I am trying to understand PTC ceramic elements. Attached is a photo of a PTC element assembly that I removed from a 12 vdc heater. It seems to consist of two PTC elements sandwiched between three aluminum heat radiators. (The black wires at the top were originally connected to each other, which means the elements were in parallel.)

With nothing connected to the wires in the photo, the resistance across both elements (in series) is 10.7 Ω, across the left element is 2.3 Ω, and across the right element is 8.5 Ω. I don't understand why there is so large a discrepancy between the resistances of the two elements. This discrepancy is reflected in a difference in the in-rush current of the two elements, but when the two elements have been under power for about 20 seconds, they both settle at about the same steady current.

My questions are (1) what determines the the resistance curve of the ceramic elements - physical size and shape and/or chemical composition? and (2) why is there such a large difference in the resistance of the two elements in the photo and (3) why does the resistance seem to equalize between the two elements when they come up to maximum temperature?

Thanks in advance.

 

If the elements are in series, the current will be the same through both heaters irregardless of resistance. This current and the voltage across each heater will determine the resistance of each heater.

 

If the elements are in series, the current will be the same through both heaters irregardless of resistance. This current and the voltage across each heater will determine the resistance of each heater.

Joe,

I measured the resistance across the elements in series, but they were never connected in series; they were in parallel until I cut the black wire at the top. Thanks for your response. The low number of responses to this post is surprising. I know that my questions have more to do with the chemical composition and physical configuration of PTC ceramic elements, but I would have expected an expert or two to be on the forum.

 

I really would like some help here. Is there no one on the forum with some expertise on PTC ceramic heaters?