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Old 03-24-2010, 11:51 PM   #1
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Default Does resistance increase with voltage

I searched the forum for this but couldnt find anything, so I thought I would try this out and ask. I understand ohms law fine, but when you run most loads at a higher voltage the amperage is lower and with ohms law if resistance is the same and you increase voltage, amperage also increases. Isn't resistance usually constant? I am missing something but just can't think of it

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Old 03-25-2010, 12:07 AM   #2
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Originally Posted by ryan902584 View Post
I searched the forum for this but couldnt find anything, so I thought I would try this out and ask. I understand ohms law fine, but when you run most loads at a higher voltage the amperage is lower and with ohms law if resistance is the same and you increase voltage, amperage also increases. Isn't resistance usually constant? I am missing something but just can't think of it
There may be a better way to explain but I will just say you are right. The resistance will be the same but the other factors, if they are there... such as Impedance will present an opposition to current flow. In real life a person would only supply a higher voltage if the equipment were rated for the higher voltage. In the case of, say, a dual voltage motor that needed to be wired for the higher voltage if you draw it on paper you would see that it looks like a regular transformer and for the higher voltage you would be hooking the windings in series and the current draw would be half as much as if it were wired for the lower voltage in which the windings would be wired in parallel.

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Old 03-25-2010, 02:37 AM   #3
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Originally Posted by ryan902584 View Post
I searched the forum for this but couldnt find anything, so I thought I would try this out and ask. I understand ohms law fine, but when you run most loads at a higher voltage the amperage is lower and with ohms law if resistance is the same and you increase voltage, amperage also increases. Isn't resistance usually constant? I am missing something but just can't think of it
Start by defining "most loads". What is usually constant in the type of loads you describe is Power. If you are operating a load at a higher voltage and are see a decrease in current chances are it was designed for that higher voltage.

In the case of a dual voltage motor, as was mentioned, the motor has 2 sets of windings intended for series or parallel operation. In series for higher voltage, parallel for lower voltage.

Take a 480-240 dual voltage motor with current ratings of 5a/10a for example.

At 240v in parallel each winding sees 240v and 5a, the current will divide between the two windings evenly, given an equal impedance, for a total of 10a.
At 480v the series connection will allow 240v to drop across each winding, but the current each winding sees will be 5a. Since current must remain the same throughout any series circuit.
The motor uses 2400VA, the windings only see 1200VA. VA being a measure of power, and also the desired constant.
I will ignore the finer points of motor theory, the above is to illustrate a point.
For a more in depth look at theory http://www.allaboutcircuits.com/.

Last edited by K2500; 03-25-2010 at 02:39 AM.
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Old 03-25-2010, 07:12 AM   #4
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Originally Posted by ryan902584 View Post
I searched the forum for this but couldnt find anything, so I thought I would try this out and ask. I understand ohms law fine, but when you run most loads at a higher voltage the amperage is lower and with ohms law if resistance is the same and you increase voltage, amperage also increases. Isn't resistance usually constant? I am missing something but just can't think of it
Voltage is inversely proportional to amperage.
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Old 03-25-2010, 01:16 PM   #5
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Put your ohm meter on a Incandescent lamp. Then work out the amperage that it would consume @ 120v
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Old 03-25-2010, 01:25 PM   #6
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Put your ohm meter on a Incandescent lamp. Then work out the amperage that it would consume @ 120v

I tried that. 60w lamp had 20.9 Ohms. At 120volts, that works out to 5 amps. And that just ain't right, it should be amp (I=P/E).
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Old 03-25-2010, 01:27 PM   #7
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I tried that. 60w lamp had 20.9 Ohms. At 120volts, that works out to 5 amps. And that just ain't right, it should be amp (I=P/E).
What you are seeing there is the changing resistance due to the filament temperature.
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Old 03-25-2010, 01:38 PM   #8
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What you are seeing there is the changing resistance due to the filament temperature.
Which is the loophole in ohms "law"
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Old 03-25-2010, 01:44 PM   #9
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Originally Posted by ryan902584 View Post
I searched the forum for this but couldnt find anything, so I thought I would try this out and ask. I understand ohms law fine, but when you run most loads at a higher voltage the amperage is lower and with ohms law if resistance is the same and you increase voltage, amperage also increases. Isn't resistance usually constant? I am missing something but just can't think of it
Resistance is constant. Running a load of a given resistance at a higher voltage draws more current. V=IR; double the voltage and you double the current. I think what is confusing you is power; power is the product of voltage and current; for a given amount of power, if you double the voltage you cut the current in half.

An ideal transformer, for example, is an equal power device - power in = power out. A 2:1 step down transformer has half the AC voltage across its secondary as the primary, while the AC current in the secondary windings is twice that of the primary.

Last edited by ggunn; 03-25-2010 at 01:53 PM.
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Old 03-25-2010, 01:56 PM   #10
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I tried that. 60w lamp had 20.9 Ohms. At 120volts, that works out to 5 amps. And that just ain't right, it should be amp (I=P/E).
120v 60w gives 240 Ohms, at rated operating temperature.
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Old 03-25-2010, 02:05 PM   #11
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Voltage is inversely proportional to amperage.
Not if resistance is constant. If resistance is constant than voltage and current are directly proportional. Power would increase ^2 with a doubling of voltage. Your statement is true only if power is constant, in which case resistance becomes variable.
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Old 03-25-2010, 04:44 PM   #12
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Same would apply to a motor.. Next to no resistance when sitting on the bench.
Then again read resistance of a CFL
Transformer with no load on it ? No resistance.. But power it up.. Next to no amps flow..
Ohms law can be very confusing in real life..
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Old 03-25-2010, 08:51 PM   #13
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Resistance is constant. Running a load of a given resistance at a higher voltage draws more current. V=IR; double the voltage and you double the current. I think what is confusing you is power; power is the product of voltage and current; for a given amount of power, if you double the voltage you cut the current in half.

An ideal transformer, for example, is an equal power device - power in = power out. A 2:1 step down transformer has half the AC voltage across its secondary as the primary, while the AC current in the secondary windings is twice that of the primary.
So with the power formula,(P=E*I, and P=I^2*R), a 60 watt load at 240V would give you 960 ohms, and at 120V it would give you 240 ohms. so thats what I'm wondering about, how does the resistance change. I've read for lightbulbs that it has to do with the filement, but what about other purely resistive loads like heaters

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Old 03-25-2010, 09:33 PM   #14
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I'm going to have to check this out. I never heard of a tungsten filament changing resistance with heat. I'm more inclined to think the ohmeter is wrong. I'll have to see.

The only thing I know of that changes resistance are thermistors, but then again, I'm pretty old. The reactant component of impedance also obeys Ohm's law. Just more variables.
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Old 03-25-2010, 09:49 PM   #15
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I'm going to have to check this out. I never heard of a tungsten filament changing resistance with heat. I'm more inclined to think the ohmeter is wrong. I'll have to see........
ALL circuits do that.
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Old 03-25-2010, 09:54 PM   #16
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I'm going to have to check this out. I never heard of a tungsten filament changing resistance with heat. I'm more inclined to think the ohmeter is wrong.
Check this out from another forum, the poster is very knowledgeable.


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Heres an actual example I did a couple of years ago using a 240V 500W linear halaogen lamp.



Voltage and Amps measured, resistance and wattage calculated, supply is genuine sine AC through a variac. As can be seen the resistance of the filament is anything but constant...
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Old 03-25-2010, 10:21 PM   #17
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so that's what I'm wondering about, how does the resistance change. I've read for light bulbs that it has to do with the filament, but what about other purely resistive loads like heaters.
Same thing with heaters, check out temperature coefficient of resistance.
A positive coefficient means resistance will increase with an increase in temperature. Which will be most elements you will see in the field, with the exception of carbon and silicon.

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Same would apply to a motor.. Next to no resistance when sitting on the bench.

Ohms law can be very confusing in real life..
I think motors are a bit more complicated than that. In that it has more to do with the counter emf developed in a spinning motor, opposing the current flow.


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