# Problem to find inductance using KVL for ac

Hello everyone. I was just playing with inductors and decided to Measure Inductance using basic ohms law V=IR and V=IZ for ac. Now my problem is the inductor i used was of 100uH but when i test it on dso by providing some frequency I’m getting it to be 4.5uH.please tell me what wrong have I done.

By using those formula you are not measuring the inductance at all you are measuring the inductive reactance. That measurement is frequency dependant, inductance is not.

To measure the inductance you can plot a graph of reactance verses frequency. Then slope of the line will give you the inductance.

What do you mean by "voltage across load"?

jremington:
What do you mean by "voltage across load"?

The diagram has L=? and rather than thinking L means inductance the person writing the words thought L means Load. Which tells you the person writing the words was an inexperienced student.

Grumpy_Mike:
The diagram has L=? and rather than thinking L means inductance the person writing the words thought L means Load. Which tells you the person writing the words was an inexperienced student.

Ofcourse i know via this method we are measuring inductive reactance. which is frequency dependend.
but if you had noticed the dso print screen the frequency is constant at 13Khz. and reactance = 2pif*L
So now please give me a logical reason why is this method incorrect.
And L=? means that we need to determine L i.e inductance if you figure that out.

jremington:
What do you mean by "voltage across load"?

sorry it was my mistake I should had written Voltage across inductor not load. please ignore that line just help me in the question it is driving me crazy

I don't think you can accurately measure a AC voltage across an inductor with a sound card. The input impedances of the cable and the sound card circuitry will strongly affect the result. Use an oscilloscope instead.

The problem you are having is that you are doing modular arithmetic on vector quantities.

When you add up the inductive reactance and the resistance you can not simply use normal arithmetic. The reactance is at 90 degrees to the resistance so you must use the vector sum.

Remember that the sound card output is not isolated from the earth (ground) of the laptop. Do you have the laptop PSU connected? You could have a short via the mains earth of the laptop PSU and the mains earth of the DSO. It would be better to unplug the PSU and run the laptop on batteries or use a differential probe with the DSO.

You are actually measuring the impedance rather than the reactance of the inductor so you should be getting a higher measurement not a lower one. You might want to Google "Maxwell Bridge" for a better method.

Russell.

jremington:
I don't think you can accurately measure a AC voltage across an inductor with a sound card. The input impedances of the cable and the sound card circuitry will strongly affect the result. Use an oscilloscope instead.

I had already used ossilloscope for measurement checkout the printshots. sound card is just for pure sine wave generation

Grumpy_Mike:
The problem you are having is that you are doing modular arithmetic on vector quantities.

When you add up the inductive reactance and the resistance you can not simply use normal arithmetic. The reactance is at 90 degrees to the resistance so you must use the vector sum.

Ya offcourse we cannot do modular mathematics.
But in my case I was using the rms current which should be same for both the component as they are in series regardless of the phase angle which will be arctan(wL/R) not 90 degree. again I had used the rms voltage across both the component which can be read from the screenshot.
My doubt is i had equated current on both component. as shown in diagram. If that is wrong please show me the correct expression please.

russellz:
Remember that the sound card output is not isolated from the earth (ground) of the laptop. Do you have the laptop PSU connected? You could have a short via the mains earth of the laptop PSU and the mains earth of the DSO. It would be better to unplug the PSU and run the laptop on batteries or use a differential probe with the DSO.

You are actually measuring the impedance rather than the reactance of the inductor so you should be getting a higher measurement not a lower one. You might want to Google "Maxwell Bridge" for a better method.

Russell.

My laptop was connected to a 19.5 v 4.5 amps hp charger. and apart from that I offcourse measure impedance rather than inductance but the impedance of inductor = wL and w is well known to us. I just want to know what wrong am i doing.

Show us how you connected your 'scope to the circuit.

You should download and learn to use the free LTSpice circuit simulator. I’ve attached a screenshot of the output you should see.

For an ideal inductor of 100 uH, ideal 25 ohm resistor and a 1V peak 13 kHz audio source with zero internal impedance, you should see peak voltages of about 312 and 944 mV across the inductor and resistor, respectively. If in the actual circuit you don’t see voltages close to that, something is seriously wrong with your assumptions, or the measurements.

jremington:
You should download and learn to use the free LTSpice circuit simulator. I've attached a screenshot of the output you should see.

For an ideal inductor of 100 uH, ideal 25 ohm resistor and a 1V peak 13 kHz audio source with zero internal impedance, you should see peak voltages of about 312 and 944 mV across the inductor and resistor, respectively. If in the actual circuit you don't see voltages close to that, something is seriously wrong with your assumptions, or the measurements.

I understand. Just one doubt what is probably wrong as far as my connections are concerned to the circuit I connected the probe as it is usually done. I mean why did this method didn't worked I'm very surprised and depressed to see that

BurntChip:
Show us how you connected your 'scope to the circuit.

I wish i could had shown you but I had broken down the circuit for now. any ways it was quite simple.
Sound card was powering the inductor and resistance(In series) and channel 1 was connected across the resistance and channel 2 was connected across the inductor. I calibrated the sound card output to be 1 v p-p at no load and load condition. I had shown the printshots.
Please confirm me What mistake am i making. i'm very surprised to see this and also very depressed because i was wondering to make a cool but cheap inductance meter using this method I know there are other more efficient method But i want this method to work.

jremington:
You should download and learn to use the free LTSpice circuit simulator. I've attached a screenshot of the output you should see.

For an ideal inductor of 100 uH, ideal 25 ohm resistor and a 1V peak 13 kHz audio source with zero internal impedance, you should see peak voltages of about 312 and 944 mV across the inductor and resistor, respectively. If in the actual circuit you don't see voltages close to that, something is seriously wrong with your assumptions, or the measurements.

I know these things work very well over simulations but i want to implement it practically.

A sound card is no substitute for an oscilloscope with a good probe.

russellz:
Remember that the sound card output is not isolated from the earth (ground) of the laptop. Do you have the laptop PSU connected? You could have a short via the mains earth of the laptop PSU and the mains earth of the DSO. It would be better to unplug the PSU and run the laptop on batteries or use a differential probe with the DSO.

You are actually measuring the impedance rather than the reactance of the inductor so you should be getting a higher measurement not a lower one. You might want to Google "Maxwell Bridge" for a better method.

Russell.

Your reply seems quite good enough let me please check out the same. because if you are saying is true then it was a very good piece of information