Author Topic: How would you measure the ohms of a light bulb?  (Read 4903 times)

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Offline Beamin

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How would you measure the ohms of a light bulb?
« on: October 29, 2017, 10:45:05 am »
To see it go from zero while is cold to high impedance when bright? Use a meter that can put out high voltage like VTVM? How would you do that with modern equipment?

And the question is MEASURE not calculate.
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Offline SeanB

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Re: How would you measure the ohms of a light bulb?
« Reply #1 on: October 29, 2017, 10:52:05 am »
Measure the current while you apply varying voltage, and then use a simple application of ohms law to get the resistance from the known voltage and current. Multimeter just does the calculations for you by using a known current and displaying the resistance after doing the math internally.
 
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Offline Rerouter

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Re: How would you measure the ohms of a light bulb?
« Reply #2 on: October 29, 2017, 10:53:30 am »
The ohms increase by the PTC of the filament,

A light bulb that expects to run of 240V AC can also be fed off 240V DC to get the same equivilent power, e.g. heat.

So use a variac or step down transformer to get about 170V AC, which when rectified gives 240V DC, then simply measure the current at the switch on vs the stead state to know your cold / hot resistance.

Of if the AC current on your multimeter is good enough resolution, simply measure in series.
 

Offline Beamin

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Re: How would you measure the ohms of a light bulb?
« Reply #3 on: October 29, 2017, 11:01:33 am »
Measure the current while you apply varying voltage, and then use a simple application of ohms law to get the resistance from the known voltage and current. Multimeter just does the calculations for you by using a known current and displaying the resistance after doing the math internally.

For this challenge you are the average American and you don't know any math; ANY.  "math is hard/sucks" and your iPhone doesn't have a calculator app, not that you would know how to use it anyways. So you have to take measurements.
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Offline BNElecEng

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Re: How would you measure the ohms of a light bulb?
« Reply #4 on: October 29, 2017, 11:12:24 am »
Since you don't mention what equipment you have available to you, an oscilloscope with a voltage and current probe would suffice. Measure the voltage at the terminals of the
bulb and the current in the wires.  Use the built-in math function to calculate a division, resulting in ohms. The time aspect can be observed on the oscilloscope screen.
 

Offline Rerouter

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Re: How would you measure the ohms of a light bulb?
« Reply #5 on: October 29, 2017, 11:23:01 am »
Ok, so how to directly measure resistance, measure it cold, stick it in a socket, heat it up, and then use a relay to switch both contacts of the light bulb to a meter measuring resistance, and disconnecting the mains, it will begin dropping quickly, but it will measure for the first fraction of a second.
 
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Offline capt bullshot

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Re: How would you measure the ohms of a light bulb?
« Reply #6 on: October 29, 2017, 11:57:30 am »
Buy an expensive SMU that can deliver enough voltage and power for your light bulb.
It should provide you with an built-in Ohms measurement function that measures the resistance at any voltage you apply.

Check the manual before buying, I don't know if an average American is able to read a manual ...
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Offline CatalinaWOW

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Re: How would you measure the ohms of a light bulb?
« Reply #7 on: October 29, 2017, 01:29:09 pm »
Aside from all of the snide remarks about Americans, what does the OP mean in differentiating measurement from calculation.  I can't think of a single "measurement" of electrical quantities that doesn't have some implicit calculation.  Old school d'Arsenval meters are based on some rather sophisticated relationships between current, voltage, magnetic field strength and Hooke's law.  The newer digital voltmeters are based on integrating current over time before the final mathematical step of a ratio.

I am guessing that one of two things is the objective.  A plot of resistance over time as the bulb heats up or a measure of the resistance at a particular operating condition in a circuit (possibly a small range of values as occurs in the classic Wien bridge oscillator).  The test set ups to get the best answer for these conditions is slightly different, so maybe it is time for feedback on the objective.
 

Offline Kalvin

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Re: How would you measure the ohms of a light bulb?
« Reply #8 on: October 29, 2017, 02:30:32 pm »
Heat the filament to the same temperature as the filament would have when the bulb was lit normally, and measure the resistance.
 
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Online Brumby

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Re: How would you measure the ohms of a light bulb?
« Reply #9 on: October 29, 2017, 02:47:06 pm »
Heat the filament to the same temperature as the filament would have when the bulb was lit normally, and measure the resistance.

That is the best answer for providing a direct resistance measurement.

Of course, there are two other questions ....
 1. Knowing what the operating temperature is
and
 2. How to heat the filament to that temperature

I take it these are left to the experimenter....
 

Offline Kalvin

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Re: How would you measure the ohms of a light bulb?
« Reply #10 on: October 29, 2017, 02:58:10 pm »
Another method is to use an analog panel meter with two needles, use the other needle for voltage measurement and the other needle for the current measurement, print a new background for the panel so that you can read the resistance from the needles' crossing, and now you have a measurement instrument for measuring the bulb resistance without calculations.

https://i.ebayimg.com/thumbs/images/g/vWcAAOxyVaBSpVKv/s-l225.jpg
 

Offline Kalvin

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Re: How would you measure the ohms of a light bulb?
« Reply #11 on: October 29, 2017, 03:03:07 pm »
Use a DC lab power supply and set the current limit to 1.000A and measure the voltage across the bulb when it is lit. Now the resistance is equal to the voltage across the bulb. No need to calculate anything.

Edit: Won't work.
« Last Edit: October 29, 2017, 04:17:39 pm by Kalvin »
 

Online Brumby

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Re: How would you measure the ohms of a light bulb?
« Reply #12 on: October 29, 2017, 03:08:39 pm »
Use a DC lab power supply and set the current limit to 1.000A and measure the voltage across the bulb when it is lit. Now the resistance is equal to the voltage across the bulb. No need to calculate anything.

Who's to say that 1A is the correct operating current?
 

Online Brumby

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Re: How would you measure the ohms of a light bulb?
« Reply #13 on: October 29, 2017, 03:10:48 pm »
Another method is to use an analog panel meter with two needles, use the other needle for voltage measurement and the other needle for the current measurement, print a new background for the panel so that you can read the resistance from the needles' crossing, and now you have a measurement instrument for measuring the bulb resistance without calculations.

https://i.ebayimg.com/thumbs/images/g/vWcAAOxyVaBSpVKv/s-l225.jpg

A thought.  Somebody just needs to design the appropriate scale.
 

Offline Kalvin

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Re: How would you measure the ohms of a light bulb?
« Reply #14 on: October 29, 2017, 03:19:53 pm »
Use a DC lab power supply and set the current limit to 1.000A and measure the voltage across the bulb when it is lit. Now the resistance is equal to the voltage across the bulb. No need to calculate anything.

Who's to say that 1A is the correct operating current?

P=U*I, thus varying the voltage one can determine the power flowing through the bulb. Of course, if the bulb won't work with 1A current, then one has to calculate something and that situation is beyond the scope of the requirements ;)

Edit: As a second though, this method may not work as Brumby noted. I will overstrike my previous message, too.
« Last Edit: October 29, 2017, 04:16:59 pm by Kalvin »
 

Offline Kalvin

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Re: How would you measure the ohms of a light bulb?
« Reply #15 on: October 29, 2017, 03:23:13 pm »
Another method is to use an analog panel meter with two needles, use the other needle for voltage measurement and the other needle for the current measurement, print a new background for the panel so that you can read the resistance from the needles' crossing, and now you have a measurement instrument for measuring the bulb resistance without calculations.

https://i.ebayimg.com/thumbs/images/g/vWcAAOxyVaBSpVKv/s-l225.jpg

A thought.  Somebody just needs to design the appropriate scale.

Sure, but that wasn't excluded from the specifications, only requirement was to be able to measure bulb's resistance directly without any further calculations. So, once a person has an instrument one can measure the resistance directly without any further calculations. ;)
 

Offline rstofer

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Re: How would you measure the ohms of a light bulb?
« Reply #16 on: October 29, 2017, 05:10:55 pm »
Aside from all of the snide remarks about Americans, what does the OP mean in differentiating measurement from calculation.  I can't think of a single "measurement" of electrical quantities that doesn't have some implicit calculation.  Old school d'Arsenval meters are based on some rather sophisticated relationships between current, voltage, magnetic field strength and Hooke's law.  The newer digital voltmeters are based on integrating current over time before the final mathematical step of a ratio.

I am guessing that one of two things is the objective.  A plot of resistance over time as the bulb heats up or a measure of the resistance at a particular operating condition in a circuit (possibly a small range of values as occurs in the classic Wien bridge oscillator).  The test set ups to get the best answer for these conditions is slightly different, so maybe it is time for feedback on the objective.

If rescaling a meter is legitimate (and I don't know how you do that without math), just scale the ammeter in Ohms assuming a constant voltage.  It's as legitimate as the rest of this thread.

You can't do electronics without math.  You simply can't avoid Ohm's Law at any level - hobby or professional.  The EEs around here have taken a boat load of math or subjects that don't have math in the title but are really math in drag.  Field Theory comes to mind as does Control Systems.  You can't avoid curl and divergence nor can you avoid Laplace Transforms.  It just comes with the paper.

Heck, you can't even calculate the ballast resistor for an LED without math.  Sure, you can copy somebody else's work but what if your LED has vastly different characteristics?  How in the world could you bias a transistor?  Calculate the gain of an op amp?  I suggest that the premise of this thread is wrong:  Everybody doing electronics can do some math.  Otherwise, they do something else.

Not a realistic proposition, doing electronics without math!
 
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Offline khs

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Re: How would you measure the ohms of a light bulb?
« Reply #17 on: October 29, 2017, 05:19:46 pm »
My suggestion:

1) Remove the glass of the bulb and measure the resistance of the filament.
2) Then take a flame and heat up the filament.
3) Measure the resistance of the filament.

The filament may not survive long, but maybe long enough to make the measurement.
 

Offline kalel

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Re: How would you measure the ohms of a light bulb?
« Reply #18 on: October 29, 2017, 05:30:54 pm »
Since those power meters you plug into a wall socket and the load into the power meter can measure voltage and current, could there be a model that also shows the resistance of the load?

There's something for USB:


Quote
USB Charger Mobile Power Current Voltage tester meter capacity 3-30V QC 2.0 + load resistance 2A/1A discharge With a switch

The above is DC 2A 0-5A 3-30V so it would need to be some low power light bulb, but the same option might be available on an AC power meter.
« Last Edit: October 29, 2017, 05:45:05 pm by kalel »
 

Offline IanB

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Re: How would you measure the ohms of a light bulb?
« Reply #19 on: October 29, 2017, 05:32:25 pm »
This is an interesting experiment that anyone who is curious should perform.

I attach the results that I obtained when I did the experiment myself some time ago. One particular point to note is that there are two "resistances" for the light bulb filament at any point on the graph. One "resistance" is the value of V/I at that point. The other "resistance" is the slope of the graph, the value of dV/dI. Since the slope of the graph is the value that remains nearly constant around any local region of operation, this is IMHO the value that is most useful where something called "resistance" is needed.


« Last Edit: October 29, 2017, 05:35:25 pm by IanB »
I'm not an EE--what am I doing here?
 

Offline Mjolinor

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Re: How would you measure the ohms of a light bulb?
« Reply #20 on: October 29, 2017, 05:36:44 pm »

Resistance is a derived unit. It is not possible to have a resistance value without calculation, it can only be "derived" using volts and amps.
 

Offline kalel

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Re: How would you measure the ohms of a light bulb?
« Reply #21 on: October 29, 2017, 05:39:01 pm »

Resistance is a derived unit. It is not possible to have a resistance value without calculation, it can only be "derived" using volts and amps.

Definitely, but unless I misunderstood, the discussed task is to have it calculated by an instrument, rather than a calculator, so that the user doesn't need to know the formula.
 

Offline Mjolinor

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Re: How would you measure the ohms of a light bulb?
« Reply #22 on: October 29, 2017, 05:45:01 pm »

Sense resistor to an A/D and a micro implementing ohms law, job done though why bother is for sure a lot more difficult to answer. :)
 

Offline IanB

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Re: How would you measure the ohms of a light bulb?
« Reply #23 on: October 29, 2017, 05:45:38 pm »
Definitely, but unless I misunderstood, the discussed task is to have it calculated by an instrument, rather than a calculator, so that the user doesn't need to know the formula.

But if you don't know the formula how is the value useful to you, other than as a trophy to stick on the wall?
I'm not an EE--what am I doing here?
 

Offline SeanB

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Re: How would you measure the ohms of a light bulb?
« Reply #24 on: October 29, 2017, 05:51:28 pm »
Having it done for you is easy, just use 2 meters that have remote measurement measuring ability, and connect to a computer, with the software there ( probably Mathlab to use a sledgehammer to open a peanut, but simpler is just Python and use a little scripting to do the simpler graphical display of instantaneous resistance) doing the scaling of the 2 measuements and the multiplication, and then giving a real time instantaneous resistance value.

If you have 2 HP GPIB meters you can get increased resolution using some nice 6 digit meters, and have the result accurate to 6 digits, but in most cases with any arbitrary lamp 2 decimal points in scientific notation will be more than enough for most uses. you will see from that how external reflectors that focus the radiation back on the filament does have an effect on the resistance, and how forced and convection cooling of the envelope also does change it.
 


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