Author Topic: 121GW - Frequency measurement inrush current  (Read 456 times)

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Offline final

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121GW - Frequency measurement inrush current
« on: June 19, 2018, 04:55:52 am »

I just received my 121GW about three hours ago. First off: very nice that it comes with batteries and a 8GB microSD preinstalled. I'm sure everybody but me was aware of that, but I was pleasantly surprised. I updated my firmware to 1.21 without issues. So far I'm very happy with the meter.

Since I didn't have a circuit lying around, I figured I'd play with the circuit that's always available - mains voltage! For the record, I live in Europe, so we're talking 230VAC @ 50Hz.

I plugged in the meter and measured some voltages and frequency and checked that it doesn't blow up on bad settings and so on and all of a sudden, I hear a "clack" and I'm in the dark. Turns out that my 10mA RCD had triggered while measuring frequency against ground. I thought this was very interesting, so I took out every meter that I own and checked if they'd trigger the RCD as well while measuring frequency. Here are the results:

EEVBlog 121GW: yes
Brymen BM235: no
Voltcraft VC890: yes
Sanwa PM300: no
Voltcraft VC-540: yes
Siglent SDM3055: no

Funnily enough, no meters triggers the RCD when measuring voltage, despite all three who do trigger the RCD when measuring frequency have the frequency on the secondary display when measuring voltage!

I thought this was quite interesting, so I went on and tried a 30mA RCD. Here, only the 121GW managed to trigger the RCD. Lastly, I hooked up two multi meters in a row, one (Voltcraft VC890) measuring current and the 121GW measuring frequency behind the VC890. It's not a very precise result, but the VC890 showed as much as 60mA current flowing initially when doing frequency measurement. This seems to behave like a kind of inrush current, as the current drops very quickly to 2-3mA. I was too lazy to bring out the oscilloscope with a shunt or a multi meter peak measurement (not sure if they work with current anyway).

Still, I find it very interesting that those meters behave quite differently and draw a significant amount of current when measuring frequency, even if only for a short period of time. But 230VAC * 60mA is roughly 14W! So there's serious power flowing through your meter when measuring frequency.

I have no idea how a multimeter actually measures frequency and whether such inrush currents are normal (I guess they are). Does anyone like to enlighten me?

« Last Edit: June 19, 2018, 05:02:17 am by final »

Offline HKJ

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Re: 121GW - Frequency measurement inrush current
« Reply #1 on: June 19, 2018, 05:58:43 am »
Many multimeters have low input impedance on frequency input with higher input voltage:

You can see a curve here from my UNI-T UT195DS review (This is typical for most meters):

Offline genghisnico13

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Re: 121GW - Frequency measurement inrush current
« Reply #2 on: June 20, 2018, 11:12:24 am »
It's a nice feature, you can test RCDs/GFCI with it ;).

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