NA shop lightbulb problem

Let'sgoflying!

Let'sgoflying!

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Dave Taylor
I bought a bunch of 200W incandescents from 1000bulbs or whatever they are and tested them all...they seemed dim so I ohmed them out and get 8.5Ω.

Is it not true that 200W, 120V should result in 1.7Ω? Line voltage is 121.

http://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htm

Seems like I'm missing something very basic but cannot figure it out. Lumens is not ∝ watts??
 
You measured DC resistance in an AC circuit. Is the element coiled at all? If so you will have some inductive resistance along with the purely resistive.
 
Did you measure the resistance when the bulb was hot? Resistance changes with temperature.

How good is your meter? Getting a reliable number down at just a handful of Ohms can be a challenge for Harbor Freight stuff.

"Long Life" bulbs are designed for a higher line voltage than nominal so on nominal line voltages they use less current, make less light, but last longer.
 
Let'sgoflying! said:
I bought a bunch of 200W incandescents from 1000bulbs or whatever they are and tested them all...they seemed dim so I ohmed them out and get 8.5Ω.

Is it not true that 200W, 120V should result in 1.7Ω?
Click to expand...

It is not true. A 200-watt incandescent bulb with a hot filament should be around 72 ohms. Current would be 1.67 amps. The cold bulb may have a filament resistance as low as 10-12% of the operating resistance, which is why they pull big surges when first lit. (Yours sounds about right.)

Lumens are a measurement of light output, not power consumption.

Some discount bulb sellers intermix 130-volt bulbs in lots of 120-volt. The 130s sometimes are noticeably yellow by comparison.

Inductance of a coiled bulb filament will be negligible at 60 Hz.
 
PW_Plack said:
It is not true. A 200-watt incandescent bulb with a hot filament should be around 72 ohms. Current would be 1.67 amps. The cold bulb may have a filament resistance as low as 10-12% of the operating resistance, which is why they pull big surges when first lit. (Yours sounds about right.)
Click to expand...

That right there. Resistance changes with temperature, usually going up. An incandescent filament is REALLY hot and its resistance will change enormously. You can't use cold resistance to work Ohm's Law here. Got to measure amperage and work it out that way.
 
aha, thanks.
Maybe I will tell them these 200W bulbs are way dimmer compared to my own 200W bulbs, see if they will trade.
 
Might they be 220V bulbs? or your ohm meter have too much spurious lead etc resistance?
 
Let'sgoflying! said:
aha, thanks.
Maybe I will tell them these 200W bulbs are way dimmer compared to my own 200W bulbs, see if they will trade.
Click to expand...

Paul was correct about both the cold and hot resistance and the bottom line is you can't tell much about an incandescent lamp by measuring the cold resistance. If you really want to know if these lamps are truly 200W @ 120 V, measure the current while operating on 120V (should be 200/120=1.67).

Incandescent lamps of the same wattage can have different spectrum and different illumination. Typically a lamp that produces more light at the same power has a shorter life although there are some tricks that have been used to circumvent this (e.g. halogen gas).

P.S. The formula for calculating resistance from power and voltage is resistance = voltage^2 / power. I think you forgot to square the voltage.
 
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