Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases. Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also feature high inrush currents relative to their steady state currents. This is typically the charging current of the input capacitance. The selection of overcurrent protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The overcurrent protection must react quickly to overload or short circuit but must not interrupt the circuit when the (usually harmless) inrush current flows.
When a transformer is first energized a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. This happens when the primary winding is connected around the zero-crossing of the primary voltage. For large transformers, inrush current can last for several seconds. Toroid transformers can have up to 80 times larger inrush, because the remnant magnetism is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop. This is caused because the transformer will always have some residual flux density and when the transformer is re-energized the incoming flux will add to the already existing flux which will cause the transformer to move into saturation. Then only the resistance of the primary side windings and the power line are limiting the current.
A resistor in series with the line can be used to limit the current charging input capacitors. However, this approach is not very efficient, especially in high power devices, since the resistor will have a voltage drop and dissipate some power.
Inrush current can also be reduced by inrush current limiters. Negative temperature coefficient (NTC) thermistors are commonly used in switching power supplies, motor drives and audio equipment to prevent damage caused by inrush current. A thermistor is a thermally-sensitive resistor with a resistance that changes significantly and predictably as a result of temperature changes. The resistance of an NTC thermistor decreases as its temperature increases[1] .
As the inrush current limiter self-heats, the current begins to flow through it and warm it. Its resistance begins to drop and a relatively small current flow charges the input capacitors. After the capacitors in the power supply become charged, the self heated inrush current limiter offers little resistance in the circuit, with a low voltage drop with respect to the total voltage drop of the circuit. A disadvantage is that immediately after the device is switched off, the NTC resistor is still hot and has a low resistance. It cannot limit the inrush current unless it cools for more than 1 minute to get a higher resistance. Another disadvantage is that the NTC thermistor is not short circuit proof.
Another way to avoid the transformer inrush current is a "transformer switching relay". This does not need time for cool down. It can deal also with power line half-wave voltage-dips and is short circuit proof. This technique is Important for IEC 61000-4-11 tests.
Another option, particularly for high voltage circuits, is to use a pre-charge circuit. The circuit would support a current limited precharge mode during the charging of capacitors, and then switch to an unlimited mode for normal operation when the voltage on the load is 90% of full charge.