| 摘要 |
A residual current operated device (10) comprises means (24, 26) for sensing an imbalance between currents flowing in line and neutral conductors (12, 14) between an a.c. source (18) and a load (20), comparators (46, 48, 66) coupled to the sensing means and arranged to provide a control signal (66c) if the magnitude of said imbalance exceeds a predetermined value in either direction, and a gated oscillator (75) responsive to the control signal to provide pulses at a frequency substantially lower than that of the a.c. source, e.g. at about 1 Hz for a 50 Hz supply for causing a thyristor (74) to actuate the switch (22). Under certain conditions, such as reversal of the line and neutral connections to the device, when the device would otherwise be activated at the supply frequency, the reduced frequency may avoid destruction of the thyristor and/or the switch, and warn the user. The gated oscillator comprises potential divider (77, 77a) cooperating with positive feedback resistor (79) to form the non-inverting input of op-amp (75), the inverting input of which is coupled via a resistor to a reference potential and via series connected resistor (79a) and diode (82) to the op-amp output. |
