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Control device and safety circuit for heating pads with PTC heater

  • US 5,422,461 A
  • Filed: 12/15/1992
  • Issued: 06/06/1995
  • Est. Priority Date: 12/15/1992
  • Status: Expired due to Term
First Claim
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1. A safety circuit for an electric alternating-current heater, said heater having a heating element provided by a web of PTC electroresistive material extending between first and second heater feed conductors, said first heater feed conductor (1) being connectable through a protecting fuse (5) to an ungrounded pole of a source of alternating electric current and said second heater feed conductor being connectable through a heater switch (T1) to a grounded pole of said source of alternating electric current, said first and second heater feed conductors, at their respective ends remote from said fuse and from said heater switch, being respectively connected to first and second safety link return conductors leading respectively to first (3) and second (4) inputs of said safety circuit, said heater switch being a triac having a control input, connected to an output of an integrated circuit (IC) unit, for duty cycle time division control of said triac through said control input of said triac in a duty cycle range varying from at most 25% duty to 100% in response to heat settings and in response to an output of said safety circuit connected to a control input of said integrated circuit unit, said integrated circuit unit being supplied with d.c. power at a reference voltage by d.c. power supply means connected to said source of alternating electric current, said integrated circuit unit also having manual control means for controlling said heater, said safety circuit comprising:

  • a first resistive voltage divider (R1, R2) having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said second heater feed conductor (2) which end is connected to said heater switch (T1) and the end of said first safety link return conductor which is remote from its connection to said first heater feed conductor;

    a second resistive voltage divider having a tap connection and a greater and a smaller resistance respectively on opposite sides of said tap connection and connected between the end of said first heater feed conductor (1) which end is connected to said fuse (5) and the end of said second safety link conductor (4) which end is remote from its connection to said second heater feed conductor (2), the greater of said resistances of each of said first and second voltage dividers being connected respectively to said first safety link return conductor and to said first heater feed conductor,the ratio of said greater to said smaller resistance being greater for said second voltage divider than for said first voltage divider;

    a first rectifier diode (D1) having a first electrode connected to said tap of said first voltage divider and a second electrode connected to a first network comprising a first capacitor (Cl) shunted by a resistor (R5) leading to ground potential, said first network having a first predetermined time constant;

    a second rectifier diode (D2) having a first electrode connected to said tap of said second voltage divider and a second electrode connected to a second network comprising a second capacitor (C2) shunted by another resistor (R6) leading to ground potential, said second network having a second time constant;

    said connection of said first rectifier (D1) to said first capacitor (C1) also being connected, through a third diode (D3), poled oppositely to said first diode (D1) in a series connection therewith interposed between said first diode and said control input of said integrated circuit unit;

    a semi-conductor inverting amplifier stage having a first main path electrode connected to said reference voltage, having a control electrode connected, through an input resistor (R8) to said second diode (D2) where said second diode is connected to said second capacitor (C2), for blocking conduction between main path electrodes of said amplifier stage by a signal rectified by said second diode (D2) and having a second main path electrode connected both to a load resistor (RIO) leading to ground potential and, through a fourth diode (D4), to said control input (9) of said integrated circuit (IC) unit (14), the polarity of said fourth diode (D4) being opposite to that of said third diode (D3) when said third and fourth diodes are considered as being in series through their common connection and being the same as the polarity of said third diode when said third and fourth diodes are considered as respectively belonging to parallel paths to ground through respective resistances (R5, R10);

    wherein said first and second networks serve to make a time that passes after initial power up of said heater before said integrated circuit unit can respond to a fault substantially equal to a time necessary for both the safety circuit and the integrated circuit (IC) unit to react to a fault.

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