Ohms law 120V halogen 50 watt bulb

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EEC

Senior Member
Location
Maryland
Testing 120V halogen 50 watt bulb with ohmmeter = 35.5 ohms
Using Ohms law: E squared/wattage= 14400/50=288 ohms
Why do I have this difference between metered ohms and calculated ohms?
 

EEC

Senior Member
Location
Maryland
To get true ohm rating?

To get true ohm rating?

Do I need to measure current while bulb is operational in order to use Ohms law P/I squared= ohms?
 

Volta

Senior Member
Location
Columbus, Ohio
That would work, it is difficult to measure the resistance directly at the proper temp. At a work bench you could get it close through a two-pole Form C relay: lamp terminals connected to the common, ohmmeter connected to the N.C., and 120v connected to the N.O. terminals. When the relay coil is energized the lamp will be brought up to temp, when the coil voltage is removed you can read the resistance on the cooling filament. That would be close, but not perfect.
 

rattus

Senior Member
Do I need to measure current while bulb is operational in order to use Ohms law P/I squared= ohms?

Forget the Ohmmeter. Just measure the lamp current and applied voltage, then divide V by I. Be aware though that this resistance changes drastically with temperature.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I think that the easiest way to measure operating resistance is to apply voltage, let the lamp stabilize, and measure the current flowing through the lamp.

An interesting experiment is to vary the voltage and generate the curve of what current flows with different applied voltages.

The rule of thumb for an incandescent lamp (and this is a very rough approximation) is that a given % change in applied voltage causes 1/2 that % change in current flowing through that lamp.

Jon
 

mivey

Senior Member
Forget the Ohmmeter. Just measure the lamp current and applied voltage, then divide V by I. Be aware though that this resistance changes drastically with temperature.
If t is temperature then the ratio must be z(t). :grin::grin::grin:
 

Cold Fusion

Senior Member
Location
way north
... Just measure the lamp current and applied voltage, then divide V by I. ...

If t is temperature then the ratio must be z(t). :grin::grin::grin:
Yes, that's true. However, but considering most fillaments are coiled and there would be a phase shift between voltage and current I'm certain there will be some arguing that the impedance goes to infinity during part of the cycle. :roll:

Unless we are dealing with 180 degree phase-opposed DC:D:D

cf
 

rattus

Senior Member
Yes, that's true. However, but considering most fillaments are coiled and there would be a phase shift between voltage and current I'm certain there will be some arguing that the impedance goes to infinity during part of the cycle. :roll:

cf
Well no, the ratio v/i will go to infinity if there is any phase shift at all, but Z is a constant and would not change.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Theory is just that, theory. If you measure the voltage and current at the fixture input, it will be much higher than 50 watts. Reason is you are assuming 100% efficiency, and neglecting the ballast consumes power in addition to the lamp.
 

dbuckley

Senior Member
Table of the voltage and current (measured), resistance and wattage (calculate) of a 240V 500W linear halogen lamp...

carparklamptable.jpg
 
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