Measuring inrush currents.

[NDG Elecs]

Hi,

Apart from spending £300 on another clamp meter (although I am tempted!) is there a cheaper way to measure inrush currents? Can't seem to find anywhere to hire one either for less than £100.00. Oscilliscope either. This is a one off job and am curious about the actual inrush on some LEDs. I have analogue and digital multimeters, clamp meters and MFT. But cannot think of anyway to measure it.

Thanks, Neil

 

[Andy78]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

 

[HandySparks]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

I spent some time a while ago measuring inrush on 24V DC LED drivers after a problem with a tripping MCB after I installed some under cupboard lighting. Turned out that the inrush on the ones I fitted first was huge. I have a Metrel MD9270 which seemed pretty good for the job.

 

[davesparks]

In an ideal world you'd want an analogue instrument with a peak hold function

 

[NDG Elecs]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

There is a Fluke clamp meter that measures the first five cycles of the sinewave (or something like that!).. So as you say, that would be the fast sampling/testing part. Anologue meters would probably just get the needle doing a quick dance.

 

[NDG Elecs]

I spent some time a while ago measuring inrush on 24V DC LED drivers after a problem with a tripping MCB after I installed some under cupboard lighting. Turned out that the inrush on the ones I fitted first was huge. I have a Metrel MD9270 which seemed pretty good for the job.

Cheers, was just looking through the spec for that, it doesn't seem to mention that it measures inrush (that's to say the quick initial surge that wants to be recorded).
Out of interest what kind of peak were you getting before it steadied? Numbers please!

 

[NDG Elecs]

In an ideal world you'd want an analogue instrument with a peak hold function

That's what I initially thought, but as the inrush is so quick I figured the needle would swing left to right really rapidly before steadying at the normal current. And consequently I am not sure how accurate the peak hold would be.

Does such a beast exist, if so please tell!

 

[HandySparks]

Cheers, was just looking through the spec for that, it doesn't seem to mention that it measures inrush (that's to say the quick initial surge that wants to be recorded).
Out of interest what kind of peak were you getting before it steadied? Numbers please!

Instruction manual here:
http://www.metrel.si/fileadmin/BAZA..._Function_Multilingual_Ver_1.3_20_751_451.pdf

I had it set to 'peak' with max hold. From memory, I was getting around 150A from three 36W drivers on one circuit. However, this may have been for less than one cycle. Nevertheless, it was a good tool for comparing one driver with another, even if the absolute values weren't necessarily that useful. I think that, momentarily, the current was only limited by the line impedance of the circuit.

 

[NDG Elecs]

Cheers Handy.

That may make a nice back up to my present ELCM. I think I will contact Metrel and see what they say. The spec seems to imply it measures the peak value for 25 micro seconds, which is a very short duration. But you have obviously got results from it. Maybe I have misread something.

The Fluke 374 covers the first five cycles ie 100 milli seconds.

 

[HandySparks]

Cheers Handy.

That may make a nice back up to my present ELCM. I think I will contact Metrel and see what they say. The spec seems to imply it measures the peak value for 25 micro seconds, which is a very short duration. But you have obviously got results from it. Maybe I have misread something.

The Fluke 374 covers the first five cycles ie 100 milli seconds.

My interpretation of the 25 micro second figure is that it samples the waveform (current or voltage) at a rate of 40kHz. I've just had a quick check. The 'peak' function has 'max hold' built in, so it displays the maximum sample measurement (not the RMS figure) since the function was selected.

Measuring the supply to my 5W LED table lamp (not during start up), the RMS current is 55mA (power factor is not 1). The 'peak' value is 300mA, so it'll be a pretty odd waveform.