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enter image description here

Case1: OUT Series res = 10 ohm -> spiky input current reaching upto 15 A -> capacitor charging fast enter image description here After a long time: enter image description here

Case 2: OUT series res = 100 ohm -> almost sinusoidal input current reaching up to 1.6 A-> capacitor charging slowly enter image description here After a long time: enter image description here

Looks like the behavior is same at steady state - whatever be the resistance.

Can the same approach (dynamically vary resistance to maintain constant current) be used to limit inrush current?

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  • \$\begingroup\$ Is this question related to one of your questions? \$\endgroup\$
    – Rohat Kılıç
    Commented Jun 13, 2022 at 13:19
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    \$\begingroup\$ You are not using/simulating a load (i.e. a restor in parallel to C1). This schematic is useful to generate a voltage, not a power supply. When a load is present, things change dramatically. \$\endgroup\$
    – linuxfan says Reinstate Monica
    Commented Jun 13, 2022 at 15:53
  • \$\begingroup\$ @Rohat: Yes, it is related. Since I didn't want to stuff to many questions in a single post, I made this as a separate question. \$\endgroup\$
    – Dynamic_equilibrium
    Commented Jun 14, 2022 at 4:38

3 Answers 3

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A common approach to make a soft start circuit is to use a PFET as a series element, with an RC that feeds the gate.

schematic

simulate this circuit – Schematic created using CircuitLab

The FET acts as a variable resistor, that starts as a very high resistor value. As the R2 C1 voltage increases the gate drive, the resistance lowers and allows more current to pass. Eventually, the PFET is fully turned on, and its resistance is only a few milliohms.

This is one of many answers on this site that discusses this topology: https://electronics.stackexchange.com/a/344821/166672

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Thermistors (NTC Type) can be made to specifically solve inrush current issues. Initially at power on the thermistor is cold and the resistance high, this limits the inrush current. Inrush plus normal operating current causes the thermistor to heat and resistance decreases, reducing operational voltage drop.

One such manufacturer is below (there are others): https://product.tdk.com/en/products/protection/current/ntc-limiter/index.html

Slightly more complex is to have a relay short the thermistor after a short period to reduce the power loss further.

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Yes, a resistor can be used, and has been used in various circuits to limit inrush current.

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