Patent GB-803352-A (GB)
Title: Improvements in or relating to high-frequency power supply arrangements
IPC: H05B6/04
Filed: 1956-05-31
Abstract: 803,352. R.F. couplings; valve oscillator circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. May 31, 1956 [June 3, 1955], No. 16861/56. Classes 40 (6) and 40 (7). In a circuit arrangement for feeding a highfrequency heating system of the type in which the inductor in the oscillator resonant circuit is inductively coupled to coil feeding the load the said resonant circuit is provided with an earthed connection positioned between the points at which the anode and cathode of the output valve are attached thereto so that the maximum voltage with respect to earth of the circuit ends is approximately half the peak voltage across the circuit. This reduces the insulation requirements of the output transformer and enables the spacing between the circuit inductor primary and the coupling coil secondary to be reduced. As the anode voltage supply source is connected to earth at one end it is necessary to insert high-frequency chokes in the leads between the source and the valve anode and cathode. In one embodiment of the invention the mid-point of the resonant circuit inductor 20 is earthed at 23 and the circuit capacity determined by capacitors 21, 22, the junction point of which is connected to the cathode of oscillator valve 1. The anode of the valve is connected to the resonant circuit by capacitor 6 and the grid by capacitor 10. A choke coil 7 is provided in the lead from the source 8 to the valve anode and a further choke 29 is provided in the valve grid connection. The grid resistor may either be connected to earth or to a source of negative bias 31 and the switch 28 for doing this provides a means for starting and stopping the supply of high-frequency energy as required. The coil .12 inductively coupled to coil 20 feeds the coil 15 surrounding the workpiece 16. The directly heated cathode of valve 1 is supplied with current from a transformer 18 and choke coils 25, 25<1> are inserted in the supply leads. The mid-point of the transformer secondary winding is earthed and the two halves of the winding short-circuited for high-frequency currents by capacitors 27, 27<1>. In a second embodiment (Fig. 3) in which parts of the circuit similar to parts of the Fig. 2 circuit have similar references the earth connection is made to a point on a capacitive voltage divider forming part of the resonant circuit; capacitor 21 being replaced by capacitors 34 and 35 with the earth connection at their junction. The arrangements for isolating the cathode for high-frequency are also modified by the omission of chokes in the leads from the transformer secondary to the cathode and the use instead of a single direct connection through choke 36 to earth. The primary winding of the transformer 18 has, in this case, to be fed through chokes and its circuit symmetrically earthed through capacitors 39, 39<1>. Capacitors 40 and 40<1> are provided to restrict high-frequency voltages in the primary of the transformer. In a further and preferred embodiment (Fig. 4, not shown) the arrangement of Fig. 3 is modified by the use of four series capacitors in place of the capacitor divider 34, 35, 22. The centre-point of the four is earthed and the two outer ends of the inner and equal capacitors connected to the anode through capacitor 6 and cathode respectively. The outer capacitors are also made equal and are attached to the inductor. One end also provides the signal fed back to the grid as in Fig. 3. In Fig. 4 the isolation of the primary of the heater transformer from high-frequency currents is effected by the use of a special transformer in which the secondary winding is arranged in an earthed brass or copper flanged screening cylinder having an axial gap to prevent it forming a short-circuited turn. A further capacitor 50 may be connected directly into the tuned circuit and the triode 1 may a provided with inductive feedback or may be excited by a preceding valve. Specification 716,891, [Group XXXV] is referred to.