# Inductor question. Why current goes up then down instead of decreasing with frequency

1. ## Inductor question. Why current goes up then down instead of decreasing with frequency

I have a question that came up during an experiment. I hooked up a function generator to a coil and to my surprise the voltage across it went from 4.48 volts at 100 kHz to 800mV at 1 mHz then back up to 4.04 volts at 4.9 mHz. I thought as the frequency went up current went down in an inductor. This one went up then down. Am I missing something?

I am using a 19.2 mH inductor and 5 volts Peak to Peak on the function generator.

Thanks for the help.

Pierre

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2. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

Hi,

1 mHz is a rather low frequency, it is quite expectable that the impedance is very low and thus the voltage of the function generator drops (because of high current)

4.5 mHz is very low frequency, too. I have no explanation why there should be 4.04V.

Btw: Are you sure you mean mHz = millihertz? Or do you mean MHz = megahertz?

Klaus

3. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

I think it's MHz.

Your real world inductor has parasitic capacitances, which result in a rapid change of input impedance towards self resonance frequency.
Have a look here: https://www.everythingrf.com/communi...with-frequency

4. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

The reported 1 MHz voltage minimum isn't expectable at first sight, it can't be explained by inductor self resonance.

But we are missing the full picture, what are the generator characteristics, how is the voltage measured? There are many ways to achieve untypical results.

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5. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

I agree with FvM. I think you should also indicate the type of inductor you are testing and the used waveform (sinewave, squarewave..). And clarify if you are speaking about milliHertz or Megahertz

6. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

You're right I did mean megahertz not millihertz

7. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

Originally Posted by albbg
I agree with FvM. I think you should also indicate the type of inductor you are testing and the used waveform (sinewave, squarewave..). And clarify if you are speaking about milliHertz or Megahertz
It's a coil I got out of a stereo amplifier. It's wound on what looks like a toroid. I was using a sine wave 5V PP. I was measuring the voltage on an oscilloscope.

Thanks again for the help.

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Originally Posted by volker@muehlhaus
I think it's MHz.

Your real world inductor has parasitic capacitances, which result in a rapid change of input impedance towards self resonance frequency.
Have a look here: https://www.everythingrf.com/communi...with-frequency

Thanks for the link. From what I understan, past a certain frequency, impedance decreases again. Is that correct?

8. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

You are seeing parallel resonance of the inductor - at a certain frequency it will look hi Z - it will then support all the volts from the source, at other frequencies it is lower Z and pulls down the volts from the ( 600 ohm ? ) sig gen ...

9. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

You are seeing parallel resonance of the inductor - at a certain frequency it will look hi Z - it will then support all the volts from the source, at other frequencies it is lower Z and pulls down the volts from the ( 600 ohm ? ) sig gen ...
That would be expectable, but post #1 describes an opposite effect, minimal voltage at mid frequency. Either the circuit is different than described or we are observing other effects like operating the generator outside its rated conditions.

1 members found this post helpful.

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10. ## Re: Inductor question. Why current goes up then down instead of decreasing with frequ

yes opposite effect described - apologies - either a typo or there is a series cap causing a min resonance in Z.

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