Ohms Law

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If I remember the story correctly, the first actual product of what became Hewlett-Packard was a high stability audio oscillator that indeed used a light bulb in that or a similar circuit for amplitude stabilization.
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Golddigger, you are correct! It was Mr. Hewlett and Mr. Packard in their garage. :thumbsup:

Larry, the model railroad power supply is using the same method as oldtimers who screw an Edison base bulb instead of a new fuse to troubleshoot short circuits. I use a headlamp and jumper clips to troubleshoot vehicle short circuits. Saves buying fuses, plus gives visual indication of when the short goes away!

Or the original electric hot dog cooker, suicide cord, two nails and a light bulb in series. When the light goes out, the dog is cooked an no longer passing current.:D
 
Frank DuVal said:
Larry, the model railroad power supply is using the same method as oldtimers who screw an Edison base bulb instead of a new fuse to troubleshoot short circuits.
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I know. I am one of those old-timers. ogy.gif I've done it many times and have described it here more than once.

I use a headlamp and jumper clips to troubleshoot vehicle short circuits. Saves buying fuses, plus gives visual indication of when the short goes away!
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Absolutely, same as above. Fuses or breakers.

Or the original electric hot dog cooker, suicide cord, two nails and a light bulb in series. When the light goes out, the dog is cooked an no longer passing current.:D
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We did that in elementary school science class.
 
I would imagine the resistance is nonlinear based on the input current. Your meter is probably using micro/milliamp currents to take readings. That bulb is not getting hot...........
 
quantum said:
I would imagine the resistance is nonlinear based on the input current. Your meter is probably using micro/milliamp currents to take readings. That bulb is not getting hot...........
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You are a bit late to this particular party. :D
 
one more just for kicks - 15 W LED bulb measures 197K ohms both directions.
A 6 W LED bulb measures 1.1 megohm in one direction an >10meg in the other:roll:
 
Nothing new here. I had thought the temperature coefficient for tungsten would not be constant from room temp to incandescent. Apparently it is pretty constant.

Alpha = .0044 1/C

As others have said, for a typical 100W bulb,
resistance at 20C = 12 Ohms
resistance at 2550 C = 144 Ohms

Attached is a graph of the data I found

Interesting thought:
The resistance cannot be measured directly.
Voltage can be. Current can be.
Meters measure V and I and calculate the resistance.

the worm
 

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iceworm said:
Nothing new here. I had thought the temperature coefficient for tungsten would not be constant from room temp to incandescent. Apparently it is pretty constant.

Alpha = .0044 1/C

As others have said, for a typical 100W bulb,
resistance at 20C = 12 Ohms
resistance at 2550 C = 144 Ohms

Attached is a graph of the data I found

Interesting thought:
The resistance cannot be measured directly.
Voltage can be. Current can be.
Meters measure V and I and calculate the resistance.

the worm
Click to expand...
You could consider that a resistance bridge, like the classic Wheatstone bridge, measures resistance directly. You are adjusting a variable resistor for a null in the bridge circuit independently of the applied voltage and the resulting current.
 
GoldDigger said:
You could consider that a resistance bridge, like the classic Wheatstone bridge, measures resistance directly. You are adjusting a variable resistor for a null in the bridge circuit independently of the applied voltage and the resulting current.
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Yes, one could. And I certainly would not say you were wrong.
Still, one had to calibrate the variable resistor.

Then again, someone also had to calibrate the first ammeter, and the first voltmeter.

Aarg -- I need one-armed engineer
 
Dennis Alwon said:
I have been going nuts all day trying to figure out why the difference yet when wired in series the voltage at the bulbs calculated correctly.--bulb were hot....


Thank you both
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Why would you wire lamps in series? Just an experiment?
 
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meternerd said:
Why would you wire lamps in series? Just an experiment?
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Yes, I am teaching 1 course at a community college and I thought it might be good to see how electricity reacts under different conditions. We will eventually wire a small cottage that the construction class builds and then they raffle it off and start over.

Series wiring is not really what I am teaching but I thought it might be fun to do in the lab.

The course is residential wiring and some code
 
190202-1940 EST

Dennis has a very good idea as a means to teach how the resistance of an incandescent bulb varies as voltage or current changes. This simply takes two series connected sockets and a variety of tungsten bulbs. Not as easy as a Variac, but cheaper if you don't have a Variac.

I might like the variety to consist of 250 W, 150 W, 100 W, 60 W, 40 W, 25 W, 15 W, and 7.5 W . Also a 100 ohm 100 W Ohmite power resistor would be useful. Directly connected to 120 V this is somewhat overloaded, 144 W, but for short times not a major problem.

Another experiment that can be performed is to demonstrate peak inrush current to a 100 W bulb. Lacking a scope and a controlled way to turn on the AC waveform one can use the following setup: However, you will be very lucky to hit the peak voltage point with manual switching.

Connect a 1 ohm 10 W Ohmite resistor in series with a socket by connecting one resistor end to the shell terminal. The tip terminal goes to a switched hot from 120 V. The neutral wire goes to the other end of the 1 ohm resistor. With the selection of 1 ohm the voltage across the resistor equals current in amps.

A Fluke 87 in its fast response peak hold DC mode might work without anything else, but I don't have the 87 at home to try.

So I used my Fluke 27 on DC in MIN/MAX. To get fast peak response I used a 1N4004 equivalent diode in series to a 25 mfd electrolytic capacitor for an initial peak hold. This is across the 1 ohm resistor. The diode produces about 1 V drop so the measured voltage across the capacitor is slightly below the actual peak. With multiple switch tries I got a reading as high as 9.5 V. About 10.5 A. This is substantially below the 17 A I can produce where I can turn on the switch at a voltage peak as I did years ago in the waveforms on my website.

Still with random switching you can show the substantial inrush to a tungsten filament on a cold start. Same approach can be used to look at transformer inrush.

.
 
In a related story...

I use white Christmas (series) minilights as general lighting in my studio. As the lights burn out they short out; this was a development many years ago to make the rest stay lit when a bulb fails. The result is that the 120VAC gets distributed over fewer lights. With a 100 bulb set the difference is negligible when the first one fails, but as more fail the rest get brighter. This continues over time, and at some point the voltage across the remaining lights gets so high that they all burn out at once, and it's like a flashbulb going off. The first time it happened it startled the heck out of me.
 
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