I have a temperature controller supplied with 230VAC via a 0.5A circuit breaker curve C. The thing is, the MCB trips as soon as power is turned on. I have measured the inrush current profile without the MCB with a current probe: up to 90A for 20ms!

I can see three options:

  1. A capacitor is first charged when power is turned on, and a second "power" switch allows the capacitor to discharge. I don't want another switch, so I'd rather avoid this option.
  2. A RC + MOSFET ramps up the voltage. If possible and safe, I'd like to avoid adding components as it is a real hassle to go through the design update process.
  3. The 0.5A nominal is changed for 10A. This is the quickest option as I have some around and it doesn't take much to change a value on a drawing and get it signed off; however that allows more potential destructive power in the downstream components (a couple of relays, a AC/DC, in addition to the controller). The technician used the same wire gauge everywhere, so up to 10A wouldn't melt the wires.

Is there any other option I am not aware of? If there isn't, then this post serves to those in the same situation. After all that's what forums are for.

  • \$\begingroup\$ What is the purpose of the circuit breaker? Normally the only purpose is to prevent fires. If you are thinking that the circuit breaker needs to do something else, you should come up with a way to simulate that case and test it. What gauge wire did the technician use? \$\endgroup\$
    – user57037
    Commented Jul 22, 2015 at 20:45
  • \$\begingroup\$ Can you get Curve D breakers at 0.5A or somewhere closer to it than your suggested 10A? If you fit a Type D breaker you need to ensure the earth impedance is adequately low (to increase the fault current if a short circuit occurs) - this should be easy at 0.5A * 20. This page shows at least one 2A D breaker (for which your earth connection would need to carry 40A until the breaker tripped) : neweysonline.co.uk/… \$\endgroup\$
    – user16324
    Commented Jul 22, 2015 at 21:36
  • \$\begingroup\$ mkeith: firstly this, for the wires, but those 0.5A I also wanted to reduce short circuit currents to avoid permanent damage in the components - but it is certainly utopic as anything with a capability of a few 10's of mA will burn out long before that, right? \$\endgroup\$ Commented Jul 23, 2015 at 9:39

1 Answer 1


NTC inrush current limiter, might be an option if you can find one that fits your requirements. The NTC inrush current limiter is a negative thermal coefficient in other words its resistance decreases as it warms up, so this devices limit inrush current by acting a series resistance and as current goes through it warms up and its resistance drops allowing higher currents to go through. You can check more info in wikipedia Edit: This previous answer looks like it might be helpful.

  • \$\begingroup\$ Don't you mean "its resistance decreases as it warms up"? \$\endgroup\$
    – brhans
    Commented Jul 22, 2015 at 20:51
  • \$\begingroup\$ You are definitely correct!!! Thanks for pointing that out \$\endgroup\$
    – Kvegaoro
    Commented Jul 22, 2015 at 20:53
  • \$\begingroup\$ Thanks. I had forgotten about NTC, that's a good idea. However in this specific case (and althought that might have been what I asked for...), isn't limiting the current going to temper with the boot sequence of the controller? Unless the inrush is purely for the bypass cap, and the current profile seems to support it's 1mF. \$\endgroup\$ Commented Jul 23, 2015 at 22:48

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