Patent ID: 6294767
Filing Date: 2001-09-25
Classification: A47L,D06F,H02H,Y10S

Abstract:
A protection circuit for a resistive heating element powered from an alternating current supply comprising:a current transformer the primary circuit of which comprises both the phase and neutral supply conductors connected across said heating element,a secondary transformer winding across which a voltage is developed which is proportional to any out of balance current between the phase and neutral conductors respectively,a third conductor forming a tertiary transformer circuit which is arranged to carry a very low bias current derived from said alternating current supply and to continuously develop a small predetermined voltage in the transformer secondary winding,a direct current supply derived from said alternating current supply,a relay coil and a transistor series connected across said direct current supply, the relay switch being connected in series with one of the alternating current supply conductors to the heating element,a voltage sensing means connected to said secondary winding which provided said predetermined voltage is present across the secondary winding supplies enough current to said transistor to cause it to conduct sufficiently to allow a current to flow through said relay coil from said direct current supply which is sufficient to hold the relay switch on and to present an alternating voltage across said heating element, but in the event of failure to detect said predetermined voltage said voltage sensing means supplies insufficient current to said transistor to allow it to conduct enough current through the relay coil to retain the relay latched on.