### Author Topic: How would you measure the ohms of a light bulb?  (Read 4866 times)

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#### Brumby

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##### Re: How would you measure the ohms of a light bulb?
« Reply #50 on: November 03, 2017, 12:36:39 am »
You are right ... theoretically, this is not an issue at all.  Building a device that can achieve the stated objective has no fundamental obstacles.  It just requires the motivation to do so.

There is no real fun in that, which is why most answers have been offered using off-the-shelf equipment and sometimes creative setups.

Some challenges so far have been the "zero calculation" requirement and the fact that resistance of a light bulb is not linear over temperature.

#### Richard Crowley

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##### Re: How would you measure the ohms of a light bulb?
« Reply #51 on: November 03, 2017, 02:42:18 am »
OK, @Beamin, answer this for us:  How do you "measure the ohms" of ANYTHING without calculations?

(Hint: You cannot "measure the ohms" of ANYTHING.  But you can measure current and voltage and calculate the resistance/impedance.)

#### Brumby

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##### Re: How would you measure the ohms of a light bulb?
« Reply #52 on: November 03, 2017, 02:58:32 am »
The measuring device does the calculation.

The  "no calculation" constraint was clearly aimed at the operator...
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.

Mind you, the thing that I find fascinating is trying to figure out what a person with such limited capabilities is going to DO with that resistance measurement.
« Last Edit: November 03, 2017, 03:00:14 am by Brumby »

#### Mjolinor

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##### Re: How would you measure the ohms of a light bulb?
« Reply #53 on: November 03, 2017, 03:54:29 am »
The measuring device does the calculation.

The  "no calculation" constraint was clearly aimed at the operator...
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.

Mind you, the thing that I find fascinating is trying to figure out what a person with such limited capabilities is going to DO with that resistance measurement.

Get a friend to work out the current?

#### Beamin

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##### Re: How would you measure the ohms of a light bulb?
« Reply #54 on: November 03, 2017, 04:47:37 am »
Hi,

The resistance of a bulb is not zero with zero voltage.  It is finite and non zero.

For example, a 100 watt 120vac bulb is around 10 ohms cold (zero current).
For a 1.5v bulb i have it is around 6 ohms cold (zero current)

When you measure you apply a voltage and measure the current and use Ohm's Law:
R=V/I

however with zero current you cant do that.  You really have to use the slope, but you can get a good idea by just using a very very low current like 1ua, but even 100ua will work in most cases and even 1ma in larger wattage bulbs.

So all filament bulbs will have a curve similar to:
R=Ro+R(v)

where Ro is the zero current resistance and R(v) is a function of the applied voltage.

There are equations that approximate the character of filament bulbs and they even can predict life expectancy in the absence of vibration.

The thread was more of an exercise in creative ways to figure things out using equipment rather than math . Its a very simple problem when using math but not so simple just by making observations. So I wasn't so much asking "I have this light bulb in my house how many ohms is it: I have to figure out what bulbs I can substitute and replace it with". More just a fun exercise where I earned a lot of creative options that I wouldn't have thought up on my own and I can use this knowledge in the future when I say "Hey remember in the light bulb thread the guy said measure the temperature? well I think that might work in this scenario. I try not to do linear single problem single solution thinking. This thread was more of a creative writing /a arts project; two thing you wouldn't think would apply to EE but apparently they can be valuable tools! A well rounded education in action solving real world problems.

For instance Verizion FIOS came to my house to install the internet. A horizontal drill head was placed under ground and the drill progress had to b measured. So that the workers (trabaho as they taught me to call it) would walk out to the site and feel around there hands on the ground for vibration. when they were doing that they weren't working  the drilling machine or digging holes because the ha to stop what they were doing every five minutes to check the progress of the bore hole. So I came out side with a bunch of small metal 2"  rods with little flags on the tops. I set the flags into the round in a grid pattern around the area of the bore holes. Now instead of stopping what they were doing and getting on hands and knees feeling the ground avery few minutes cutting into work time: NOW they could just take a quick peak at which flags were shaking at the top from a distance and see where the drill was without leaving their equipment. Saving tie and saving backs.
The asked if they could keep the lags so I gave it to them.

EDIT add the following letters to my post: AAADNFFKRNESSSS
« Last Edit: November 03, 2017, 11:57:14 am by Beamin »
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#### Brumby

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##### Re: How would you measure the ohms of a light bulb?
« Reply #55 on: November 03, 2017, 06:17:10 am »
The measuring device does the calculation.

The  "no calculation" constraint was clearly aimed at the operator...
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.

Mind you, the thing that I find fascinating is trying to figure out what a person with such limited capabilities is going to DO with that resistance measurement.

Get a friend to work out the current?

#### MrAl

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##### Re: How would you measure the ohms of a light bulb?
« Reply #56 on: November 03, 2017, 02:46:28 pm »
Hi,

The resistance of a bulb is not zero with zero voltage.  It is finite and non zero.

For example, a 100 watt 120vac bulb is around 10 ohms cold (zero current).
For a 1.5v bulb i have it is around 6 ohms cold (zero current)

When you measure you apply a voltage and measure the current and use Ohm's Law:
R=V/I

however with zero current you cant do that.  You really have to use the slope, but you can get a good idea by just using a very very low current like 1ua, but even 100ua will work in most cases and even 1ma in larger wattage bulbs.

So all filament bulbs will have a curve similar to:
R=Ro+R(v)

where Ro is the zero current resistance and R(v) is a function of the applied voltage.

There are equations that approximate the character of filament bulbs and they even can predict life expectancy in the absence of vibration.

The thread was more of an exercise in creative ways to figure things out using equipment rather than math . Its a very simple problem when using math but not so simple just by making observations. So I wasn't so much asking "I have this light bulb in my house how many ohms is it: I have to figure out what bulbs I can substitute and replace it with". More just a fun exercise where I earned a lot of creative options that I wouldn't have thought up on my own and I can use this knowledge in the future when I say "Hey remember in the light bulb thread the guy said measure the temperature? well I think that might work in this scenario. I try not to do linear single problem single solution thinking. This thread was more of a creative writing /a arts project; two thing you wouldn't think would apply to EE but apparently they can be valuable tools! A well rounded education in action solving real world problems.

For instance Verizion FIOS came to my house to install the internet. A horizontal drill head was placed under ground and the drill progress had to b measured. So that the workers (trabaho as they taught me to call it) would walk out to the site and feel around there hands on the ground for vibration. when they were doing that they weren't working  the drilling machine or digging holes because the ha to stop what they were doing every five minutes to check the progress of the bore hole. So I came out side with a bunch of small metal 2"  rods with little flags on the tops. I set the flags into the round in a grid pattern around the area of the bore holes. Now instead of stopping what they were doing and getting on hands and knees feeling the ground avery few minutes cutting into work time: NOW they could just take a quick peak at which flags were shaking at the top from a distance and see where the drill was without leaving their equipment. Saving tie and saving backs.
The asked if they could keep the lags so I gave it to them.

EDIT add the following letters to my post: AAADNFFKRNESSSS

Hi,

Ok then just glue little flags to the top of the light bulb and when you see them start to shake you know the bulb resistance :-)

Seriously though sometimes you need math, at least a little.  With this project all you have to do is know how to divide.  Divide measured voltage by the measured current.

BTW the temperature of most bulbs will be nearly the same.  That's because temperature is not a measurement of power even though it takes power to raise the temperature.  The temperature of something is dependent not only on power but also on surface area so you would need to know the surface area and then you might be able to estimate the power.  For example a thicker filament would require more power than a thinner filament but the two might reach nearly the same temperature because the envelope of the thicker filament bulb might be larger and thus have more surface area.  The power, temperature and surface area are related exponentially and we have other things to consider also such as unforced or forced convection cooling.  This means this would be a partly physical measurement, and when looking for results to electrical questions (such as Ohms) it is almost always best to stick to electrical measurements and calculations because they are inherently so accurate.

« Last Edit: November 03, 2017, 02:47:59 pm by MrAl »

#### MK14

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##### Re: How would you measure the ohms of a light bulb?
« Reply #57 on: November 03, 2017, 11:22:48 pm »
If someone insisted that I make such a device (OP), I would maybe do the following:

Design and make a custom bench power supply, with a maximum voltage output rating to suit the desired maximum working voltage of your bulbs, and give it an adjustable voltage control knob.

Connect the bulb under test, to the power supplies output. Then the program (if it has an MCU) inside the custom bench power supply, can calculate the resistance across its output, and hence calculate and display both the set voltage and resistance in Ohms.

Maybe call it the "LazyOhm" range of power supplies.
« Last Edit: November 03, 2017, 11:24:28 pm by MK14 »

#### innkeeper

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##### Re: How would you measure the ohms of a light bulb?
« Reply #58 on: November 05, 2017, 02:45:34 am »
there are already devices that do this
Hobbyist and a retired engineer and possibly a test equipment addict, though, searching for the equipment to test for that.

Smf