In the last posting, relating to the design of an isolated power supply – Designing an isolation power supply – an inrush current limiter was identified to solve a problem… (There most definitely was! The 2 A slow blow fuses didn’t stand a chance with the 300 VA toroidal transformer having a primary winding DC resistance of 3.92 Ohm at 230 V RMS.) So what to do?

The solution is some form of inrush current limiter and has led to the Inrush current limiter kit and Inrush current limiter PCB now available in the online store.

There are a couple of options:

  1. Negative Temperature Coefficient (NTC) thermistor.
  2. Fixed value resistor, switched out after the initial inrush period.
  3. Something more complex (depending upon the switching circuit power supply, etc.)…

A point to note here is as follows: the different options typically have fault modes which leads to any solution, upon the fault mode happening, becoming “interesting”. For example, option 1 uses an NTC thermistor which has a limited energy rating lifetime, after which the device fails. Similarly for option 2, if the controlling circuitry switching out the fixed value resistor fails, then the resistor will surely fail. As for option 3, it depends upon the details, but basically the situation is similar to options 1 and 2 – there’s a high voltage and current involved and something has to give…

For a review of the different possibilities see the discussion on the page:

Anyway, a relay based system using a fixed value resistor is straightforward and relays are reliable devices. Therefore this is the approach followed for the circuit detailed here.