Abstract: A maintenance termination unit (10) comprises a voltage sensitive switch (30,50) connected permanently in series with each loop conductor (11,13) and an impedance termination unit (18) connected permanently across the loop conductors. Each voltage sensitive switch (30 or 50) comprises a triac (32) connected in series with a loop conductor (11 or 13), a capacitor (34) and resistor (36) connected across the terminals of the triac, and a bilateral switch (60) connected between one of the terminals and the gate electrode of the triac. In response to a signal having a voltage greater than the threshold voltage of the switch, the capacitor is charged to the threshold voltage, thereupon the capacitor discharges insuring that the triac is gated to the ON state.

Abstract: DC-to-DC converter circuitry which provides input-to-output isolation, sensing of load conditions and minimal standby losses comprises: a blocking oscillator including switch means (10) and energy storage means (50); and control circuitry (12,32,46,47) for sensing load demands and reducing the oscillator frequency under no-load conditions.