Measuring inductance with DC amps flowing through

[KevinVZ]

I have a spec requirement to check the inductance of some potted inductors used in a custom switching power supply. Their value is between 20 and 200 micro-Henrys but the spec calls out DC bias currents up to 6 amps while measuring!

Spec calls out f=10KHz 0.1Vrms measurement. Need 0.1uH resolution and 3%-5% accuracy.

What circuit can I use to isolate that big bias current from my RLC bridge? A simple capacitor?

Just to make it interesting, I need to measure them while running a temperature sweep from +85C to -40C and back!

Thanks! Kevin VanZuilen "KevinVZ"

 

[pvventura]

You do get DMMs that can measure L but are very expensive. However you can built your own circuit that can do the same. Exploit some feature of inductance such as its frequency response. Send it through a uA741 and use the relation Vout=Vin(e^(TL/R)) or something like that. Look up a book about Op-Amps . That should help you.
Let me know if you get anything.
PVVentura

 

[cbarn24050]

Hi, you cant pass current through a capacitor, you need an inductor that is much larger than the 1 you are measureing in series with the bias supply.

 

[Nehemiah]

Check the date of the first post. He has probably figured it out by now or has given up.

 

[KevinVZ]

Thank you everyone for the good replys. As someone surmised in one reply, I have moved on from this problem. Solution was to ignore the DC bias requirement and take a reading of the inductance with a normal LCR bridge, so noting the deviation from the spec requirement in my tech report. At the low amplitude the LCR bridge sends AC into the inductor to do its measurement, the inductor's magnetic flux is barely wiggled so there is no magnetic saturation to mess up the reading. Results will be identical to what I would have measured with DC bias applied, if I could have figured out how to do that. Duh! I should have thought of that much earlier. Good practical lesson.