Patent GB-938655-A (GB)
Title: Improvements relating to transistor d.c converters (inverters)
IPC: H02M3/338
Filed: 1959-05-21
Abstract: 938,655. Converting. E. I. BARDOS, and F. BARDOS. May 20, 1960 [May 21, 1959; May 28, 1959], Nos. 17232/59 and 18133/59. Class 38 (2). [Also in Group XL (c)] A single-phase, push-pull or polyphase inverter or converter circuit comprising a low voltage unidirectional source of supply; one or more semi-conductor devices in which current flow through a pair of electrodes can be regulated by a third electrode; a transformer having an individual tapped primary winding for the or each phase; and automatic starting means operative only in the absence of self-maintained oscillation to upset the equilibrium of the circuit to initiate self-maintained oscillation is characterized in that one end of the or each winding is connected to one electrode of a respective positive temperature coefficient non-linear resistor, the other electrode of the resistor is connected to the control electrode of the or a respective one of said semi-conductor devices, the other end of the or each winding is connected to one pole of the said low voltage source, the tapping on the or each winding is connected to one of said pair of electrodes of the said respective semi-conductor device, and the other of said pair of electrodes is connected to the other pole of the said low voltage source, and is further characterized in that the or each said control electrodes is connected to one electrode of a respective semiconductor diode, which can be switched by means of an electromagnetic or semi-conductor switching device to connect to said control electrodes a supply source of suitable polarity and magnitude to produce a reverse bias and hence mute oscillation. Starting.-In a first embodiment, Fig. 3, a push-pull D.C.-A.C. converter comprises a pair of transistors T1, T2 with non-linear temperature sensitive resistors HR for stabilizing the base current. Starting is effected by a relay R1 having a contact RC connected in the base circuit of transistor T1, which is closed to produce asymmetry in the circuit when no current flows through the relay. In a modification, Fig. 4 (not shown), instead of being connected directly in the base circuit the contact is connected across the D.C. supply in series with a further winding on the transformer. In a second embodiment, Fig. 5, instead of a relay a further transistor T3 is connected to the further winding W6 to produce the initially required asymmetry. On switching on, transistor T3 conducts to produce a starting pulse in winding W6. Once oscillation commences the voltage drop across resistor 4 due to the transistors T1, T2 conducting cuts off transistor T3, disconnecting winding W6. In modifications of this circuit, Figs. 6, 7 (not shown), the reverse bias for cutting off transistor T3 is produced by rectifying a part of the oscillator output produced in a still further winding, and using this to oppose the forward bias on the transistor. The push-pull arrangements described above, may be modified for polyphase working and a D.C. output may be produced e.g. by a Scott connection rectifier circuit. As an overload protection, the starting relay may be provided with interlocks. Muting.-In order to mute the operation of the circuit the bases of the oscillator transistors are connected via diodes D1, D2, &c.. Fig. 9, to a source of reverse biasing potential via a switching circuit X1 X2. Muting signals applied to the base of X2 switch X1 on connecting point P to the muting supply (+). The collector of X1 may either be connected directly to the muting supply, or via a bias source. In a first modification, Fig. 10 (not shown), the switching circuit X1, X2 is replaced by a zener diode, or similar two terminal device. The source of muting signals may be a bi-stable circuit, a photoelectric cell or a radio receiver. Specification 937,231 is referred to.