Circuit to Test Inductance with Idc

I think you may be better to move the sense resistor to the positive side of the inductor or move the current source there. As it is, the currents from your control inputs pass through the sense resistor.

A small sine wave makes a better test signal if you want to do accurate measurements. Pulses have harmonics above resonance. This leads to problems as the resonance frequency rises for increasing DC currents.

If you have a scope that will do averaging, you can get very good results by carefully setting the generator and the averaging so that the mains noise doesn't show up in the results.

If you electronically cancel most of the signal, you can get even higher accuracy in your measurements. If you are after the current that makes a 5%, 10% and 20% drop in the inductance, you need to do fairly accurate measurement to correctly place the 5% number.

I wanted to monitor Idc on the scope too. Note: Vbias is a floating supply connected to the scope ground.

It's a very damped circuit when the MOS switches. Still, there are transient effects so I just measure di/dt after settling.

I was testing with /_\\I = 20mA to 100mA to swamp noise currents.

I meant the circuit to be a magnetics learning tool. I'm just shooting for something to roughly check my math and magnetics theory. Perhaps I might tweek the design and set up for more precision if needed.

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

Another trick is to put two identical inductors in series, run the DC through both, and measure the inductance at the junction.

John

I think I got it..

large +-------||------+ | | +---Idc>--+-L---+---L-----+ | | Vgen | | | +---------+--gnd Vgen = 50 ohm source. Scope at junction and ref gnd. Idc is bench supply in current mode.

Zcoil/2 = 50 when junction at -6db Convenient, but I don't have to use that f.

At resonance, junction = Vgen. Ok.

Neato... :)

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

The Idc can be any bench power supply with current limit, or in fact any supply that can push the required current with some control. Watch out for any DC voltage at the junction, which could fry the signal generator.

John

I posted a reply but...I haven't seen it yet?? Misclick...time delay??? Anyways..

Yup...I like that.. I can even use a reference inductor. The bench supply in I mode can be bypassed with a cap.

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

I have also done this using a big air core inductor. You measure the parallel combination and then do a little math.

" D*****ad from Backwoods Canada "

Someone ought to bullet a oxygen thief like you in the head.

F*CK OFF !!!!

...... Phil

Yikes..don't let Phil see that. He'll be flipping out for a week. :)

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

One of these days you could look up a "bias tee".

Here's one.

ftp://jjlarkin.lmi.net/tee.zip

This wouldn't in general be very useful for the present problem.

John

Like I care what phil thinks of me? :)

It was a decent circuit at doing what it was supposed to-- energize inductor, wait for current to reach 700mA, recycle.

I added a 10k pullup to the Trigger pin just to clean that waveform up a little.

Cheers, James Arthur

I think I might be able to get on of those fancy mosfet drivers to act the same way. Those chips can have current sensing guts.

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

Or just an adjustable pulser driving an N-ch MOSFET. Say 50Hz rep rate, 2-200uS pulsewidth. A CMOS 555 might manage it.

Varying both pulsewidth AND supply voltage lets you cover a wide range of inductance.

Cheers, James Arthur