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What does Rush current mean?

Latest Updated:08/01/2005

Question:

Rush current

Answer:

This is a current that flows like when short-circuiting occurs, because of charging and discharging of a capacitor.

One example of a rush current is a high current that flows out of the power supply unit at power-on because a smoothing capacitor or a bypass capacitor is charged.
Another example is a case where a capacitor is connected between a digital signal line and ground to prevent noise.

In this case, a high current flows when the signal changes because the driver side is short-circuited, causing degradation of characteristics and malfunctioning.
Inserting a current-limiting resistor is effective to suppress such a rush current.

When the power is turned off, the input voltage rises beyond the supply voltage (0 V) and exceeds the absolute maximum rating because the capacitor is charged.
Therefore, a protection diode that discharges the current to the power supply is required. A simple countermeasure against noise consists in inserting a pull-up resistor.

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