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Self limiting electric heating element and method for making such an element

  • US 5,057,674 A
  • Filed: 08/08/1990
  • Issued: 10/15/1991
  • Est. Priority Date: 02/08/1988
  • Status: Expired due to Fees
First Claim
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1. Self limiting electric heating element including two outer semiconductive layers (1, 2;

  • 11,

         12) having zero temperature coefficient (ZTC) separated from one another by a continuous positive temperature coefficient (PTC) layer (3;

         13) and energized by two parallel electrodes (4, 5;

    14,

         15), one of which is in contact with one end of one of the ZTC layers and the other parallel electrode is in contact with the other ZTC layer at its end furthest removed from said one end, characterized in that the resistances of the PTC and ZTC components have the following characteristics;

    at room temperature;

    
    
    space="preserve" listing-type="equation">RPTC1<

    <

    RZTC<

    <

    RPTC2,and at control temperature;

    
    
    space="preserve" listing-type="equation">RPTC1=RZTC,where RPTC1 is the electrical resistance measured across the PTC layer,where RZTC is the resistance of the two ZTC layers connected in parallel, each having a resistance of 2·

    RZTC,where RPTC2 is the resistance measured between the electrodes (4, 5;

    14,

         15) along the PTC layer,so that at the control temperature the heat generated per time and unit area, i.e. the watt density, of the PTC layer and the watt density of the two parallel ZTC layers are essentially equal.

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