Electric Heating Element

[LarryFine]

Dissimilar loads in series causes dissimilar voltages across each load, inversely proportionate to each load's resistance. The larger heater will cause the small one to see the most voltage.

When two equal load are wired in series and supplied by twice one unit's rated voltage, they will each see approximately their rated voltage, about half the total, and operate as they should.

However, two dissimilar loads in series, if supplied by twice one unit's rated voltage, will each see a different voltage, often destroying the one with the higher resistance due to over-voltage.

This is exactly what happens when a neutral opens. The line-to-line voltage remains the same, but whichever half of the line-to-neutral loads has less load will be subject to higher voltage.

 

[LarryFine]

OK, thank you. I think I get it.

By George, I believe you've got it!

Note in your correct math that doubling (or halving, etc.) the voltage applied to a given (constant) resistance quadruples the power, because the current also doubles (or halves, etc.).

 

[Sea Nile]

Of course, all of this is taken into account when designing a supply for a given known load.

Yes, I made a mistake. I was simply warning of the dangers of putting a 600w element in a heater designed for a 300w element. I'm sorry for the confusion and will do my best to read the question more carefully before posting.

 

[GoldDigger]

Hi there, I am looking at two heating elements, one is 12V, 300W and the other is 12V, 600W. The only physical difference is the size.

If I supply each heater with the same amount of current will I get the same amount of heat?

Ex. they are both 12V and if I supply 25A I should get 300W. Would the 600W element produce the same amount of heat and just have capacity for more? Will the 600W just heat slower?

Thank you!!

Looking at the OP's exact wording, if the two elements are both rated for 12V and are supplied with 25A of current (say from a current limited solar cell array) then the 300W element will produce 300W, while the 600W element will have an applied voltage of only 6V when supplied with 25A of current.
The output of the 600W nominal element will be only 150W because of the impedance mismatch between the source and the load.

 

[LarryFine]

Yes, I made a mistake. I was simply warning of the dangers of putting a 600w element in a heater designed for a 300w element. I'm sorry for the confusion and will do my best to read the question more carefully before posting.

No apologies needed. I was adding to what you said for the OP, not to admonish you.

 

[LarryFine]

Looking at the OP's exact wording, if the two elements are both rated for 12V and are supplied with 25A of current (say from a current limited solar cell array) then the 300W element will produce 300W, while the 600W element will have an applied voltage of only 6V when supplied with 25A of current.
The output of the 600W nominal element will be only 150W because of the impedance mismatch between the source and the load.

You consider a constant-current source, which varies the voltage to maintain a given current, because you work(ed) with solar. Most of us have little or no experience with constant-current sources. I'm familiar with them because I've been an electronics enthusiast since I was a young kid.

I believe the goal here is to try to give simple yet accurate explanations to questions, especially those about theory and math. Thus, we should use the voltage as the constant, the resistance as the variable, and the current and the power as results. Just my ever-so-humble opinion.

 

[LarryFine]

I was adding to what you said for the OP, not to admonish you.

Wow, did I mix tenses there!

 

[JoeStillman]

I think the point here is that heaters are rated in watts, but only at their nominal voltage. The wattage is not constant when voltage varies, but the resistance is.

 

[jaggedben]

...

If I supply each heater with the same amount of current will I get the same amount of heat?

I would have just said ...

"Maybe, but I'd like to see how you plan to do that. What current source are you going to use? "

 

[jaggedben]

I think the point here is that heaters are rated in watts, but only at their nominal voltage. The wattage is not constant when voltage varies, but the resistance is.

Actually the resistance isn't entirely constant either, it increases as the element heats up. Which is why heating elements have inrush current. The rating is based on where the element reaches equilibrium.

 

[GoldDigger]

Actually the resistance isn't entirely constant either, it increases as the element heats up. Which is why heating elements have inrush current. The rating is based on where the element reaches equilibrium.

True, but a typical alloy used for heating elements will have a relatively low temperature coefficient and a relatively low working temperature, even when red hot.
The tungsten filament of a light bulb, on the other hand, will have a large multiplier of resistance between room temperature and white hot operating temperature.