Electrical – Inrush current limiter resistor power dissipation calculation


Theoretically, I have read and understood that during the initial start-up of a circuit, the circuit will draw high current (known as inrush current), so as to charge the capacitors in its direct line to its input voltage value.

My questions :

  1. But, without performing any sort of testing, is there a way to estimate the value of the inrush current, provided we have the capacitance values? Or only after testing, do we get to know the inrush current value for the circuit?

  2. Suppose, we have figured out the inrush current. To limit the magnitude of the inrush current, we may employ a small value of resistor so as to not dissipate high power in the resistor during the standard operating conditions.
    But how to calculate the peak power dissipation during the inrush condition?

Suppose, I have an inrush current of 2.5A from 0 to 5ms, and then the current decreases to my actual load current of 500mA, how to calculate the peak pulse power dissipation of the resistor during that 5ms.

Is it simply ((I^2 * R) * 5ms)? Or what is the formula for calculating the inrush power dissipation?

Best Answer

Decades ago I was tasked with analyzing the power dissipation of switch-cap circuits.

As part of that, I wrote the integrals of energy in the switch and in the capacitor.

Turns out the SWITCH(RESISTOR) energy dissipation is exactly the energy stored in the capacitor.

Thus if you have a large audio amplifier with 100,000 uF capacitors on +50v and -50v rails, the energy stored in each will be

Energy = 1/2 * C * V^2 = 0.5 * 0.1Farad * 50 * 50 = 2500 * 0.1 * 0.5 = 125 joules

Assume your surge-limited resistor dissipates that energy in 1/120 second.