GB894860A - Protective apparatus for a high frequency generator - Google Patents

Abstract

894,860. Protective cut-out arrangements. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Sept. 15, 1960 [Aug. 31, 1959], No. 29672/59. Class 38 (5). [Also in Groups XXXVI and XL (c)] An arrangement for protecting a microwave frequency generator G comprises a directional coupler F provided in the output path from the generator and arranged to detect energy reflected back to the generator, e.g. in the absence of a load L formed by a strip of material to be heated, and a switch to reduce or cut-off the output from the generator when the reflected energy reaches a predetermined magnitude. The switch may be a relay having its operating coil 4 connected in series with thermistors T1, T2 in one limb of a Wheatstone bridge 3. A standing bias current i is applied to the bridge such that the thermistors have a resistance which just balances the bridge. The coupler F is connected to the waveguide wall by a resistor R and through a coaxial cable 2 to the junction between the thermistors, the outer conductor of the cable being connected between the waveguide wall and through capacitors C1, C2 to the outer terminals of the thermistors. The current produced in the coupler F by reflected energy flows through the thermistors in parallel and reduces their resistance value to cause operation of the relay. A meter M may be connected across the bridge to give a measure of the reflected energy.