When electrical equipment is first turned on, a large current flows that exceeds the steady-state current value. This current is called an inrush current.
Why does this inrush current occur?
There are many factors that cause it, and the following are some examples:
- In equipment with large-capacity smooth capacitors or decoupling capacitors, when the power is first turned on, a large current flows through to charge those capacitors - a necessity when first powering up the equipment.
- Immediately after the power is turned on, the filament and other parts have low resistance, and a large current flows. (As they begin to generate heat and warm up, their resistance increases and the current drops to the steady-state current.)
To provide a more easy-to-understand image of inrush current, Figure 1 shows the current waveform when the power is turned on. When the power is turned on, current begins to flow, and the initial current flow reaches the peak current value that is larger than the steady-state current value. Following this, the current value gradually decreases until it stabilizes at the steady-state current. The part during which a large current flows before reaching the steady-state current is the inrush current. If the size of the inrush current exceeds that allowed by the part in use, depending on the magnitude of the inrush current (difference between the peak current value and the steady-state current value) and length of its duration (the length of time until the peak current value converges with the steady-state current value, hereafter called the pulse width), the part used in the circuit may overheat, potentially causing the electrical device to malfunction or break down.
Figure 1. Current waveform when the device is powered up.