Basic Question

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Steve McKinney

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The more I learn, the more I find out I don't know. 90 watt incandescent lamp reads 15.5 ohms continuity. Apply 120 vac and and should have 7.75 amps according to Ohms Law. But we all know we only have 3/4 of an amp. I plugged it in for a while to see if the property of the filament changed. It did. It went down to 12 ohms! I understand it is a pure resistive load, or is it?

Can anyone shine any "light" on this? (Pun intended)

Steve
 
Besoeker

Besoeker

Senior Member
Location
UK
Steve McKinney said:
The more I learn, the more I find out I don't know.
Click to expand...
And that's how it should be. You won't learn otherwise.
Steve McKinney said:
90 watt incandescent lamp reads 15.5 ohms continuity. Apply 120 vac and and should have 7.75 amps according to Ohms Law.
Click to expand...
From cold, it probably would. It also equates to about 900W.
Then the filament rapidly heats up and, with it, the resistance. By a huge factor. Hot resistance for your 120V, 90W lamp would be 160 ohms.
Steve McKinney said:
But we all know we only have 3/4 of an amp. I plugged it in for a while to see if the property of the filament changed. It did. It went down to 12 ohms! I understand it is a pure resistive load, or is it?
Click to expand...
It is resitive, just not constant resistance.
 
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Steve McKinney

Member
Wow! thnx for the quick responses. I'm impressed and I appreciate the responses.

Anyway----------3 different Fluke digital meter read the lamp 15.3 - 15.5 ohms. Plugged it in for a few minutes and the resistance actually went down.

So what we are saying is the resistive circuit(inductive I have a better grip on) obviously changes with the current flow, but we really can't measure that.(?)

It would be nice if I understood this because I teach it! ha
 
drbond24

drbond24

Senior Member
Location
Barboursville, West Virginia
Steve McKinney said:
The more I learn, the more I find out I don't know. 90 watt incandescent lamp reads 15.5 ohms continuity. Apply 120 vac and and should have 7.75 amps according to Ohms Law. But we all know we only have 3/4 of an amp. I plugged it in for a while to see if the property of the filament changed. It did. It went down to 12 ohms! I understand it is a pure resistive load, or is it?

Can anyone shine any "light" on this? (Pun intended)

Steve
Click to expand...

In general, resistance increases changes with temperature (i.e. higher temperature = higher resistance). This is true for all substances as far as I know.

As far as your lamp, I tend to agree with Jljohnson that you were reading your multimeter with the decimal in the wrong place. :wink:
 
mistermudd

mistermudd

Senior Member
Location
Washington State
It is fairly well known that a cold light bulb filament has less resistance than a hot one. Therefore, a light bulb draws excessive current until the filament warms up.
Since the filament can draw more than ten times as much current as usual when it is cold, some people are concerned about excessive energy consumption from turning on light bulbs.
The degree of this phenomenon has become a matter of urban folklore. However, the filament warms up very rapidly. The amount of energy consumed to warm up a cold filament is less than it would consume in one second of normal operation.



http://members.misty.com/don/bulb1.html#bp - See also for more info on tungsten
http://hypertextbook.com/physics/electricity/resistance/
Interesting stuff, the filament of a bulb is hotter than the sun, resulting in the large increase in resistance with temp.

I am sure you are reading your digital fluke correctly, since they do the decimal point thing for you
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
081226-2151 EST

Steve:

I just ran an experiment on a 100 W tungsten filament bulb.

It read 9.8 ohms witha Fluke 27 before I applied power.
On removal and after a minute or so of cool down it read 9.6 ohms.
Cycled it again and it returned to 9.6 ohms. My initial measurement may have some extra contact resistance between the ohmmeter leads and the plug.

I have no theoretical reason to expect the room temperature resistance to become less after heating unless some of the filament coils short together.

Have you tried the experiment of measuring the resistance while applying external radiant heat energy. A fire place might do it as well as a heat lamp, or try the sun.

.
 
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Steve McKinney

Member
Thanks again for all the input. Tell ya what started all this. Customer installed about 8 can lights in his remodeling kitchen project. I went over and wired them all to a 12/2 that would be coming from the eventual switch. (I just threw a cord cap on the romex and plugged them in so they could evaluate the "effect" of their new kitchen lighting. Which brings to mind, does anyone know of a software program that a blue collar guy like me could afford that will let me super impose lighting options on a digital picture of the room? Meanwhile, back at the ranch, they were not happy with lighting. Customer pulls them down and installs more larger ones. "I wired them just like you did". So I temporaried them into a cord cap and connected 2 of the 3 wires.(hint, hint). Plugged them in, Wow, we like those. So my son and I contiuned to run the other kitchen circuits. We then wire them into the new dimmer and restore power for a very very brief moment, if you know what I mean. "Houston, we have a problem". So I get out my trusty Simpson 260 (I know, old skool) and find that I have 2 ohms between the grounded(white) and ungrounded (black) conductors. 2 OHMS! You kidding? I just had them on 15 minutes ago plugged into a receptacle! Then for grins and giggles, and being the next logical step, I checked from hot to bare,(Grounding conductor) Dead short, almost. Maybe 1/2 ohm. Owner had clamped down so hard when rewiring the new cans, when he installed the romex basically on edge and clamped down like he wasn't ever coming back. Well, he wasn't, but I was. Found two, possibly three piercing the jacket and black insulation. As you have probably figured out by now, I never connected the "grounding" conductor when I temped in the cord cap. Okay, so we got that one behind us, as far as the fault, but it left a lingering question as to why I only have 2 ohms resistance in a "good" circuit. Thanks for all your inputs gentlemen. As I originally stated.............I'm s'pose to know this stuff!

BTW.....learned something else yesterday. Can't meggar a GFCI receptacle and get a true reading for circuit insulation. Must be internal circuitry of the GF.

It is kinda' like hearing the reason for a Code requirement. When you learn more about the reasoning, it makes more sense.

Anyway, thanks again guys and Happy New Year!

Steve
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Steve McKinney said:
BTW.....learned something else yesterday. Can't meggar a GFCI receptacle and get a true reading for circuit insulation. Must be internal circuitry of the GF.
Click to expand...

Steve did you mega a circuit line to line with a GFCI down line?

If so you might want to go back and change the GFCI.
 
mdshunk

mdshunk

Senior Member
Location
Right here.
iwire said:
Steve did you mega a circuit line to line with a GFCI down line?

If so you might want to go back and change the GFCI.
Click to expand...
Oh my. Yeah, pretty good chance you toasted the GFCI. Start with an ohm meter or 50/100 volt megger before you throw real juice to the system.
 
S

Steve McKinney

Member
I set the Biddle hand crank style meggar on 250 volt and never really got up to speed.

I read from ungrounded to ground, reason being stuffing devices into a steel box, with the GF being one of them.

Fired it up and it tested ok.

I have found the fewer surprises in my life, the better it seems to go. (Is that too philosophical this morning?.................Maybe it is just the time of year that reflection kicks in)..................oh, and I did look up how to spell that one correctly....ha
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090101-1051 EST

Steve:

The GFCIs that I have opened have an MOV (Metal Oxide Varistor) across the input terminals, and no connection to the ground terminal.

An MOV is a nonlinear resistive device relative to voltage. With a Fluke 27 I measure about 20 megohms into the line input terminals of a Leviton GFCI. I believe this meter has less than 1 V applied to the device under test for resistances below 10 megohms.

With GFCIs in the circuit you should avoid peak test voltages over 190 V. This would correspond to the peak value of a 135 V sine wave.

.
 
mikeames

mikeames

Senior Member
Location
Gaithersburg MD
Occupation
Teacher - Master Electrician - 2017 NEC
Remember.... the lights are in parallel so for every light you place in the circuit you reduce the resistance. Add enough lights and you may be very close to 0 ohms especially cold.

If all the lamps are the same wattage or the same resistance then take the value of one and divide by the number you have in the circuit. For example if the resistance of one is 10 ohms cold and you installed 10 fixtures and took a resistance reading then you should see 1 ohm total resistance. If you have 20 fixtures then you will have .5 ohms. Keep adding and you will eventually come close to 0 ohms...... Impossible no thats a short circuit..... Add enough lights and you drop the resistance to zero.... which trips the breaker.

Wire them in series and you increase the resistance. Eventually they wont light unless you up the voltage. Wire 4 120 volt lamps in series and connect to 480 and your good to go.

Resistance is proportional to temperature thus the theory of superconductivity at 0 kelivins.
 
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