moamps
Well-known member
abbey road d enfer said:
Try to use this on-line calc and report back what did you get. I can't get correct result.
They placed sqrt3 to numerator instead in denominator.
Inductance measurement problem
- Thread starter DaveP
- Start date
Help Support GroupDIY Audio Forum:
Try to use this on-line calc and report back what did you get. I can't get correct result.
They placed sqrt3 to numerator instead in denominator.
I never did try the calculator. All I say is the formula is somewhat correct, within limits.moamps said:
In fact, it all boils down to an EDN article:moamps said:
http://www.edn.com/design/test-and-measurement/4363759/Circuit-measures-capacitance-or-inductance
See the two last comments, hinting that the method may be adequate for caps but not for inductors, and that the method neglects too many factors.
When you think one person wrote it and two others edited it...
EDN used to be a trustworthy resource...
There is no calculator in the article.
This Daycounter, Inc Engineering services, they added the faulty calculator.
"Daycounter provides contract engineering services."
I guess I would never trust them for any kind of design.moamps
Well-known member
I have already quoted the EDN article in the beginning of this thread as example of the correct calculation.
From my experience this method is adequate for measurement in audio frequency spectrum.
From my experience this method is adequate for measurement in audio frequency spectrum.
OK,
I have attached PRR's method and I'll also place it in the Technical Documents section for reference.
CJ,
You are of course right about your method, I have used at work a Tektronix pulse generator that could output a 60V square wave from a pair of 6L6's in the output stage! But your level of professional equipment is not available to most of us, so I have just stuck to PRR's method which at least gets the ball rolling.
Best
DaveP
I have attached PRR's method and I'll also place it in the Technical Documents section for reference.
CJ,
You are of course right about your method, I have used at work a Tektronix pulse generator that could output a 60V square wave from a pair of 6L6's in the output stage! But your level of professional equipment is not available to most of us, so I have just stuck to PRR's method which at least gets the ball rolling.
Best
DaveP
moamps
Well-known member
To be precise, your equation should be
Lx(H)=R/(2*π*f) for Vout=0.707Vin
where:
R- resistance in ohms,
f- frequency in Hz.
Lx(H)=R/(2*π*f) for Vout=0.707Vin
where:
R- resistance in ohms,
f- frequency in Hz.
CJ method, does this look correct for a 25EI 6.5mm stack?
I put 1000 turns on the bobbin and here's what I'm finding.
I put 1000 turns on the bobbin and here's what I'm finding.
> you will not see a series resistor being used with too may transformers
Triode plate resistance; pentode current sources parallel with load resistance.
We rarely "add" resistance, true. But we can never get rid of all resistance.
Frequently enough we are "stuck" with significant source resistance. The "15K" spec on that transformer strongly suggests tubes. A 15K resistor is a convenient and repeatable approximation of tube OT resistances.
Yes, measuring current-at-voltage is just as valid. And I think any audio-geek who can not massage his gain-set to read small audio current needs to expand his test skills. But when working a transformer between resistances (as we usually do), approximating those resistances is just as valid.
I don't see that my approach is novel, or needs publication. If a tranny says "15K:600", pad-out your signal generator to 15K (add 14,400 to my 600r Heath) and load with 601r 1% (we used to shovel those around the studio). That's clearly 6dB loss with perfect iron, so calibrate the midband (1KHz) loss and call that "zero". Then work to 20Hz and 20KHz, see what more loss you find. Vary the level over the useful range: big deep bass will saturate, while very low bass will lose inductance.
We don't hear Henries. We hear frequency response.
Henries may be handy for "what if?" thinking.
As CJ and Abbey say, "precision resistors" are totally a waste when doing iron-core coils which vary 1,000% with frequency and level.
They do not even measure milliHenry IF coils much better than 10%. Even though in the final product they must be right to <1% (2KHz out of 445KHz). You can't count turns that close, and you sure can't press powdered iron that exact. Instead there's a trim (C or L) and you adjust so the tuning is spot-on (as close as your boss wants to pay for fiddling).
Triode plate resistance; pentode current sources parallel with load resistance.
We rarely "add" resistance, true. But we can never get rid of all resistance.
Frequently enough we are "stuck" with significant source resistance. The "15K" spec on that transformer strongly suggests tubes. A 15K resistor is a convenient and repeatable approximation of tube OT resistances.
Yes, measuring current-at-voltage is just as valid. And I think any audio-geek who can not massage his gain-set to read small audio current needs to expand his test skills. But when working a transformer between resistances (as we usually do), approximating those resistances is just as valid.
I don't see that my approach is novel, or needs publication. If a tranny says "15K:600", pad-out your signal generator to 15K (add 14,400 to my 600r Heath) and load with 601r 1% (we used to shovel those around the studio). That's clearly 6dB loss with perfect iron, so calibrate the midband (1KHz) loss and call that "zero". Then work to 20Hz and 20KHz, see what more loss you find. Vary the level over the useful range: big deep bass will saturate, while very low bass will lose inductance.
We don't hear Henries. We hear frequency response.
Henries may be handy for "what if?" thinking.
As CJ and Abbey say, "precision resistors" are totally a waste when doing iron-core coils which vary 1,000% with frequency and level.
They do not even measure milliHenry IF coils much better than 10%. Even though in the final product they must be right to <1% (2KHz out of 445KHz). You can't count turns that close, and you sure can't press powdered iron that exact. Instead there's a trim (C or L) and you adjust so the tuning is spot-on (as close as your boss wants to pay for fiddling).
looks good MicDaddy, note saturation at 20 Hz,
core gone at 2 K so no need to measure air core inductor unless you want to,
vector graph of resistor method,
you can use a low R for simulating mic or line in and whip out the trig function calculator to get an answer for R not equal to X-L.
core gone at 2 K so no need to measure air core inductor unless you want to,
vector graph of resistor method,
you can use a low R for simulating mic or line in and whip out the trig function calculator to get an answer for R not equal to X-L.
Attachments
As I mentioned earlier, this method has been discussed earlier in this group, like hereDaveP said:
http://groupdiy.com/index.php?topic=51672.msg656683#msg656683
and there
http://groupdiy.com/index.php?topic=47515.msg672676#msg672676
here is a chart showing what voltages you should get if you want to scale down the series resistance.
you would keep 1 volt or 10 volts or whatever across the Series RL circuit, then you would turn down the pot as far as you like and read the voltages off the coil and resistor, checking them with this chart, then just multiply your resistor value by the factor shown and you have your XL.
this chart is derived by dividing R by XL, taking the arctan to get the phase angle in degrees, and then finding the sin and cosine of the angle, which is your voltages across R and the Coil.
you would keep 1 volt or 10 volts or whatever across the Series RL circuit, then you would turn down the pot as far as you like and read the voltages off the coil and resistor, checking them with this chart, then just multiply your resistor value by the factor shown and you have your XL.
this chart is derived by dividing R by XL, taking the arctan to get the phase angle in degrees, and then finding the sin and cosine of the angle, which is your voltages across R and the Coil.
Attachments
Yes Abbey, you are right, I should have looked on the forum first.
But having seen many "how to" videos on Youtube I mistakenly thought that would be the best option. But I am glad it was discussed again and the method is now recorded for future use. 51 people have already downloaded it so the effort was worth it IMHO.
One of those topics is 11 pages and 52 posts long, that is a lot to plough through to get a decent method. If there is a method already in Technical Docs then it makes your job as moderator easier as you can refer people to that first.
Thanks to you, CJ and PRR for helping me get to grips with this, now I can get on with what I do best, building stuff.
Best
DaveP
you can always add to previous topics,
tonight we will do a test which compares the resistor method with no resistor,
wondering if the DCR of the transformer is additive or in parallel with the transformer,
in either case, both methods are better than an LCR meter as you can control level and frequency with the signal generator,
tonight we will do a test which compares the resistor method with no resistor,
wondering if the DCR of the transformer is additive or in parallel with the transformer,
in either case, both methods are better than an LCR meter as you can control level and frequency with the signal generator,
Indeed it is in series. If it was in parallels, DC ohm measurement would be zero, and in operation the dissipation in this resistance would be enormous.CJ said:
The equivalent Rloss is in parallels, but frequency and level-dependant.
jeezus h, ???
Bo Deadly
Well-known member
Make an RLC notch filter with it and look at the frequency response. Then model the same circuit in LTSpice adjusting the values and parameters of the inductor as necessary to match the measured response. Then you'll know the inductance and possibly other things depending on how sophisticated your model is.
> jeezus h, ???
That's all in there when you wind.
Rp is primary DCR, what you were asking.
Rc is core loss, small on thin lams (but on huge power transformers can add-up to a bunch of idle heat).
Xm is primary inductance. On a laminated iron core, this inductance *drops with frequency*, as you have often shown by test. (The reactance rises, but less steeply than expected for a constant inductance.)
> Make an RLC notch filter
At what frequency??
I think he tried to resonate it, at hundreds of Hz, and got answers he did not like (implied poor performance). Testing again with simple resistance reference, he got numbers he liked (implied performance better than specified, and that is what we expect from good parts). I think if he tried to resonate it in the 20hz or 40Hz area, he'd get the same/similar inductance numbers. (Different because iron-core inductance varies with level, another CJ curve, and resonances change level quickly.)
Tuning a big coil and core to 20Hz is likely to require dozens of uFd. Which will probably be an Electrolytic cap. (Hardly worth buying huge film caps for one test.) E-caps are perhaps the second most UN-ideal parts in our bins. I remember +100%/-50% tolerances. There's some voltage coefficient (not as much as the worst ceramic dielectrics). They leak like my water cooler. They love to eat reverse voltage and die.
Put it between the resistances you are gonna use it at and take a full response curve. Same as any passive black-box which you suspect of being un-flat.
That's all in there when you wind.
Rp is primary DCR, what you were asking.
Rc is core loss, small on thin lams (but on huge power transformers can add-up to a bunch of idle heat).
Xm is primary inductance. On a laminated iron core, this inductance *drops with frequency*, as you have often shown by test. (The reactance rises, but less steeply than expected for a constant inductance.)
> Make an RLC notch filter
At what frequency??
I think he tried to resonate it, at hundreds of Hz, and got answers he did not like (implied poor performance). Testing again with simple resistance reference, he got numbers he liked (implied performance better than specified, and that is what we expect from good parts). I think if he tried to resonate it in the 20hz or 40Hz area, he'd get the same/similar inductance numbers. (Different because iron-core inductance varies with level, another CJ curve, and resonances change level quickly.)
Tuning a big coil and core to 20Hz is likely to require dozens of uFd. Which will probably be an Electrolytic cap. (Hardly worth buying huge film caps for one test.) E-caps are perhaps the second most UN-ideal parts in our bins. I remember +100%/-50% tolerances. There's some voltage coefficient (not as much as the worst ceramic dielectrics). They leak like my water cooler. They love to eat reverse voltage and die.
Put it between the resistances you are gonna use it at and take a full response curve. Same as any passive black-box which you suspect of being un-flat.
just pick up a used Huey Packer on evilbay,
but for ten grand i want 5 hz or lower,
ever read a Jensen datasheet? they go way low, asked Whitlock what piece of amazing equipment they used to get down there, the sheepish reply was that it was some kind of musical synth,
yes, core loss, parallel plate resistance,
air core inductors are linear til stray c and copper eddies take over, iron has that weird BH loop, so we try and make a connection between linear air core inductors and complex iron inductors, that is where the math gets sticky and we retreat to the lab,
but for ten grand i want 5 hz or lower,
ever read a Jensen datasheet? they go way low, asked Whitlock what piece of amazing equipment they used to get down there, the sheepish reply was that it was some kind of musical synth,
yes, core loss, parallel plate resistance,
air core inductors are linear til stray c and copper eddies take over, iron has that weird BH loop, so we try and make a connection between linear air core inductors and complex iron inductors, that is where the math gets sticky and we retreat to the lab,
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