Tagged: Section 16 Inrush Current
September 22, 2017 at 9:01 pm #2405
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!September 23, 2017 at 2:35 am #2406
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 18.104.22.168) 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!September 23, 2017 at 6:53 pm #2408
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 😁September 23, 2017 at 7:19 pm #2409
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.March 6, 2018 at 2:53 pm #4577
Could you please elaborate and explain in further detail how step 1 determines the inrush limits? I have a product with nominal operating voltage of 28VDC. Should I connect my EUT to the power supply and adjust the voltage to 30.3V, 22V, and 18V per the table in 22.214.171.124 and then record the current draw at each of the voltages to find the maxim steady state load?
For example, if the current draw measured at 18V is higher than the current draw measured at 30.3V, 28V, and 22V then I should use the current measured at 18V as the maximum steady state load value in the requirements section when it says the peak inrush current shall be less than 9 times the maximum stead state load for 3.0 milliseconds. Do I multiply the current measured at 18V by 9?
Thank you.March 7, 2018 at 2:24 am #4578
You are correct. I forgot to mention section 126.96.36.199 AND 188.8.131.52 are steady state conditions. Find the “steady state” condition that has the worst case current draw between 18.0V and 32.2V.
Use that worst case current draw to set your limits of
9 times the maximum steady state load for the first 3.0 milliseconds,
4 times the maximum steady state load up to 500 msec, and
2 times the maximum steady state load up to 2 seconds and nominal thereafter.March 8, 2018 at 7:38 pm #4579
Thank you very much Patrick,
DO-160F goes on to say:
“At the maximum steady state load current draw, with the power source with-in it’s unloaded voltage tolerance, add a resistor between the power input hot lead and each EUT power input hot lead to adjust the measured voltage drop between the power source and EUT to the following…. 1.0V +/-0.1V dc for 28V dc equipment… During inrush test, with the appropriate series resistance to the EUT installed, the lab power source is to provide:…. 23.0 to 30.3 V dc…”
Does that mean I use the maximum steady state load current draw measured at the worst case current draw between 18.0V and 32.2V to determine the resistor value? For example, if I measure 1.53Amps at nominal operating voltage of 28Vdc and I measure 2.11 Amps at 20.5Vdc do I use the 2.11Amps to calculate the value of the resistor? My understanding so far has been that I use the 2.11 Amps to calculate the limits and then use the current draw measured at 28Vdc to determine the resistor necessary to create the 1V drop at 28Vdc. In this case R = 1V/1.53A = 0.65 Ohms. Is that correct? Or should it be R = 1V/2.11A = 0.47 Ohms? It doesn’t make sense to me to use 0.47 Ohm resistor it would not have a 1V drop when 28V is applied. But I just wanted to check with you first to confirm my understanding and see if you have any other feedback or guidance.
Thanks again for all your help and guidance.March 9, 2018 at 5:01 pm #4581
DO-160 says that you should be using the current at the maximum current draw to determine your voltage drop. That voltage drop should be “measured” (you can calculate to pick a value of course). Then, when using 28VDC is says “with the appropriate series resistance” and is no longer requiring a measured value.
– Add resistance to get measure voltage drop at your highest current
– Run test with selected resistance at nominal voltage
“At the maximum steady state load current draw with the power source with-in its
unloaded voltage tolerance, add a resistor between the power source and each EUT power
input hot lead to adjust the measured voltage drop between the power source and EUT to
4.0V +/- 0.3 Vrms ac for 115V ac equipment
8.0V +/- 0.6 Vrms ac for 230V ac equipment
0.5V +/- 0.1 V dc for 14 V dc equipment.
1.0V +/- 0.1 V dc for 28 V dc equipment
9.0V +/- 0.9 V dc for 270 V dc equipment
This voltage drop from the wiring to the EUT is to be measure within 7 inches (0.18 m)
of the regulation point of the power source and within 7 inches (0.18 m) the EUTs input
During the inrush test, with the appropriate series resistance to the EUT installed, the lab
power source is to provide:”
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