# Inrush Current Depends on the Power Supply Rise Time

I have a query.

I understand that the inrush current is the current that appears during the initial turn ON of the Board so as to charge all the Capacitors present in the board.

But I think that this initial inrush current not only depends on the capacitors that are present on the board but also on the Power Supply rise time.

I found that there is a difference in the inrush current values when my power supply rise time is 300us and when my rise time is at 100ms.

My questions

1. What should my actual power supply rise time be so as to calculate the maximum initial inrush peak current of my board? Is there any sort of standard regarding this Power Supply rise based on application?

Based on the power supplies that one is using, power supply rise times tend to differ. I assume it is due to the presence of the capacitors at the outputs of the Power Supply.

In real life, my PCB board runs on a 12V Lead Acid Battery. How to simulate the Power Supply rise time of the Lead Acid battery using a normal 32V/5A power supply.

2. How does the inrush current vary when one uses a Bench Power Supply and when one uses a 12V Lead Acid Battery.

• How will your circuit become connected to the battery? How long are the wires, what impedance are the wires? Sep 10, 2020 at 9:38
• The Wires connected to the battery are 25 sq.mm and 4 feet each
– user220456
Sep 10, 2020 at 9:40
• I never see a different approach: expect external Power supply's to deliver maximum. Do not expect external Power supply's to have a soft start or such. If you want to control inrush current do it on your board. Sep 10, 2020 at 14:56
• Yes I agree. But in general, I am asking. Say my power supply maximum voltage for the board is 16V. What should be the ideal Rise Time for a power supply from 0V to 16V or in the case of the 12V Lead Acid battery from 0V to 12V?
– user220456
Sep 10, 2020 at 15:17
• well without further constraints its zero nanoseconds. seriously. if your load needs more current than available only at turn on, ramping up the power supply might do the job. But then it is determined by the current. Also you might not wan't a fuse to trigger. But there is no general rule: a 16V power supply should have a rise time of X. Sep 10, 2020 at 16:55