I'm looking at powering an embedded board PC using a 12V PSU and I'm having some trouble due to inrush current. The board draws 5A for about 1ms on startup and unfortunately this trips my PSU's overcurrent protection. I did some research online and it looks like I can get an inrush current limiter from Digikey (http://www.digikey.com/product-search/en/circuit-protection/inrush-current-limiters-icl/656273).

My question is will these inrush current limiters work for a DC output, or is there a better way around this? I've only seen people talk about using the inrush limiters at the input side of AC-DC converters, although some places mention it can be used in DC scenarios, but with no examples.

If these inrush current limiters will work for my purpose, do I just pick one with a steady state max current that's lower than my PSU's maximum output, and pick one with an R@25c value that gives me an acceptable current limit at 12V?


1 Answer 1


They work on dc just fine and your reasoning about choosing the correct one is also sound. You probably know this but inside is basically a thermistor that has a negative temperature coefficient so when cold they will exhibit higher electrical resistance and reduce inrush current. They rapidly warm and the resistance drops.

You might also try inserting the equivalent to the warm resistance in series. This might be enough on it's own.

One caution; if you switch power off then restore it too quickly you may still get an inrush problem because the device hasn't cooled down sufficiently. There are mosfet circuits that do the same thing too.

  • \$\begingroup\$ Hi Andy, thanks for the quick reply! I know about how they are essentially an NTC thermistor, but I guess I have some more questions now that I think about it more. If I were looking at this unit (digikey.com/product-detail/en/MF72-005D5/317-1144-ND/1190919), it says the steady state max is 1A, but the resistance at cold is 5 ohms. How does this prevent the 5A being drawn from my PSU? You're supposed to connect these in series between your supply and load, right? \$\endgroup\$ Feb 7, 2014 at 0:22
  • \$\begingroup\$ when the trip current limit of the power supply is close to the normal running current, these devices become less effective. That's the bottom line. You should concentrate on finding a device with as much series hot resistance as your load can tolerate then, check it won't be over powered by normal currents then work backwards to see what the cold inrush current will be. If this doesn't stack up you should consider a different technology. \$\endgroup\$
    – Andy aka
    Feb 7, 2014 at 0:28
  • \$\begingroup\$ you may have to bite the bullet and use a mosfet with a CR delay and ramp on the gate. Yes they go in series \$\endgroup\$
    – Andy aka
    Feb 7, 2014 at 0:31
  • \$\begingroup\$ if you need reverse polarity protection, the standard mosfet circuit that doe that can be easily modified to have inrush limiting with an RC circuit. You might also want to think about how long the inrush situation prevails and this will help figure out a mosfet circuit. You can of course shunt the thermistor with a mosfet once inrush period is nearly completed. \$\endgroup\$
    – Andy aka
    Feb 7, 2014 at 0:35
  • \$\begingroup\$ Okay, I will take all that into consideration. I'll also do some more research, the working principle made sense earlier but before I blindly use something, I want to make sure I know what's going on. \$\endgroup\$ Feb 7, 2014 at 0:36

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