Ohms Law

[Besoeker3]

Agree - I suspect he thought a hands-on experiment would more useful to drive home the concept in this case.

I think he takes the "hands on" approach. Not a bad thing. But let's listen to what he has to say.

 

[kwired]

measure resistance of the lamp immediately after turning it off, though it is probably dropping rapidly if you connect your meter to it fast enough you may catch it while it is still dropping.

 

[steve66]

You just had to throw that 43/60 in there. I was feeling good with the answers now tell me how you got 43/ 60

That was probably not quite correct. I was just trying to say that newer bulbs may be labeled "60 watts", but they are really only 43 watts. So you may need to use 43 watts in your calculation instead of 60.

​

:thumbsup:

Of course. However, these are older bulbs that have 60 watts on them. If I remember the others stated 43 watts or whatever.​

I believe newer bulbs are still labeled with 60 watts, although they only use 43 watts. Its "truth in advertising" turned on it's head - manufacturers are actually required to lie about the wattage a bulb uses. A 43 watt bulb will be labeled "60 watts".

However, you said you used an ammeter and you got the correct reading for current. That seems to confirm you do actually have older lamps (probably made before the 2010-2014 range when they first started to phase out incandescent), and that they probably do use 60 watts.

As others have mentioned, the resistance of the filament is much lower when its at room temperature. And the resistance goes up a lot when the filament starts glowing red hot. So that's why you just can't measure the resistance. However, if you use an ammeter to measure the current when the bulb is operating, you should get the expected current which is 60W / 120V = 0.5 amps.

Then if you use R=V/I, you can calculate the resistance for the "hot" filament which is 120V/0.5A = 240 ohms, as you expected from your first post.



​

 

[Gary11734]

That was probably not quite correct. I was just trying to say that newer bulbs may be labeled "60 watts", but they are really only 43 watts. So you may need to use 43 watts in your calculation instead of 60.

I believe newer bulbs are still labeled with 60 watts, although they only use 43 watts. Its "truth in advertising" turned on it's head - manufacturers are actually required to lie about the wattage a bulb uses. A 43 watt bulb will be labeled "60 watts".

However, you said you used an ammeter and you got the correct reading for current. That seems to confirm you do actually have older lamps (probably made before the 2010-2014 range when they first started to phase out incandescent), and that they probably do use 60 watts.

As others have mentioned, the resistance of the filament is much lower when its at room temperature. And the resistance goes up a lot when the filament starts glowing red hot. So that's why you just can't measure the resistance. However, if you use an ammeter to measure the current when the bulb is operating, you should get the expected current which is 60W / 120V = 0.5 amps.

Then if you use R=V/I, you can calculate the resistance for the "hot" filament which is 120V/0.5A = 240 ohms, as you expected from your first post.



​

As a consumer, the wattage was always a comparison between bulbs and my reference of what I saw as light as installed.

This thread tells you that cold resistance and hot resistance in light bulbs is bigger than we once thought. I never thought it would be this wide. Good experiment here.

 

[kwired]

That was probably not quite correct. I was just trying to say that newer bulbs may be labeled "60 watts", but they are really only 43 watts. So you may need to use 43 watts in your calculation instead of 60.

I believe newer bulbs are still labeled with 60 watts, although they only use 43 watts. Its "truth in advertising" turned on it's head - manufacturers are actually required to lie about the wattage a bulb uses. A 43 watt bulb will be labeled "60 watts".



​

I think most (if not all) are labeled 43 watts, in the 43/60 example, but packaging may indicate it is 60 watt equivalent/replacement in some way or another. Lamp itself hardly ever states any "equivalent" rating from what I recall seeing.

 

[winnie]

There is a classic circuit called a Wien Bridge Oscillator which uses the changing resistance of an incandescent lamp to provide for stable operation.

Oscillators depend on positive feedback through an amplifier to get oscillation. The problem is that this positive feedback quickly drives the system into saturation. You need some technique for having positive feedback until the signal amplitude gets high enough and then limiting the gain so the amplitude doesn't get too high.

In the Wien Bridge, an incandescent lamp is used as a resistor in the feedback path setting gain. As the signal amplitude increases, more current runs through the lamp, it heats up, and its resistance goes up, reducing the gain. A very clever approach to keeping the system in its linear range.

-Jon

 

[Gary11734]

There is a classic circuit called a Wien Bridge Oscillator which uses the changing resistance of an incandescent lamp to provide for stable operation.

Oscillators depend on positive feedback through an amplifier to get oscillation. The problem is that this positive feedback quickly drives the system into saturation. You need some technique for having positive feedback until the signal amplitude gets high enough and then limiting the gain so the amplitude doesn't get too high.

In the Wien Bridge, an incandescent lamp is used as a resistor in the feedback path setting gain. As the signal amplitude increases, more current runs through the lamp, it heats up, and its resistance goes up, reducing the gain. A very clever approach to keeping the system in its linear range.

-Jon

How would a tungsten lamp not be linear as it heats up in the reference to resistance?

If you know the cold resistance, the hot resistance, wouldn't it basically be a straight line between the two?

 

[winnie]

How would a tungsten lamp not be linear as it heats up in the reference to resistance?

If you know the cold resistance, the hot resistance, wouldn't it basically be a straight line between the two?

The thermal time constant of the lamp was much longer than the period of the sine wave, so the lamp was responding to the RMS of the sine wave, not the instantaneous value.

As the RMS value of the sine wave would climb, more current would go through the lamp, and the filament would heat up, and its resistance would go up and the feedback gain would go down. The other resistors in the feedback path were selected so that at the desired output amplitude the gain would be unity.

As far as the _signal_ itself was concerned, the lamp was a nice linear resistor.

As far as the envelope of the signal RMS averaged over time, the lamp was a non-linear resistor (resistance changing with amplitude) such that you got the right amount of feedback at the right signal amplitude.

Note that the temperature coefficient of resistance does change with temperature, but that is secondary to the point above.

-Jon

 

[Gary11734]

The thermal time constant of the lamp was much longer than the period of the sine wave, so the lamp was responding to the RMS of the sine wave, not the instantaneous value.

As the RMS value of the sine wave would climb, more current would go through the lamp, and the filament would heat up, and its resistance would go up and the feedback gain would go down. The other resistors in the feedback path were selected so that at the desired output amplitude the gain would be unity.

As far as the _signal_ itself was concerned, the lamp was a nice linear resistor.

As far as the envelope of the signal RMS averaged over time, the lamp was a non-linear resistor (resistance changing with amplitude) such that you got the right amount of feedback at the right signal amplitude.

Note that the temperature coefficient of resistance does change with temperature, but that is secondary to the point above.

-Jon

Jon, you are way past my pay grade... I see a bulb turning on, and it goes from cold to hot.

 

[GoldDigger]

There is a classic circuit called a Wien Bridge Oscillator which uses the changing resistance of an incandescent lamp to provide for stable operation.

Oscillators depend on positive feedback through an amplifier to get oscillation. The problem is that this positive feedback quickly drives the system into saturation. You need some technique for having positive feedback until the signal amplitude gets high enough and then limiting the gain so the amplitude doesn't get too high.

In the Wien Bridge, an incandescent lamp is used as a resistor in the feedback path setting gain. As the signal amplitude increases, more current runs through the lamp, it heats up, and its resistance goes up, reducing the gain. A very clever approach to keeping the system in its linear range.

-Jon

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.

 

[ggunn]

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.

There is a line of semipro audio loudspeakers that uses snowmobile headlights in series to protect them from overdriving. As a FOH guy (sound man), when you start seeing lights behind the speaker grills, it's time to turn things down a little.

 

[winnie]

Jon, you are way past my pay grade... I see a bulb turning on, and it goes from cold to hot.

I can't explain it nearly as well as others. The bulb does not see full voltage and isn't fully turned on; in fact it doesn't produce visible light. It just gets warmer than room temperature.

Here is a fun video of someone playing with the circuit:

https://hackaday.com/2012/11/18/jeri-uses-light-bulbs-in-an-oscillator/

-Jon

 

[Dennis Alwon]

We use the 60 watts as a fixed known. Then, this happens!

Based on the new information, 43/60. I would energize this light. Let it get to operating temperature, then put an amp probe on it.

Now you can determine what the true resistance is at operating. And, what the wattage is.

Also, you can now measure the resistance cold and see the difference between room temp and operating resistance for clarity.

This is what we did. We were able to measure the amps and we got the resistance which matched pretty closely to what ohms law was saying. The problem was that I was expecting an unlit bulb to measure around the same as ohms law would yield. The ohms law calculated came very close to the actual ohms when the circuit was energize fully.

Gar I do get most of it but I am not setup to do experiments at the level you do them. i understand that the resistance changes when the bulb is energized then when it is off.

I just wanted a simple experiment to show people using a series circuit. We have accomplished that.

Thanks all but go ahead with discussion if you want. I am happy with my minimal knowledge of it all. :lol:

 

[LarryFine]

Years ago, one of the electronics hobbyist magazines I read had a project that was a PWM model train power pack that would start a train at crawling speed by varying the duty cycle of a chopped full 12v DC.

They used a paralleled pair of high-current 12v bulbs (#93?) in series with the output as a track short-circuit protector. A jeweled red lens mounted in front of one of the bulbs acted as a rack-short indicator.

The cold filament resistance was so minimal that the voltage drop was negligible, but hot, the bulbs limited the current to less than the current capacity of the power pack, ~ 4 amps, if I remember correctly.

 

[gar]

Radiant energy detector

Radiant energy detector

190131-2102 EST

An interesting experiment is:

Put a 50 W tungsten incandescent reflector bulb in a socket with no wires or switches, and directly connect a Fluke or similar DVM directly to the socket terminals. Tighten the screws tight. Use ohms on the meter. Response time is relatively quick.

DVMs compared to a Simpson 260 apply very little voltage to the resistance under test at low resistances.

My bulb read 21.3 ohms at room temperature. The next two measurements were using eyeball estimate of distance. A 100 W 120 V bulb in a reflector fixture was used as an energy source.

At 2 ft R = 21.8 ohms. Delta R = 0.5 ohms.
At 1 ft R = 22.8 ohms. Delta R = 1.5 ohms.

Different room, different ambient temperature, and a non-fan radiant space heater at about 2 ft.
20.7 ohms to 21.4 ohms. Delta R = 0.7 phms.

Its night time so viewing sun is not possible..

 

[MAC702]

With only conduction and radiation heat through the glass and gas, how long before the filament reaches room ambient temperature?

 

[gar]

190131-2211 EST

A few seconds for one time constant. Stable within possibly 10 to 20 seconds for the measurementds I made.

.

 

[winnie]

I forgot something else I used incandescent lamps for: precharge on a lab inverter that I was part of building. As the capacitor bank charged, the voltage across the bulbs would drop, and their resistance would drop, speeding the precharge.

-Jon

 

[Besoeker3]

This is what we did. We were able to measure the amps and we got the resistance which matched pretty closely to what ohms law was saying. The problem was that I was expecting an unlit bulb to measure around the same as ohms law would yield. The ohms law calculated came very close to the actual ohms when the circuit was energize fully.

Unlit bulb is about 20°C at room temperature.
Energised it is about 25000°C

The ohms are much different.
Ohm's law not violated.

 

[SG-1]

Another interesting experiment is to take two or three incandescent light bulbs of different wattage & wire them in series.

Apply 120V & see which one is the brightest & why.