# Estimating load capacitance from inrush current

I need to estimate the capacitance of a load for choosing a PSU for it, as they state the maximum capacitance. It is a strobe light so I suspect it will have quite a bit of capacitance. I've tried measuring the capacitance but this is much lower than I suspect is in there at 6uF. The strobe is 24V and the inrush current is 4.8 amperes.

• If you want to estimate an equivalent capacitance then you need to know how long this inrush current lasts. Strictly speaking, you need to integrate the current over time to find the total charge delivered to the load. Nov 10, 2021 at 16:19

## 3 Answers

It is very difficult to estimate the capacitance of a load just by measuring the inrush current, since it depends on the ESR of the capacitor plus the elements it may have in series. What you can do is insert a known series resistor and using an oscilloscope, take a shot of the input current waveform, and studying its shape you may be able to calculate the RC time constant of the inrush current decay, and the involved maths will give you the value of the input capacitor.

The inrush current is merely a measure of the step input voltage impedance, which for simply a capacitor is the Vin/ESR=Imax. Assuming the supply is lower impedance, and constant voltage, then the ESR*C=63%T can be measured by the 63% below peak time and Thus C can be estimated from that.

If your PSU lab supply dips during power on, then you are looking at the ratio of source to load cap ESR.

for PSU power specs ...

By knowing the type of capacitor and ESR*C=T time constant (film vs ceramic vs electrolytic) and measuring the start surge, you can estimate the energy lost in charging it up from $$\1/2CV^2\$$ and thus knowing V can estimate losses of $$\(I^2*ESR + V*I)*T/3\$$ for a triangular approximation using 1/3 the peak current. (Assuming no load regulation dip, otherwise factor this as well)

Then knowing the energy of the strobe pulse VI*t and it's repetition rate and equivalent resistance compared to the ESR of the cap, determine how long the cap can maintain voltage regulation say within 10% before the supply must restore voltage with current. specs are needed for Vsag, mJ of pulse and f rep rate before PSU power can be determined then add margin and verify Zout/Zload which is your supply load regulation error (%).

Almost all modern PSU's have a soft-start feature that allows the pwm controller to control the current delivered to the load at start-up.

• Thanks, yes, but I'm going to be introducing this high capacitance every 300ms or so. Nov 11, 2021 at 7:16