Power supply circuit protection - senses current drawn by fluorescent tube circuit and shuts off power if current surge or loss of current occurs in any branch

摘要

A power supply protection circuit for a flourescent tube (L) having a series inductance (I) and parallel capacitance (C) comprises two current sensors (51, 52) in the capacitance and inductance branches, coupled to respective rectifier circuits (MF1, MF2) which provide a d.c. signal proportional to the sensed current. Each rectifier circuit applies its signal to a respective input of a three-input AND gate (ET) and a level detector (1, 2), the output of the latter being applied through an OR gate (OU1) and a memory inverter (M) to the AND gate third input. The absence of current or current surge in either branch prevents the AND gate from firing a thyristor (T) in the voltage supply, the memory (M) inhibiting the AND gate even if the current surge is only temporary. A time base circuit (CT) maintains the thyristor (T) conductive on initial switch on but is then rendered inoperative through the OR gate (OU1) if current surge occurs.