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Electronics => Projects, Designs, and Technical Stuff => Topic started by: TheAmmoniacal on June 23, 2016, 03:49:21 pm

Title: Cheapest way of measuring inrush current
Post by: TheAmmoniacal on June 23, 2016, 03:49:21 pm

I am interested in measuring the inrush current to a transformer, it's not a measurement I've done before or plan to do again - so won't invest in a clamp meter with that functionality. So can it be done cheaply? With tools I already have? (U1272A DMM and Fluke/Philips PM 97 scopemeter)

Would a simple clip-on current transformer like this http://www.ebay.com/itm/SCT016-Split-Core-Current-Transformer-Input-0-01-120A-Output-40mA-1-3000-Black-/162103229324 (http://www.ebay.com/itm/SCT016-Split-Core-Current-Transformer-Input-0-01-120A-Output-40mA-1-3000-Black-/162103229324) work? Hooked up to a the U1272A in min/max mode? Or put an appropriate resistor in series and trigger on the voltage across it with the scopemeter?

Title: Re: Cheapest way of measuring inrush current
Post by: ebclr on June 23, 2016, 04:46:59 pm

http://www.ebay.com/itm/30A-range-Current-Sensor-Module-ACS712-Module-Arduino-module-NEW-/281683074851?hash=item41959e9f23:g:W2IAAOSwBLlVSKtj (http://www.ebay.com/itm/30A-range-Current-Sensor-Module-ACS712-Module-Arduino-module-NEW-/281683074851?hash=item41959e9f23:g:W2IAAOSwBLlVSKtj)

Title: Re: Cheapest way of measuring inrush current
Post by: rob77 on June 23, 2016, 05:10:32 pm

just use a regular current transformer (e.g. Talema AC1010) - feed the wire in which you want to measure the inrush though it and terminate the output winding with a 100ohm resistor. then hook a scope across that resistor , set the trigger, hit one-shot and power up the load/transformer whatever. the CT i mention as an example has a ration of 1:1000 so for 10A current through the wire you'll get 10mA in the output winding - yielding 1V across that 100ohm resistor.

Title: Re: Cheapest way of measuring inrush current
Post by: uncle_bob on June 24, 2016, 12:16:45 am

Hi

Is this an AC transformer? :)

The inrush current will vary a bit depending on where in the AC waveform the "switch" is closed. If you want to do a reasonable job of characterizing a part, the switch may be the most exotic item in the test set.

Bob

Title: Re: Cheapest way of measuring inrush current
Post by: Circlotron on June 24, 2016, 01:13:24 am

Worst case inrush current to the transformer will be no higher than dictated by the DC resistance of the primary e.g. 120VAC and 2 ohms = 60 amps rms worst case. Depends on where in the AC cycle you switch it on. Zero crossing is worst. I used the word worst too much.