Measuring the capacitance of an inductor or Inductors

[belwar]

Slowly but shirley I have been teaching myself many electronic principles and im now at the point that I want to start making some measurements..

I have the following gear

USB Oscilloscope with Built in Function Generator
A DMM w/ Freq.
Older HP Audio Oscillator
Extech LCR Meter

Can someone point me in the right direction from here? Do I have what is needed to measure the capacitance of an inductor directly, or indirectly via calculations.

where to start....

[Joe Gwinn]

Measureing self-capacitance of an inductor

From post 86 of the Lane Poor thread:

Get a collection of film capacitors, measure their capacitance with the LCR meter, and write the value of each capacitor on the capacitor with a fine-point Sharpy pen. One by one, connect a capacitor in parallel with the pickup under test, and determine the zero-phase resonant frequency for each film capacitor. Multiply all resonant frequencies by 2 Pi to convert them from Hertz to radians per second. Plot added capacitance versus the square of resonant frequency in radians per second. If the inductance does not vary with frequency, the plotted points will fall on a straight line, but more likely they will fall on a curve. Draw a best straight line through the points. Where the line intersects the capacitance axis is the negative of the self capacitance. (There is a more complicated algorithm to compensate for the variation of inductance with frequency, but it doesn't seem worth the effort here.)

This method is mentioned in Terman, but he didn't invent it.

[belwar]

Get a collection of film capacitors, measure their capacitance with the LCR meter, and write the value of each capacitor on the capacitor with a fine-point Sharpy pen.

That I can do.. though can you explain / answer a couple of things?

1 - Why Film? I have hundreds of old PIO caps (.024, .047, and 0.015).. Is there something about using film which adds or detracts from the final calculation?
2 - How many different values should I use? 2? 4? 10? Is the idea to have as many as possible to get a more accurate reading?
3 - Any particular Range of Cap I should stay in?

One by one, connect a capacitor in parallel with the pickup under test, and determine the zero-phase resonant frequency for each film capacitor.

You showed me how to measure the Resonance frequency using an oscillator, resistor, and a good DMM. Is this method the same method used to measure the Zero Phase Resonance Frequency? Is there a difference between the two? If so, how do you measure the Zero Phase Resonance Frequency .. i.e. calculations required..

Multiply all resonant frequencies by 2 Pi to convert them from Hertz to radians per second. Plot added capacitance versus the square of resonant frequency in radians per second. If the inductance does not vary with frequency, the plotted points will fall on a straight line, but more likely they will fall on a curve. Draw a best straight line through the points. Where the line intersects the capacitance axis is the negative of the self capacitance. (There is a more complicated algorithm to compensate for the variation of inductance with frequency, but it doesn't seem worth the effort here.)

This method is mentioned in Terman, but he didn't invent it.

Ok, this part seems like the easy part - I think once I have the first two parts answered I can handle this.

Thanks a million Joe for take the time to answer. You're a wonderful contributor on this forum and im sure I speak for many when I say your contributions are appriciated by many.

[belwar]

So I went over to frys at lunch and bought a few Film Capacitors, and a new Fluke 179 DMM... I was able to get a handful of different caps. Here are the readings.. I cant post a table on the forum so its a screen cap.

[belwar]

This method is mentioned in Terman, but he didn't invent it.

Which Terman book are you refering to? I think I might pick up a copy. I noticed some books from 1958 that reference "electronic measurement". Do you have an ISBN or Amazon Link?

[belwar]

4th damn reply! Everywhere I look there are more questions.

So I looked at what you read more, and if I understand you right, you are suggesting I do the following. (Part of this is just thinking out loud)

1 - Hook up a Capacitor & the inductor in Parallel.
2 - Connect this to the LCR Meter
3 - Measure the Inductance & Capacitance .. In parallel (because they are Film based based).
4 - Use the resonance frequency formula to determine the resonance frequency with each cap.

Fr = 1 / (2*Pi*sqrt(LC))

5 - Take each of the Resonance Frequencies and multiple them by 2Pi to covert the frequency from Hertz to Radians per Second

** I'm with you up to this point, but you lose me after here **

For the graph I am about to make, what are the two axis?.. Capacitance, and Freq in Radians per second?

Which capacitance points am I putting on the graph? The values of the capacitor alone? or the Value of the capacitor with the inductor in parallel?

So when the line is drawn, where the line goes through the vertical axis @ 0 radians/second is the negative of self capacitance .. or Cap@0Radians = -(self capacitance). I didnt really know how to explain that any better...

matt

[bbsailor]

Slowly but shirley I have been teaching myself many electronic principles and im now at the point that I want to start making some measurements..

I have the following gear

USB Oscilloscope with Built in Function Generator
A DMM w/ Freq.
Older HP Audio Oscillator
Extech LCR Meter

Can someone point me in the right direction from here? Do I have what is needed to measure the capacitance of an inductor directly, or indirectly via calculations.

where to start....

Belwar,

You can't directly measure the capacitance of a coil's self capacitance but there is a simple way to calculate the equivalent coil capacitance by knowing the pickup's self resonance and inductance. There is only one capacitance value that can cause a known inductance to resonate at a particular frequency. See my pulse induction metal detecting coil article for the hook up diagram. http://www.geotech1.com/pages/metdet...s/FastCoil.pdf Just put the pickup in the same place as the pulse induction coil in the diagram.

Generally, you want to inject a variable frequency into a guitar pickup through a high resistance such as 1 meg ohm resistor in series with the signal generator and the pickup. This resistor isolates the pickup impedance from the low (50 ohm) impedance of a typical signal generator. Then you want to use an oscilloscope to observe the voltage across the pickup coil (after the resistor) using a 10X probe setting which has a lower capacitance than the loading of a 1X probe which could alter the accuracy of the reading.

Sweep the audio generator from 1KHz to 10 KHz and observe the scope. When the pickup is at the resonance frequency the scope pattern voltage will go high at the resonance point and be lower below and above the resonance point.

Note the frequency of the resonance point. Note the inductance from the Extech meter reading. Use a free program such a MiscEl (http://www.miscel.dk/MiscEl/miscel.html) and enter the inductance and the resonant frequency to reveal the capacitance that causes that particular pickup inductance to achieve that resonant frequency.

Note that this is the resonant frequency of a naked pickup without the coax loading. If you want to see how the coax affects the pickup resonance, just measure the capacitance of the guitar-to-amp coax cable with the Extech meter and then substitute a capacitor of that capaitance across the pickup and now the resonance reading will reflect the sound that you hear. You may want to put a 250K ohm to 500K ohm resistor in parallel with the equivalent cable capacitance to represent the loading of the volume pot. This will reduce the sharpness of the resonance point a bit.

I hope this simple method helps you. I can measure the resonance of a pickup, or any coil in about 30 seconds using this method.

Joseph Rogowski

[Joe Gwinn]

That I can do.. though can you explain / answer a couple of things?

1 - Why Film? I have hundreds of old PIO caps (0.024, 0.047, and 0.015).. Is there something about using film which adds or detracts from the final calculation?

Because the capacitance values are more stable, and largely unaffected by voltage and frequency.

PIO (Paper In Oil) caps are not as bad as ceramics, which are not as bad as electrolytics, but they are all pretty bad as a capacitance standard. And film caps are cheap, so there is no reason not to use them.

The most stable common film caps are those made of polystyrene, polypropylene, or teflon.

2 - How many different values should I use? 2? 4? 10? Is the idea to have as many as possible to get a more accurate reading?
3 - Any particular Range of Cap I should stay in?

The typical self-capacitance of a pickup coil is about 50 pF (picofarads), so at least five values between 100 pF and 1000 pF will work. It isn't very critical.

You showed me how to measure the Resonance frequency using an oscillator, resistor, and a good DMM. Is this method the same method used to measure the Zero Phase Resonance Frequency? Is there a difference between the two? If so, how do you measure the Zero Phase Resonance Frequency .. i.e. calculations required.

The method using the DMM is the peak-response method, not the zero-phase method. With low Q inductors, they don't yield the same answer. You can use the peak method with the multiple parallel capacitors, although with some loss of accuracy.

You measure zero-phase frequency with an oscilloscope. I posted the method some time ago, in an answer to Andrew Campbell (Tonerider). I don't have time to find it right now, but searching for oscilloscope and resonance should lead you there.

[Joe Gwinn]

Which Terman book are you referring to? I think I might pick up a copy. I noticed some books from 1958 that reference "electronic measurement". Do you have an ISBN or Amazon Link?

The book is "Radio Engineers' Handbook", F.E.Terman, McGraw-Hill 1943. This predates ISBN coding, but there are many copies available in the used book market accessible from Amazon.

The measurement method is described on pages 922-924, starting at section 10 Miscellaneous at the bottom of page 922.

[Joe Gwinn]

So I looked at what you read more, and if I understand you right, you are suggesting I do the following. (Part of this is just thinking out loud)

1 - Hook up a Capacitor & the inductor in Parallel.
2 - Connect this to the LCR Meter
3 - Measure the Inductance & Capacitance .. In parallel (because they are Film based based).
4 - Use the resonance frequency formula to determine the resonance frequency with each cap.

Fr = 1 / (2*Pi*sqrt(LC))

1-Yes, exactly. 2-No, the LCR meter is used to measure the capacitances of the film capacitors in isolation, unconnected to anything except the meter itself. 3-No, one measures only the resonant frequency of the inductor and the current capacitor in parallel, and this will work with any kind of capacitor. 4-Not quite; more below.

5 - Take each of the Resonance Frequencies and multiple them by 2Pi to covert the frequency from Hertz to Radians per Second.

** I'm with you up to this point, but you lose me after here **

For the graph I am about to make, what are the two axis?.. Capacitance, and Freq in Radians per second?

Exactly. The horizontal axis is the capacitance of the film capacitors, and the vertical axis is the inverse of the square of the frequency in radians per second.

The method is based on the resonance equation f= 1/(2 Pi Sqrt[L*C]), so one can work out the math where Ci=Cself+Ci, having Ci be the various film capacitors and fi be the corresponding resonant frequencies.

Which capacitance points am I putting on the graph? The values of the capacitor alone? or the Value of the capacitor with the inductor in parallel?

The capacitor alone.

Although the LCR will report the "value of the capacitor with the inductor in parallel", the reported capacitance is not the self capacitance. The easiest way to see this is to observe that at resonance, an LCR meter operating at that frequency would report a pure resistance (zero capacitance, zero inductance), even though the resonance is with that self capacitance.

So when the line is drawn, where the line goes through the vertical axis @ 0 radians/second is the negative of self capacitance .. or Cap@0Radians = -(self capacitance).

I have started to explicitly parallel Terman here, so the horizontal and vertical axes may be reversed from prior descriptions. But it won't matter, so long as one is self-consistent.

For each film capacitor, connect it in parallel with the inductor, measure the resonant frequency, compute the inverse of the radian frequency, and plot the point on the graph. When all capacitors have been used, look at the graph. A more or less straight line should have emerged. Draw the line. It will intersect the horizontal axis (capacitance) in the negative-capacitance region at some value. The negative of this value is the self-capacitance of the inductor.

Damn The Torpedoes! Just try it. Even if things are not quite clear (and the answers are nonsense), it will sharpen the questions, and you cannot hurt anything. Use the peak method for now, so only one new thing is being thrashed out at once.

[belwar]

The book is "Radio Engineers' Handbook", F.E.Terman, McGraw-Hill 1943. This predates ISBN coding, but there are many copies available in the used book market accessible from Amazon.

The measurement method is described on pages 922-924, starting at section 10 Miscellaneous at the bottom of page 922.

Thanks alot .. I found it for 6 bucks plus 3 bucks shipping. I also picked up the Electronics Measurement book he did for 7 dollars.. Supposedly a sort of followup book to Radio Engineers handbook detailing the difficulty of measuring certain items .. and im assuming methods to measure!

[belwar]

You measure zero-phase frequency with an oscilloscope. I posted the method some time ago, in an answer to Andrew Campbell (Tonerider). I don't have time to find it right now, but searching for oscilloscope and resonance should lead you there.

Joe,

If you happen to come accross it i'd appriciate it. I spent about 40 minutes looking for it last night and came up blank. I'm sure its a good read. I did all kinds of searches using keywords like "zero-phase" "resonance" "Tonerider" and "Joe Gwinn" and they all returned nothing but this thread! :>

[Joe Gwinn]

Joe,

If you happen to come accross it i'd appriciate it. I spent about 40 minutes looking for it last night and came up blank. I'm sure its a good read. I did all kinds of searches using keywords like "zero-phase" "resonance" "Tonerider" and "Joe Gwinn" and they all returned nothing but this thread! :>

Maybe the thread is on the old AMPGE forum.

[Joe Gwinn]

Measuring zero-phase resonant frequency with an X-Y scope

I found the original posting. This is a partial reposting of my 3 June 2005 posting to the thread "Re: Strat Resonant Peak", later posted on 25 June 2006 as "Measuring resonant frequency with an X-Y scope" in response to a request:

Those that have access to a dual-input oscilloscope capable of X-Y display, even a cheap one, plus a signal generator (even one with somewhat distorted sine waves), can use the standard electrical method to measure resonant frequency. This works with or without the added "cable capacitor" (which simulates the effect of the cable between guitar and amplifier).

First an explanation. The actual definition of resonance is not that the voltage peaks, although that certainly happens. The actual definition is that the inductive reactance and the capacitative reactance are exactly equal, so the resonant circuit acts like a pure resistor of very high value at this one special frequency. At this frequency, the current through the resonant circuit will be in phase with the drive signal, just as if one replaced the resonant circuit with a carbon resistor.

Now, the measurement method. Hook the pickup to the signal generator through a 1 Mohm resistor. (Exact value not critical.) Hook the scope X input to the generator output, and shield to the generator ground terminal (or shield). Hook the Y input to the junction of the resonant circuit and the 1 Mohm resistor. Let everything warm up. Adjust frequency for maximum Y amplitude. This peak may be very wide and indefinite, but great accuracy is not required, so don't worry about getting this exactly right. Just get into the neighborhood. Adjust scope so that the X and Y displays are both of reasonable amplitude on the screen. Switch the scope to X-Y mode. You should see an ellipse, perhaps a very narrow one, perhaps a circle. Adjust the frequency. You will see that the shape of the ellipse varies as the generator is tuned. This ellipse is also known as a Lissajou Figure. The frequency at which the ellipse collapses into a straight line (the phases of X and Y being identical) is the true resonant frequency. In low Q circuits, this zero-phase-difference frequency will differ noticably from the peak-Y frequency, and the lower the Q the greater the disparity, further confounding attempts at understanding and comparing pickups.

[bbsailor]

Slowly but shirley I have been teaching myself many electronic principles and im now at the point that I want to start making some measurements..

I have the following gear

USB Oscilloscope with Built in Function Generator
A DMM w/ Freq.
Older HP Audio Oscillator
Extech LCR Meter

Can someone point me in the right direction from here? Do I have what is needed to measure the capacitance of an inductor directly, or indirectly via calculations.

where to start....

Belwar,

Try this using the equipment that you have.

1. Connect your pickup under test (PUT) to your old HP audio oscillator with a 1 Meg ohm resistor in series with that connection. The grounds are all a common connection point.
2. Connect your USB scope to the pickup (after the 1 meg ohm resistor and the common ground point).
3. Set your scope to read the range of 1KHz to 10KHz.
4. Sweep the HP audio oscillator from 1KHz to 10 KH and observe the scope pattern. When you hit the pickup resonance point you will see the voltage rise sharply.
5. Note the frequency on the oscillator where the pickup peak resonance occurs.
6. Use your Extech LCR meter to measure the capacitance of the probe. Note this measurement.
7. Use the free program MiscEL to enter the resonant frequency and the inductance in Henries of the PUT measured with the Extech LCR meter.
8. The result of this calculation will be the self capacitance of the pickup and the scope probe.
9. Subtract the capacitance of the scope probe and you will now have the self capacitance of the PUT.

This should do it for you with the equipment you have on hand.

Joseph Rogowski

P.S. If your HP oscillator frequency readings are not very accurate, attach your DMM, with frequency readout, to the oscillator output and use that DMM frequency reading as your frequency to enter into the MiscEL calaculator.