Measuring Inductance

[Madscientist267]

Dunno where else to put this, and since its being applied to a buck converter, I figured this was the best place. So here goes.

Is there a dead simple way of making an adapter of some kind so that the average DMM can read (or otherwise provide calculable information for) determining the value of an inductor?

I need to measure/tweak toroids, and have no idea how to go about getting their stats.

Please! Help!  I'm lost here...

Steve

[joestue]

do you have an o'scope and a signal generator?

Tim does an excellent job explaining this stuff.
http://webpages.charter.net/dawill/tmoranwms/Elec_Inductor.html
http://webpages.charter.net/dawill/tmoranwms/Elec_Magnetics.html

[Madscientist267]

Scope, yes. Calibrated sig-gen? no... But I'm assuming you're saying make a tank with known capacitance, hit it with a signal generator and look for resonance?

Guess in that case I wouldn't need a calibrated source, the scope would tell me the frequency...

Would a 555 work?

Steve

[joestue]

you want something a little better than a 555 to drive the inductor, feed the 555 into a flipflop, use that to drive a push-pull transformer with two mosfets and you got yourself a sig gen that can drive double digit watts into just about anything. stick a resistor in the dc supply feeding the transformer and you won't fry it shorting it out.

it will output a 50% square wave though, which is fine for measuring inductors.

[Madscientist267]

Sounds good... will give it a shot...

Steve

[commanda]

I found a cct online for an add-on unit for a DMM to do this. Uses one quad nand gate. I built it, and it works great.
If I can re-find it, I'll post the link.

Amanda

Here it is .......

http://freecircuitdiagram.com/2009/05/12/inductance-meter-circuit/

[Madscientist267]

Excellent - Thanks!!! This has been a thorn in my side for a LONG time, finally said enuf's enuf!! 

It will come in handy I'm sure for this whole buck thing... I keep seeing different values, all in the 150-400 uH range, but have no way of telling!!!

Now I do. 

Steve

[commanda]

Furthermore, you may find this interesting.

http://www.vk2zay.net/article/200

I haven't built it yet myself, but I do have a nice function generator that should drive it.

Amanda

[Opera House]

Saturation is worth mentioning.  Just because you have the right inductance doesn't mean the inductor will do its job.  Once it saturates the coil is just a resistor.

I've used that saturation technique to determine the mechanical speed of relays and solenoids.  As the core pulls in the inductance changes and therefore the slope.

[wooferhound]

Here is a fairly inexpensive Inductance Meter Kit . . .
http://www.electronics-diy.com/store.php?sel=kits

[SparWeb]

Are you sure you don't just want to get a DMM that measures inductance?  200$ or so?  Other useful features such as Hz and capacitance and logic levels galore?

I'm all for DIY, and I'd find it interesting to build it myself, too.  But if I did build one, I wouldn't believe the results until I'd calibrated it six ways to Sunday.... a process that requires the o-scope etc.

[Madscientist267]

Yeah thats ultimately the direction I went... Didn't want to bother with all the crazy calibration stuff.

And even as it is, I don't completely trust the one I got... it's a 'Sinometer' LMFAO

Cheap made in hong kong crap... but whatever. haha

Steve

[gizmo]

I remember doing this as a kid. To measure inducance all you need is a AC step down transformer to provide 6 to 12 or so volts. A fillimant transformer is perfect. Connect the inductor in series with a resistor, low ohm, somewhere between 1 and 10, depends on the inductor, 10 or so watts. Apply AC to the circuit, then use a multimeter to measure the AC voltage across the inductor and also the resistor, write down both readings. Then disconnect it from the power cause it will be getting warm. You play around with the resistance value to get both readings about the same, just more accurate that way.

Then we use maths. We know the value of R, our resistor, and we can work out the circuit current (I) with I=V/R, V we measured across the resistor. Once we know I, and V across the inductor, we can work out its Impenance (X_L) at the AC frequency. X_L=V/I.

Once we have that, we work out inductance with L=X_L/(2*3.14*Hz). Hz is the AC frequency, depending on country, 50 or 60Hz.

I'm pretty sure that maths is right, someone tell me if its not. Yeah I should have spent more time playing sports as a kid!

Glenn

[joestue]

i suppose it depends on why you need to know the inductance.

about the only inductance test at low current that remains accurate in real life circuit conditions is air core inductors and ferrite inductors operating at low flux.
powdered iron is always run at high flux, low permiability and high eddy current losses. about the only thing you can use an inductance meter for is figuring out what material the core is.

really its all iterative.. manufactures still provide tools that help you find the optimum core.. using "rules of thumb"