Just goofing around with my new meter...

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zappy

Senior Member
Location
CA.
I have a 60W bulb 120V. I ohm it, and it says like 20.0 ohms. But according to ohms law, it should be 240 ohms. So whats going on here? Thank you for your help.
 
J

janagyjr

Guest
I have a 60W bulb 120V. I ohm it, and it says like 20.0 ohms. But according to ohms law, it should be 240 ohms. So whats going on here? Thank you for your help.

Incandescent bulbs have two resistances. "Cold" resistance, which is the resistance of the filament when it isn't energized. We did an experiment in class last trimester because one of my fellow students had about light bulb resistance (we've gone through this once a trimester with the new guys, waiting for it to happen again). As the filament heats up it will change resistance (if I'm not mistaken, as a conductor heats up its resistance also goes up) to it's "hot" resistance.

Run an experiment. Now that you have the cold resistance measurement, put the bulb in a base, turn it on for about 10 minutes and then take it out (being careful not to burn oneself). You should find that the filament's hot resistance is different then it's cold resistance. The wattage @ volts is its running numbers.

As for the toy, what sort of meter? I have an Amprobe 38XR-A and am loving it. Need an ampclamp accessory or just get a clamp-meter (Greenlee has one for $70-80 that I could live with).
 

zappy

Senior Member
Location
CA.
Got the Milwaukee 2205-20 fork meter

Got the Milwaukee 2205-20 fork meter

It's really nice! It's has more features then the fluke, and if you buy one before sept. 30, you get a free radio. It was a no brainer.
 

zappy

Senior Member
Location
CA.
Incandescent bulbs have two resistances. "Cold" resistance, which is the resistance of the filament when it isn't energized. We did an experiment in class last trimester because one of my fellow students had about light bulb resistance (we've gone through this once a trimester with the new guys, waiting for it to happen again). As the filament heats up it will change resistance (if I'm not mistaken, as a conductor heats up its resistance also goes up) to it's "hot" resistance.

Run an experiment. Now that you have the cold resistance measurement, put the bulb in a base, turn it on for about 10 minutes and then take it out (being careful not to burn oneself). You should find that the filament's hot resistance is different then it's cold resistance. The wattage @ volts is its running numbers.

As for the toy, what sort of meter? I have an Amprobe 38XR-A and am loving it. Need an ampclamp accessory or just get a clamp-meter (Greenlee has one for $70-80 that I could live with).

I tried that, but maybe I didn't leave the bulb in long enough. Reading didn't really change.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
I tried that, but maybe I didn't leave the bulb in long enough. Reading didn't really change.
Actually, the filament cools off way too fast for you to measure it's 'hot resistance,'especially if you are taking the time to unscrew it from the holder.
 
J

janagyjr

Guest
Hrm, I know we measured the hot resistance...I might have the experiment steps wrong.

Try this:

You have cold resistance measured directly with DMM on ohms.

Get a cheater cord and make your meter part of the circuit and measure the current when it's hot (get a voltage measurement from the receptacle so you can adjust based on actual) and use ohms law to figure the hot current (voltage nominal 120, measured current...).
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Resistance vs Impedance

Resistance vs Impedance

I have a 60W bulb 120V. I ohm it, and it says like 20.0 ohms. But according to ohms law, it should be 240 ohms. So whats going on here? Thank you for your help.

Ohms law only works out mathematically when DC voltage is applied to a load. The meter is reading the DC resistance (not impedance)of the bulb.
When AC voltage is applied to the bulb the R in the equation becomes Z (impedance) and that is why the numbers don't come out.
Impedance (capacitive and inductive reactance) cannot be measured with a meter, only calculated.
Try this:
Apply DC voltage to the bulb and measure the DC current. Then use ohms law to find R. Plug R into the power formula and the numbers should come out (I think :D)

TT
 

dkidd

Senior Member
Location
here
Occupation
PE
Ohm's law is valid for the RMS value of alternating current. An incandescent lamp filament is essentially a pure resistance. As previously stated, the resistance is highly temperature dependent. If you could heat the filament to its working temperature, you could measure its resistance.
 

handy10

Senior Member
measure vs. calculate

measure vs. calculate

Ohm's law is valid for the RMS value of alternating current. An incandescent lamp filament is essentially a pure resistance. As previously stated, the resistance is highly temperature dependent. If you could heat the filament to its working temperature, you could measure its resistance.

I need to nit pick the terminology a little. Since no one has suggested a way to heat the filament to its working temperature except to fully energize the bulb, there is no way to "measure" the resistance. However, it is simple to measure the current and voltage when the bulb is energize. From those measurements, you can calculate the resistance. Is this a fair summary?
 
J

janagyjr

Guest
I need to nit pick the terminology a little. Since no one has suggested a way to heat the filament to its working temperature except to fully energize the bulb, there is no way to "measure" the resistance. However, it is simple to measure the current and voltage when the bulb is energize. From those measurements, you can calculate the resistance. Is this a fair summary?

That's essentially what I suggested.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Resistance vs Impedance

Resistance vs Impedance

Ohm's law is valid for the RMS value of alternating current. An incandescent lamp filament is essentially a pure resistance. As previously stated, the resistance is highly temperature dependent. If you could heat the filament to its working temperature, you could measure its resistance.

OK DKIDD,
You got me on a technicality. The Filament is for the most part resistive and your point is well taken.
BUT........... if he uses his DMM on ohms scale and measures a complex load and is expecting to work out the numbers with ohms law then what..............? I contend the numbers won't come out. :cool:

TT
 

dkidd

Senior Member
Location
here
Occupation
PE
As I stated, IF the filament could be heated to operating temperature, then an accurate measurement of resistance could be made. There is essentially no reactance. OP did not ask about measuring a load with a complex impedance.
 
J

janagyjr

Guest
See all my replies, please. I revised the experiment steps I originally gave.
 

ronaldrc

Senior Member
Location
Tennessee
Hello

I am retired and I guess by now everyone on here knows I have way too much time on my hands,
I have been known to change the air in my tires.


That said I do have a way to prove the fact not theory that a incandescent light bulbs resistance
goes up when you apply voltage to it.


If you check the resistance of a 100 watt 120 volt light bulbs it reads around 12 ohms. This is around
the resistance of a 4500 watt water heater Element.


Below is a link to a wire identifier I built it works with a resistance bridge which applies a 36 volts to the
conductor under test. On the #2 setting the resistance has to fall within a range between 10 and 20 ohms.
Which I also use to test water Heater Elements. If the Element is good the tester light goes out and stays out
which means the resistance is within the 10 to 20 ohm range not shorted and not open, either one of those and
the tester light will remain on.


On the #2 setting with the wire Identifier when you first put the leads on a 100 watt light bulb the tester light will extinguish for about 1/2 of 1 second.


In other words once the 36 volts from the tester heats the bulbs filament the resistance goes out of 10 to 20 ohms
range which takes about a 1/2 of a second and the testers light bulb comes back on.



The water heater tester will not work because the applied 3 volt voltage is to low to heat the light bulb filament so the
Tester light stays off of course this is just more proof the heat is what does it.

You can actually feel the bulb get warm with the 36 volts applied to it with the wire Identifier.


Now I expect everyone to build both of these testers just to see if this is true. :roll:


Oh I all most forgot here are the links.


Click here for wire Identifier

Click here for Water Heater Tester


Thanks:Ronald :)
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
As I stated, IF the filament could be heated to operating temperature, then an accurate measurement of resistance could be made. There is essentially no reactance. OP did not ask about measuring a load with a complex impedance.

If the temperature coefficient of the metal the filament is made from known, then a simple math conversion could be made to correct for the resistance at operating temperature.

Or, you could just trust the scientists and apply Ohm's Law to figure out the resistance at the operating temperature.

Unless the little coils in the filament present some reactance.....then....all bets are off...
 
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