across articles that say the resistance of the filament changes with temperature. Fine, but that doesn't explain why the resistance changed after I broke the bulb open. It also doesn't explain why I measured 220 ohms on a cold light bulb if the resistance changes when it gets hot because 220 ohms is what the hot bulb should read. One article said the resistance of a cold bulb is about 18 ohms and about 200 ohms when it gets hot. Can anyone sort this out?? I'd sincerely appreciate it.

Thanks!

 

[jbartos]

Suggestion: Generally speaking, you approximately stated correct ohms values. The cold ohms should be much lower than the hot ohms. However, one would have to measure several bulbs to be able to draw some more accurate conclusions. There may have been something wrong with the bulb or with the measuring method.

 

[electricpete]

jbartos was right on the money on this one. That's one of the reasons light bulbs often blow immediately when you turn on the switch.... there is a short "inrush" current until the filament warms up.

 

[don01]

hi all
This rings a bell from somewhere in the dark recesses of my mind. The characteristics of a tungsten filament do change dramatically when it heats up. (thats why it glows as we approach melting piont) But my experience with small globes and low volts is that the ohms (cold)will give me a ball park figure for the load when in use. These are say 12v 5 w for toys and hobby applications.

One thing that has consistantly happened to me is that unless I am carefull with the meter probes& scratch the "solder" tarnish on the bottom contacts I do get a high reading.
This has shut down more than one of my power supplies during testing. I would agree with light bulb that if it measures x cold with the galss on then it should stay about that in air (cold). I think we are assuming a standard filament type globe not a fancy one

Any other ideas?
Regards
Don

 

[electricpete]

As already discussed there are 2 unexpected results from your measurements:

#1 - A common light bulb resistance should have measured much lower than would be computed from the watts (but was not).
#2 - The cold resistance should not change (my opinion... agree with Don) when you break the bulb leaving filament intact (while deenergized of course).

These two contradictions can be explained by one measurement error. I must assume that actual resistance was on the magnitude of 5 ohms all along and you incorrectly measured it at 220ohms the first time. Perhaps you could try again with another bulb?

 

[don01]

hi guys
after the last post I scurried out to the w/shop and guess what?
I was wrong
the lamps do seem to read v low ohms compared to the calculated value
damn & curses must learn to shut up one day !!!

regards Don

 

[movietechie]

I would have said go back and measure again. I have usually found that the operating resistance is about 10X the cold for a variety of small instrumentation bulbs.

I once built a unit which worked fine until I installed (just before shipping) the POWER ON lamp, which ran from 5VDC. On power up the lamp inrush pushed the supply into foldback and at the reduced voltage the lamp never got hot enough for its current to drop.

Fortunately we had some negative temp coefficient thermistors around of nearly suitable size. An NTC thermistor in series with the lamp limits the inrush and extends the life of the lamp emormously. Once the lamp is operating the thermistor heats up, its resistance drops and most of the voltage is applied to the lamp.

It is really hard to select the thermistor .. the ones I had were just a bit high in resistance so the lamp was a bit dim. To make it brighter, I paralleled 2 thermistors and to my suprise the lamp became DIMMER! Think about it!

 

[jbartos]

Suggestions to the previous posting:
1. Indeed, the high resistance of thermistor can cause the bulb to be dim. Perhaps, a thermistor with the smaller resistance at the bulb rated current would help. The paralleling of thermistors increases their individual thermistor resistances since they share the bulb rated current by a half. The thermistor resistance is supposed to decrease with its current flowing through it.
2. Reference 1:
Fink D. G., Carroll J. M., Standard Handbook for Electrical Engineers, 10th Edition, McGraw-Hill Book Company, 1968, Sections 19-11 through 19-25 haveSuggestions to the previous posting:
1. Indeed, the high resistance of thermistor can cause the bulb to be dim. Perhaps, a thermistor with the smaller resistance at the bulb rated current would help. The paralleling of thermistors increases their individual thermistor resistances since they share the bulb rated current by a half. The thermistor resistance is supposed to decrease with its current flowing through it.
2. Reference 1:
Fink D. G., Carroll J. M., Standard Handbook for Electrical Engineers, 10th Edition, McGraw-Hill Book Company, 1968, Sections 19-11 through 19-25 have related and very informative content pertaining to this posting, e.g. lamp equations, filament treatment, characteristic curves for large gas-filled lamp forms, filament evaporation, lamp life, mortality and depreciation, etc.
Subsequent editions of Reference 1 also have a good treatment of lamps.
related and very informative content pertaining to this posting, e.g. lamp equations, filament treatment, characteristic curves for large gas-filled lamp forms, filament evaporation, lamp life, mortality and depreciation, etc.
Subsequent editions of Reference 1 also have a good treatment of lamps.

 

[CentauriX]

If anyone is still reading this thread, maybe the problem is in the measurement of the resistance. The first measurement was made between the positive terminal and ground while the second measurement was made across the filament. Maybe the rest of the resistance was in the terminal itself.

Cent

 

[jbartos]

Yes, there is a possibility of having an additional resistance in series between the filament and socket contacts. The manufacturer may also protect the filament while it is cold with lower resistance by an in-series device.