A current-to-current isolator is utilized in a monitoring system between a signal transmitter and a control room instrument. The isolator has a master oscillator, including a first transformer, a slave oscillator, a current transformer and a rectifying circuit, so that the current signal at its input is closely reproduced at its output. The isolator is powered by the signal current from the transmitter and does not require an external power source.
A telephone apparatus powered directly from the telephone lines without external power includes a current converter in which current from a telephone line is limited by a transistor, current-sense resistor, and optical coupler acting together as a constant current device. This limited current is then converted from direct current (d.c.) to alternating current (a.c.) at the primary of a current transformer by power switching transistors driven by a flip-flop which is in turn driven by a free-running constant frequency oscillator. The alternations are symmetrical and their period or timing controlled so the current transformer does not saturate, thus maintaining maximum efficiency of conversion. The transformer outputs, after being rectified and filtered, are isolated from the primary, and provide d.c. current to maintian the charge on a battery across one output and to power telephone circuitry within a telephone device connected to the d.c. outputs. An optical coupler acting as an isolated current switch turns the constant current device off and on to facilitate pulse dialing. The current converter boosts the low telephone line current to a higher current or voltage for use by telephone devices such as pay phones, key phones, modems, or other telephone line connected devices.
A turn-off relief network for a direct voltage converter includes a transformer having a primary winding. At least one clocked power transistor is connected in series with the primary winding. A first series circuit connected parallel to the primary winding includes a diode and a capacitor. A second series circuit connected parallel to the capacitor includes a reversing inductor, a reversing diode for a capacitor voltage and another transistor.
A testing apparatus tests the performance of an electronic device having an operation circuit for providing a useful output signal. A demodulator configured to provide a phase or frequency demodulated signal related to the output of the operation circuit is packaged with the operation circuit. The gain of the demodulator is controllable from outside the package. The testing apparatus analyses the demodulated signal and controls the gain of the demodulator.
