This topic contains 3 replies, has 2 voices, and was last updated by  Patrick Albersman 2 months, 3 weeks ago.

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    John Header

    Hi I’m John and have been doing this many years and I’m stumped with the inrush current test in Section 16. This forum looks pretty bare…maybe I’ll get lucky.

    Anyway the test requirements call for the max steady state current and specifies the source power supply voltage at a normal 28V. However a note explains that the max steady state current is typically at lower power voltages, WTF?

    How do we to determine the max current? Do we measure it at a low voltage or just use the ‘rated’ design value such as on the spec sheet? I think I understand this current is used to determine the a resistor that is placed in-line with power lead so that a 1.0V drop is produced in the test setup, right?

    I even paid and downloaded the DO-357 user guide and I want my money back, well maybe it will be useful as a learning tool.



    Thanks for posting on the forum. I have received a question similar to this so maybe this will help someone googling in the future. This section is confusing. It is too bad the user manual says nothing of value here.

    This section is really confusing for what I think is no reason. Here is what you should do.
    1) You should test your unit for the highest current draw. Do this by putting it in the mode AND voltage (within steady state operating condition defined in that creates this condition. This will determine the inrush limits.

    2) Set the power supply to 28.0V +/- 0.25 V dc to and your EUT to the highest current mode (but not the voltage). Tune the resistor so that you see a 1.0V +/- 0.1 V dc for 28 V dc equipment. This is now your setup.

    3) Apply both test conditions with 23.0 AND 30.3 V dc at the power supply regulation point. Since the power on inrush might have a different worse case than the steady state current draw test earlier.

    Post future questions here. If I can’t answer them, I will get someone who can!


    John Header

    Patrick, Thank you very much for this missing vital info for this test. Totally makes sense now. I’m a aerospace pal fan for life know 😁


    Thanks John!

    Just one last bit of info. If you don’t pass inrush the first time it can be successfully argued that the limit should be set by the spec limit since that is how the fuse is sized.

    Still, it is best to start with the tested max since that is more conservative being that it should be under the product specification limit.

    Thanks again for posting.

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