Coil measurement instrument question - Page 1
Forum section: electronics / testgear

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 17, 2019, 09:39:32 am
content: So, we are looking for an instrument capable of measuring the following parameters of a coil:- inductance- self capacitance- DC resistanceIt should be able to perform these measurements across a frequency of up to 2MHz. We are mainly going to use it with basic air coils. DC resistance measurement is not a mandatory feature as we can do that with our current basic multimeters that we have.It seems a "usual" LCR meter like the U1733C from keysight is not really suitable, as it can only go up to 100kHz. And I am also not sure if it can report the self-capacitance of the coil, it is not obvious from its datasheet (we are talking about capacitances in the range of 200...300pF @ 2Mhz, considering an inductance value of some 2.5mH).The 3260B Series Precision Magnetics Analyzer from Wayne Kerr could be an option (http://www.waynekerrtest.com/products_detail.php?indexs=9), but it is prohibitively expensive and it can measure so many more things that we are not really ever going to use. We know about it because one of our coil suppliers is using it.What are your suggestions?Best regards,Cristian

author: DaJMasta
author_rank: Super Contributor
timestamp: May 17, 2019, 03:20:15 pm
content: Well such an instrument is usually called an LCR meter, but can go by an impedance analyzer, especially for fancier, more modern models, and starts to get to the realm of a low frequency network analyzer. The cheapest option would probably still be the LCR meter named ones, though you pay a good premium for that kind of speed. You can sometimes find older bench LCR meters with a good bandwidth, I ended up picking up a Hioki 3532-50 a while back (goes to 5MHz) for a few hundred bucks, and the inexpensive chinese made fixtures/tweezers have worked well with it.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 17, 2019, 04:12:46 pm
content: Any good benchtop LCR meter, even if they are a little older, will measure your requirements for inductance and DC resistance.The challenge is your requirement for the inner windings capacitance.How important is this measurement for you?Look at an older HP or Agilent LCR meter or the Keysight LCR metershttps://www.keysight.com/en/pc-1617851/bench-top-lcr-resistance-meters?nid=-34124.0&cc=US&lc=eng

author: ogden
author_rank: Super Contributor
timestamp: May 17, 2019, 04:50:20 pm
content: Quote from: brumbarchris on May 17, 2019, 09:39:32 amCheck Hioki IM3536 then. If LCR meter with measurement at fixed frequency is not what you are looking for, then Hioki IM3570. [disclaimer: I have no affiliation whatsoever with said company]

author: bob91343
author_rank: Super Contributor
timestamp: May 17, 2019, 05:08:38 pm
content: For such measurements, a Q meter is ideal. You can select the frequency and measure the inductance over the range and locate the self resonant frequency (which permits calculating distributed capacitance). For some measurements, a Q meter can't be beat. The Q of course relates to the losses, or equivalent resistance.There are LCR meters that have a wide frequency range and would also work well. However, the usual ones only measure at a few frequencies, which makes it more questionable.I was lucky recently and acquired two Q meters, one for HF and one for VHF. I believe the HF one starts at something like 50 kHz. For lower frequencies I can cobble up a simple setup with a 'scope and generator and capacitor, or use my LCR bridge for 1 kHz or 120 Hz. The advantage of the LCR bridge is its accuracy.

author: Kean
author_rank: Unknown
timestamp: May 17, 2019, 05:54:21 pm
content: If you have a limited budget maybe take a look at one of the Chinese brands with a decent reputation in this area, like Tonghui. They have bench LCR meters that can measure up to 2 & 5 MHz.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 17, 2019, 06:19:59 pm
content: I don't know of a way to directly measure the self capacitance of an inductor. The usual way is to measure the self resonance frequency and calculate the self capacitance from that.An air core inductor of 2.5 mH will have a lot of turns. I found such an air core coil in my junk box and made a measurement using a Hioki IM3570 impedance analyzer. The measurement shows the inductance and ESR of the coil over a frequency range from 1 kHz to 5 MHz. The green curve is inductance and the yellow curve is ESR. Marker A shows the low frequency inductance of about 4.15 mH at about 10 kHz. The first self resonance (marker B) occurs at about 936 kHz. Using the usual formula: f = 1/(2 PI sqrt(L*C)), we can calculate the self capacitance to be 6.97 pF.A low cost way to make your measurement would be to use a meter like the U1733, or a DE5000, to measure the low frequency inductance. Then use a signal generator to apply a sine wave to the inductor and dial the frequency around until you find the self resonance frequency.It might make sense to specify the required self resonance frequency of the coil rather than the self capacitance.

author: bob91343
author_rank: Super Contributor
timestamp: May 17, 2019, 09:16:45 pm
content: It seems to me that measuring at frequencies well above self resonance should yield capacitance directly. After all, the inductance becomes a high reactance and so doesn't shunt the distributed capacitance much.Distributed capacitance of an air core coil is in the realm of 1 - 2 pF per turn. Those are all in series but of course it's more complex because there is the capacitance from the first to the third turn, and so on. So a wild guess of the capacitance of a 30 turn coil might be around 0.1 pF. These figures are all off hand, so don't quote me.Most measurement equipment has problems with less than 1 pF although some, like my Q meters, have dials you can interpolate down to below 0.1 pF. Just breathing near a device under test seems to make the values change. My high end LCR bridge has a residual reading of about 1.6 pF so I subtract that from whateveer measurement I make. I do the same for inductance. A four terminal measurement is important to eliminate the stray values from the test fixture.One of the most satisfying feelings comes from having two measurements from two different setups agree.

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 17, 2019, 11:58:43 pm
content: If we are discussing a model where we have an pure inductance in parallel with a capacitor, this model does not represent what the impedance at the terminals looks like above the first self resonance.The impedance analyzer can display the impedance it sees at the fixture in many ways. It can show it as impedance and angle (Z-theta), resistance and reactance (real part and imaginary part). It can treat the imaginary part as a parallel capacitance or a series capacitance, or as a parallel inductance or series inductance, or combinations of these.If the analyzer is set to display an impedance as inductance and if the phase angle is positive, then the value of the inductance shown will be positive. If the phase angle of the impedance is negative, then if the analyzer is set to display the inductance of the impedance, the value of the inductance will be negative; in this case the operator of the instrument will realize that the impedance is not inductive, but rather capacitive.Here I've set the analyzer to show the impedance as a parallel capacitance (green) and show the phase angle of the impedance (yellow). Since the vertical scale is logarithmic for the capacitance display, if the impedance is inductive, the green curve will be below the bottom of the display. The phase angle, however is shown with a linear scale with 90 degrees at the top, -90 degrees at the bottom, and 0 degrees at the middle of the display. Thus, when the impedance is inductive, the yellow curve is above the middle of the display, and below the middle if the impedance is capacitive.I've changed some of the display parameters in what follows. The left edge of the display is 100 kHz, the right edge is 5 MHz. The scale (logarithmic) for the capacitance (green) is .1pF at the bottom and 10 pF at the top.Marker A shows where the impedance just changes from inductive to capacitive (self resonance). The green curve to the right of this frequency is showing the apparent capacitance of the coil at the frequencies above its self resonance.We see that the apparent capacitance is not constant above self resonance, so the question is "which value on that rapidly varying green curve is the self capacitance?"The value calculated from the self resonant frequency and the low frequency inductance was 6.97 Pf. The highest value on the green curve doesn't even get that high.The 3 following images show marker B positioned at different frequencies so we can see the values of the apparent capacitance there.I think it's conventional to consider the self capacitance to be the value calculated from the self resonant frequency; at least that procedure only gives a single value.

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 20, 2019, 07:58:55 am
content: Thank you all for the replies, I really appreciate both the estimations and the measurements. As it seems, the suggested Hioki instruments would be suitable although the IM3570 is on par in terms of prices with the 3260B Series Precision Magnetics Analyzer from Wayne Kerr mentioned in my OP. The IM3536 seems to be a bit more reasonable in price.Anyway, what I have discovered in the meanwhile is that we have a R&S ZNL6 Vector Network analyzer already available. And I am wondering if it would not be possible maybe to use this one to measure perhaps indirectly the parameters. I did a bit of reading on the subject, seems not to be trivial. There are some indications that VNAs like the Keysight E5061B can be used for that, but I think it has specific in-built functions which the R&S ZNL6 is not equipped with.Best regards,Cristian