Measuring DC resistance of an inductor

Thread Starter

Tera-Scale

Joined Jan 1, 2011
164
Okay now I know why there is the low pass filter :). Measured the output from the op amp and now I am only seeing about 10mV of AC. I will be sure about it tomorrow in the lab at school and I will try to capture the waveform as well. thanks a lot for now.
 

Ron H

Joined Apr 14, 2005
7,063
Okay now I know why there is the low pass filter :). Measured the output from the op amp and now I am only seeing about 10mV of AC. I will be sure about it tomorrow in the lab at school and I will try to capture the waveform as well. thanks a lot for now.
The oscillations are probably caused by the LM317. They need to be prevented, not just eliminated. I'll look into it when I have more time.
 

Thread Starter

Tera-Scale

Joined Jan 1, 2011
164
Just a short question. So when you designed the low pass filter, you did not have these oscillations in mind? Because I think the filter is doing the trick.

Thanks for your patience.
 

Ron H

Joined Apr 14, 2005
7,063
Just a short question. So when you designed the low pass filter, you did not have these oscillations in mind? Because I think the filter is doing the trick.

Thanks for your patience.
No, I didn't. Whenever you make a high gain amplifier, it just makes sense to filter it as much as possible. It reduces the chatter from your A/D.
I wouldn't count on your current source being reliable is it is oscillating.
 

Thread Starter

Tera-Scale

Joined Jan 1, 2011
164
Today I will find out how it is performing exactly. I will use an high end Ohm meter to measure the DCR of a range of Inductors and match with my results.

It would be a good idea if the frequency was of oscillation was constant throughout all inductor range ;). Because my LRC meter will be targeted for modern SMPS design which will run at these frequencies (100kHz-120kHz). Current source at these frequencies would also include real world parameters such as skin effect and maybe slight hysteresis. But lets just stick to this method for now.

Any further hints would be appreciated a lot. thanks
 

Ron H

Joined Apr 14, 2005
7,063
Skin effect is an AC phenomenon. Filtering eliminates any manifestations of skin effect.

EDIT: Try putting 100nF across the inductor. I think this will stop the oscillation.
What is the range of inductance that you are testing?
 
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Thread Starter

Tera-Scale

Joined Jan 1, 2011
164
Yep I tried that at school with 2 inductors. The two of them are in the 100μH's range. My inductor range would vary from 1nH to at least 2mH. With the two values mentioned, only a ripple of about 1mV remained which I think is tolerable considering it will pass through the low pass filter. What if I try adding some smaller nanos and picos caps in parallel...would it help to reduce that 1mV?
I am considering it since my input resolution is 0.1mV.

I wont mind waiting a bit if I try to use a higher capacitance and cover the required range.

I added a 10Ω 2W resistor in series with my parallel LC to help a little what do you think? Should I experiment with this resistor?
 

Ron H

Joined Apr 14, 2005
7,063
Add 100Ω in series with the cap you put in parallel with the inductor. This should stop the oscillations.
 

Ron H

Joined Apr 14, 2005
7,063
Below is a link of Coilcraft inductors datasheet. At the end of the table there are these testing specs:

1. Tested at 15.75 kHz, 0.1 Vrms, with DC bias applied up to the Irms current.
2. Current that causes a 40°C temperature rise from 25°C ambient.

Can you give me a brief explanation of what do they mean exactly?


http://www.coilcraft.com/pdf_viewer/showpdf.cfm?f=pdf_store:pcv.pdf

thanks
I interpret (2) to mean that they have tested each inductor value, and Irms is the current require to cause the temperature of that particular part to rise 40°C. They then measure the inductance with that DC current flowing through it, and 0.1Vrms at 15.75kHz applied across it (presumably through a capacitor, to prevent the AC voltage source from shorting out the DC current source).
 

Thread Starter

Tera-Scale

Joined Jan 1, 2011
164
Hi Ron,

today I tested the current source circuit and discovered the parallel RC circuit only reduced the small oscillations by 2mV .. with about 4mV remaining.

Surprisingly when I switched of the power the DC component of 25mV disappeared leaving the 4mV ac component there. When I zoomed into the waveform I discovered that is it a 101MHz sinusoidal waveform matching the intermediate frequency of a local powerful radio station... What do you think?

I will try the experiment this week with the circuit in a box with foil and earthed
and see the results. I think that this showed the the RC/C parallel circuit was doing all the work.. but we have no control over external EMI.

I assume these small factors are corrected by the sallen-key .. am i right?
 
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