I'm tinkering with what amounts to a "smart switched outlet"... which is really just a glorified way of saying I have a bunch of devices that run off 110VAC and need 12VDC for internal "stuff". Emphasis on "bunch".

I'm looking at e.g. IRM-03-12, which is stated as having 10A inrush current.

Now, for just one, that wouldn't be an issue, but I might have 4-8 of these on a circuit. Arranging for them to stagger-start isn't really an option (too much added complexity, and adding a manual process is highly error-prone anyway), which means my wiring is exposed to potentially 40A-80A...

What are my options for limiting inrush? What are the pros and cons of each, especially if I've trying to keep cost down? How would I "size" each option?

As I understand it, options include:

  • An inductor.
  • A simple passive resistor. (One on "L"? Two, with one each on "L" and "N"?)
  • A passive NTC. (Needs to be sized carefully or will be very inefficient?)
  • Something something PTC. (Not clear how these achieve the goal.)
  • Various combinations of the above plus a bypass relay. (How "beefy" does the relay need to be?)
  • Other?

I've read maybe a dozen articles, but have ended up with more questions than answers. I'm hoping to generate a comprehensive, novice-friendly answer that explains things in a way that's generally useful (at least with respect to AC/DC convertors as the cause of inrush). Also, please quantify drawbacks like "wastes power", e.g. how much power?

p.s. I'm ideally looking for something that is within 150mm², $10 and 4 components.

  • 1
    \$\begingroup\$ Why do you have to do anything at all about inrush? So what if you have 80A of inrush, what problems would that cause? \$\endgroup\$ Commented Jan 7, 2023 at 18:16
  • \$\begingroup\$ @DanielChisholm, tripping breakers, blowing fuses... is that safe on 15A wiring? What if these are on a switch; will the switch handle that much inrush? What's it going to do to other things plugged into my house? What if I want to use one of these downstream of a 5A relay? You actually raise a fair point for this specific case, but I'd still appreciate a better understanding of inrush limiting. 🙂 \$\endgroup\$
    – Matthew
    Commented Jan 7, 2023 at 19:21
  • \$\begingroup\$ You might take a look here and here as a starting point. For further calculation it would be nice to know the maximum and average current drawn on the DC side. A good matching NTC ICR might be found then. \$\endgroup\$
    – datenheim
    Commented Jan 7, 2023 at 20:53
  • \$\begingroup\$ @datenheim, "within the ability of the stated power supply". I'm not sure if I actually trust that my application is going to draw <5mA, but it's basically an indicator LED and a couple of MOSFETs, so... maybe. Might see very short and very infrequent maximums of twice that, but most of the time average and instantaneous current should be nearly identical. (BTW, I'd already seen that first link... and as to the second, I haven't a clue what the capacitance is, unless it's somewhere in the data sheet and I missed it.) \$\endgroup\$
    – Matthew
    Commented Jan 7, 2023 at 22:04
  • 1
    \$\begingroup\$ The inrush current may be 10A, but for how long? For something like an electric motor, the inrush current might be for seconds. In the case of the psu module, probably milliseconds. The mains wiring has impedance, circuit breakers have a trip curve based on current and time, same with fuses. The wiring rating is based on temperature rise in order for the insulation not to fail. In the inrush is only for milliseconds, then the instantaneous temp rise would be insignificant. Current and time are the critical values. \$\endgroup\$
    – Kartman
    Commented Jan 8, 2023 at 12:45


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