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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Inrush-current-control circuit
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Overview
The document presents an inrush-current-control circuit developed by Steven Cole at Caltech for NASA’s Jet Propulsion Laboratory. This circuit is designed to mitigate the high inrush current that occurs when voltage is abruptly applied to capacitors in electronic instruments. Such inrush currents can disrupt the power supply bus, potentially affecting the operation of other instruments connected to the same bus.
The circuit operates by gradually ramping the voltage on the instrument bus from zero to the full power-supply voltage, thereby controlling the inrush current. This is achieved through a sequence of events initiated upon the application of power. Initially, all capacitors are discharged. When power is applied, a transistor (Q1) turns on, allowing a capacitor (C1) to charge through a resistor (R1). As C1 charges, another capacitor (C2) charges to the full power-supply voltage. Once C1 is fully charged, Q1 turns off, which allows C2 to discharge partially. When the voltage across C2 reaches a certain threshold, another transistor (Q2) activates, causing the output voltage to ramp up towards the full power-supply value.
The document emphasizes the simplicity and automatic reset capability of this inrush-current-control circuit, which is a significant improvement over prior art. The circuit effectively addresses the problem of high current spikes during power-up, which can disrupt the flight power bus in space instruments. By applying power in a controlled manner, the circuit ensures that the current drawn by the capacitors is a square pulse, thereby minimizing the risk of disruption.
Overall, this inrush-current-control circuit represents a critical advancement in the design of electronic systems used in space applications, enhancing their reliability and performance. The technical disclosure highlights the motivation behind the development, the problem it solves, and the innovative solution it provides, making it a valuable contribution to the field of electronic instrumentation.
