The electronic circuit shown in the figure regulates the inrush current that arises upon initial application of voltage to capacitors. This inrush-current-control circuit is intended principally to be incorporated into an electronic instrument in which capacitors are used to filter out current spikes and noise that would otherwise be impressed on the instrument power-supply bus. In the absence of a circuit like this one, voltage would be applied to the capacitors abruptly - typically by closing a relay; the resulting high inrush current could disrupt the power-supply bus and thereby also adversely affect the operations of other instruments connected to the same bus.
Shortly after turn-on, the inrush-current-control circuit causes the voltage on the instrument bus to ramp approximately linearly up to the full power-supply potential, so that the inrush current is constrained to be an approximately square pulse of controlled amplitude. In more detail, the sequence of events is the following:
Before power is applied, all capacitors are discharged. Upon initial application of power to the input terminals, Q1 becomes turned on, and C1 starts to charge through R1. The turn-on of Q1 causes the charging of C2 to full power-supply voltage. When C1 reaches full charge, Q1 becomes turned off; this allows C2 to discharge partially. When the potential on the left side of C2 reaches the threshold voltage of Q2, the output voltage begins to ramp up toward the full power-supply value.
This work was done by Steven Cole of Caltech for NASA's Jet Propulsion Laboratory. NPO-20403
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