Test gear......Saturation testerII

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It's fairly easy to measure the inductance of things in the junkbox or check one you have just made but what about the saturation current ?? Well eventually I was forced to make a tester and am very pleased I did, well worth the effort and thanks to the junk box cost me nothing except time! It woks in conjunction with a DSO (If you don't have one go buy a good USB one at least, I use an OWON).

A capacitor bank is switched across the coil under test by a MOSFET and the current observed with a low value (e.g. 0R025) resistor. I found on my scope there was to much noise to reliably trigger on the current channel so instead another channel is used to measure the voltage applied (15V) and this gives a reliable trigger.

The capacitor bank needs to be large enough so there is not to much voltage sag during the measurement time to avoid corrupting the resulting current slope.

As for the control circuit it contains a timer set to 10mS to ensure the mosfet remains within its pulse rating, an UVLO circuit to ensure there is always sufficient gate voltage and an overcurrent cutoff to both protect the mosfet and the load, this latter is in my case set to 32A. This allows a variety of inductances to be tested (I have tried 2uH to 10mH) without any pot twiddling. The scope is simply set to single shot mode and triggered from the voltage channel but of course only the current is of interest. The whole rig is powered by my 15V/2A bench supply.

The scope shot clearly shows the knee of saturation at roughly 12A on this ~470uH choke wound on an old ETD54. Current is the red trace at 4A/div.

The scope can also be set to 50mV/div giving a sensitivity of 2A/div.

Hope this inspires you to be able to re-use coils and cores from your junkbox and/or recovered from s/h equipment especially if your to poor like me to be able to afford new parts all the time!
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  1. Silvio's Avatar
    I guess it would be more feasible for a beginner like me to upload a schematic and show us what you exactly did.
    I am interested in building one as I have quite a few cores and chokes that I would like to test,


    Updated 01-24-2017 at 10:37 PM by Silvio
  2. fourtytwo's Avatar
    Did you notice your PM Silvio ?
  3. scormonel's Avatar
    I want the schematic please.