An improved radio frequency cautery instrument comprising a radio frequency signal generator for supplying both monopolar and bipolar radio frequency signal outputs for medical applications. A control unit is interposed between the radio frequency outputs of the signal generator and the hand-held electrode assembly, and further interposed between a finger-operated switch on the electrode assembly and the power circuit of the signal generator, whereby the application of radio frequency energy to the electrode of the electrode assembly can be controlled by the user through manipulation of the finger-operated switch on the electrode assembly.

In a circuit arrangement for drive of a thyristor with a phototransistor, the collector-emitter path of the phototransistor is between one of two alternating voltage terminals and a gate terminal of the thyristor. In order to prevent an activation of the unilluminated phototransistor due to a steep voltage edge (dv/dt) load), its base current given such a load is carried off via an IGFET of the enhancement type. This IGFET is controlled by an IGFET of the depletion type acting as a current source which lies between a gate terminal and source terminal of the enhancement type IGFET. A capacitor is connected in series with the depletion-type IGFET. This series connection lies between the two alternating voltage terminals.

A circuit comprising an A.C. source, a load and a TRIAC for supplying A.C. power to the load from the source. The TRIAC includes first, second, and gating electrodes with the first electrode being connected in series with the load. The A.C. source is applied across the TRIAC and the load. A trigger circuit is provided for the TRIAC comprising a photodarlington circuit and a circuit for biasing the photodarlington circuit to its quiescent state in the absence of a light signal and for making the photodarlington circuit conductive and enabling it to apply a negative voltage to the gate electrode of the TRIAC to make the TRIAC conductive in the presence of a light signal.

An electrical switch which preferably can be remotely conditioned to an "ON" or "OFF" state by the use of light energy radiations, but which can also be controlled at the switch location, if desired.

A power supply receives and rectifies mains voltage using bridge D1-D4. This is used by a pair of transistors Q1 and Q2 to alternate the supply to a transformer T via capacitor C8. Outputs are available from secondary windings T2, T6 and T7. Further windings T4 and T5 provide an internal power source for system circuitry which includes optical regulation via IC1 and optical switching control using IC2.