(Not) measuring inrush current with the GW-INSTEK GMP-8310 power meter

[RBBVNL9]

Hi all,

Recently, a viewer of my YouTube channel asked wonderer if the GW-INSTEK GMP-8310 could be used to measure inrush current (a.k.a. switch–on surge, or surge current). This instrument does not have a specific measurement mode for this (and the manual does not mention inrush current measurements), but it still may be possible, I thought. After all, this device has sufficiently fast DACs (after all, for a signal with a fundamental frequency of 67Hz it can determine up to the 50th harmonics…), so, in principle, it should be able to pick up fairly short spikes. I was also interested in this, also to see whether we could determine, for a given DUT, how inrush current depended on the power supply phase point when powering on. But anyway, let’s start with the basis.

Today, I did a couple of experiments and it seems to measure inrush current is not that easy indeed. As a DUT, I used a USB-C power adapted that shows visible spikes when I put it into an outlet, so I expected to see some quite notable inrush values.

Firstly, the device does boost an ‘MAX Hold’ mode. When playing around with this, I realized that this mode only takes the max values at the maximum screen update rate of 0.1s. Not sure how the values in this interval are exactly determined, but it seems its not reporting the highest peak in that period.

Second, I tried the data logging mode, as described here. Also this way, the values we get are limited to a maximum screen update rate of 0.1s, so again, not showing the real peaks.

Third, I tried the Windows software utility. It's quite a challenge to get the interface working (I never got the USB working; did eventually get the LAN connection going after many error messages). With this utility, logging gets a bit more interesting. It saves a set of 8 different files:

• One file contains measurement values at (again) measurement intervals of 0.1s or higher, and an associated graphics file is added with the voltage component.
• Another file contains measurement values of the 50 harmonics, and two associated graphics files are added with bar graphs for voltage and current harmonics, respectively.
• And finally, one file contains higher sample rate measurements of voltage and current, and two associated graphics files are added with graphs for voltage and current (oscilloscope-like waveforms). For these measurements, I observe about 240 measurement values for a single cycle of a 50Hz signal, so about 12k/s. Great, probably good enough to capture inrush current. But there is a problem. We get only 600 measurement values in total (so about 2.5 cycles). As far as I can see, these represent the last couple of milliseconds of the total measurement interval. If you wish to observe an inrush current here, you somehow would need to figure out how to turn on the DUT in the last few milliseconds of the measurement interval, an interval which you need to set in advance in the Windows app.

So, all in all, I did not find a doable way to measure inrush current yet with the  GMP-8310 yet. Too bad, as its hardware should be able to do this, and this type of measurement is available in some other digital power meters. Within the same price category, the R&S HMC8015 has such a mode, and also the Itech IT9121E can measure inrush currents, according its specifications

Maybe I’m overlooking something and someone knows how to do this? If so, let me know!

Best, Rudi

[_Wim_]

Hi, I do not own an GW-INSTEK GMP-8310, so I cannot give much advice, but did you try those tests with manual or auto-ranging? Typically range switching is not fast enough to capture inrush events, so that might be the cause. If it is really related to display update rate, that would be a shame for an instrument with these kind of specs. 

[adam4521]

I've done some experiments in this area, although not with a power meter like the GW Instek. There's three things I'd note:
1. The inrush into X and Y capacitors is very high compared to the nominal current. It's like a short circuit of very short duration. Are you wanting to see that? You will need very high dynamic range to observe both nominal and peak inrush currents. Probably need to look at one or the other, on separate captures.
2. I've seen very high frequency transient oscillation at the inrush, like 10MHz+ ringing, due to the interaction of the capacitor with other system elements. This won't be visible at all on the lower capture rates used for mains+harmonic current measurement.
3. The probing, especially if it uses a shunt resistor, could change circuit parameters enough to significantly affect the measurement accuracy.

[RBBVNL9]

Dear Wim, adam4521,

Thanks a lot for the replies!

First, some good news: eventually, I managed to find out a way how to measure inrush currents with this instrument. It can be done by selecting a measurement called “IpK” (there is both a + and – variant), which is not among the 10 measurements shown by default on the main screen, but can be selected to replace one. While the normal “I” measures the rms value over the measurement window (set by the selected screen update time), “IpK” measures the current peak in that period. What works best for me is setting the update rate to 1s or 0.5s only, activating MAX Hold, and then observing the maximum peak current.

Concerning the comments made, they are all well taken.

Typically range switching is not fast enough to capture inrush events, so that might be the cause

Good point. I did indeed select manual range modes to prevent issues when the device has to go to another autorange. In this meter, one furthermore can select Crest Factor values that allow a (peak) measurement to be 3 times the maximum set range value, or 6 times, or even higher (the “6A” mode).

1. The inrush into X and Y capacitors is very high compared to the nominal current. It's like a short circuit of very short duration. Are you wanting to see that? You will need very high dynamic range to observe both nominal and peak inrush currents. Probably need to look at one or the other, on separate captures.

True, and my experiment was to explore this a bit. Once I found the IpK measurement, I indeed started to observe quite high values: while the DUT (a USB-C GaN power supply) has a nominal current draw of 7mA, I observed inrush current values of over 3A. Putting aside other questions about whether it is correctly measured (see below), this indeed calls for a great dynamic range. With its 16 bit A/D converters, the GMP-8310 seems to have quite a good range. On the top of my head, I think I was able to measure continuous values around 7mA nicely at the same time peaks of up to 6A (2A current range plus CF3 Crest Factor, allowing for 2x3=6A maximum values).

Also, I’d be interested in seeing how much inrush current is as a function of the phase of the AC signal at which the device is turned on. I have some DIY ideas about to automate that ;-)

2. I've seen very high frequency transient oscillation at the inrush, like 10MHz+ ringing, due to the interaction of the capacitor with other system elements. This won't be visible at all on the lower capture rates used for mains+harmonic current measurement.

Interesting. The next step I might want to take is measurements with a faster capturing device (oscilloscope) and using current probes.

Don’t have that right here: every year, during seasons holidays at relatives abroad, I take just one instrument with me which I want to get to know better, and this year, it’s the GMP-8310 ;-)

3. The probing, especially if it uses a shunt resistor, could change circuit parameters enough to significantly affect the measurement accuracy.

Indeed, so that’s why I want to use some current probes for some future experiments.

Thanks again, merry x-mas,
Rudi

[RBBVNL9]

As noted, GW INSTEK does not mention at all inrush measurements in relation to the GPM-8310… I have been wondering why. Perhaps because of some of the challenges mentioned above?

This also made me wonder how it compares to the Rohde & Schwarz HMC8015, a power meter whose documentation does explicitly mention inrush current measurements. The HMC8015 retails here at €2400, the GPM-8310 at €1700 (both including VAT).

The documentation of the HMC8015 (see here) explains inrush current measurements can be made in the two following ways:

• A peak current measurement (called IPPeak) that can be used in combination with the Peak Hold function. This measurement seems similar to using “IpK” and MAX Hold with the GPM-8310.
• A special graphic inrush screen showing a triggered display of single shot waveform. This is something the GPM-8310 does not offer.

Both these functions, however, are not standard on the HMC8015 but require an additional license HVC151, which costs another €500 (inc. VAT).

According to the HMC8015 brochure, for the graphics inrush mode, “8192 samples are acquired at all times with a logging period of 16 ms to 67 s”. That suggests a sampling rate of ~512kSa/s. This is a bit higher than that of the GPM-8310 (300kSa/s, according to its User Manual), but it is not an order of magnitude higher. So, it seems as if neither instrument will be able to pick up high-frequency components as commented by adam4521 above.

Best, Rudi