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Alternative ways to measure self resonating freqency of coil?

ronsimpson

Well-Known Member
Most Helpful Member
This coil/inductor is wound with flat wire not round wire. You can see how each turn makes a capacitor. (copper insulator copper)
120230
Look at the huge gaps in between the wires. The inter winding capacitance is much smaller.
120231
Just saying you probably have a SRF that is very high with out doing strange things.
 

dr pepper

Well-Known Member
Most Helpful Member
Your lcr meter will tell you the capacitance/inductance so long as the test freq is below the resonant freq, and you have variable test tones so you can tell if one of them is at resonance.
Another thing you can do is to build an oscilator from a transitor and see what the actual resonance is.
The resonance will change a bit with the final circuit.
Having said all that the self resonance is most likely well above audio freq's.
 

Signalglow

New Member
Why build such an antique design?, anything remotely 'modern' uses gyrators instead of inductors, and these cure all the problems with inductors (none of which is parasitic capacitance though).



So why all the concern about imaginary parasitic capacitance?, of vastly greater concern is that the inductors are all going to be coupled to each other as they are one big transformer. How tiny is your design?, there should be plenty of room for multiple inductors, which should be spaced apart anyway, in order to prevent interaction.
Well this is true, the reason why i did it in this old design is because i really like the sound of the original units, i am using the original transformers and so on, i think it would be a shame if i did not keep this project as close to the original as possible :)
i know the caps!? well they have been tested for leakage and the electrolytics are new Sprague Atoms. huge ones, two in each can, so in order to make it fit better, i had to use some motor capacitor-cans, i know this is very old school, but overall a very fun but expensive project, the inductor should be in the round brass dome, the only thing that is new in this thing is the PT, the electrolytics, and the output transformer. i will try to make this inductor, i have already bought the items i need to make one, core, wire, mylar tape and so on. I will give it a try, but yes i am concerned about the results, this could maybe be wasted cash.











 

Signalglow

New Member
This coil/inductor is wound with flat wire not round wire. You can see how each turn makes a capacitor. (copper insulator copper)
View attachment 120230
Look at the huge gaps in between the wires. The inter winding capacitance is much smaller.
View attachment 120231
Just saying you probably have a SRF that is very high with out doing strange things.
Yes i will probably have a high SRF, but its also just to be sure about this, for future projects.
It makes sense. thanks for the tip!

Your lcr meter will tell you the capacitance/inductance so long as the test freq is below the resonant freq, and you have variable test tones so you can tell if one of them is at resonance.
Another thing you can do is to build an oscilator from a transitor and see what the actual resonance is.
The resonance will change a bit with the final circuit.
Having said all that the self resonance is most likely well above audio freq's.
Thanks for the tip.
Okay i think i understand.
So one way is to measure L and get a reading at the lowest test frequency possible on the LCR meter, and then test L at the highest frequency possible on the meter, then the lower reading at a higher frequency is the capacitance reducing L, or do you mean that i should hook up my signal generator in series with my meter?
 

Signalglow

New Member
If I would/could read …..
I thought about putting the signal generator across the coil and also putting a a.c.-meter across the coil. But many meters will not measure 100khz ac.
what to do? ...….
Make a "RF probe" or a "demodulation probe" for your meter. Use the circuit from the internet. (ok most internet circuits do not work)
probe
Because we want to use it at a lower frequency than "RF" you might use 10x or 100x larger capacitors than normal.

Put the signal generator across the coil. The coil should short out the signal. Put the demodulation probe across the coil and set the meter to low voltage DC.
Sweep the signal generator and try to find a frequency where the coil 'opens up" and you get a large signal.
Sorry i did not see this post, i know the SRF would proberbly be very high anyway but i would like to try this method, but i dont think my meter can measure 100khz AC, more like 60-50hz the link you provided is not valid, but it sounds very interesting!
 

ronsimpson

Well-Known Member
Most Helpful Member
the link you provided is not valid, but it sounds very interesting!
PROBE
Will try another sight.
There are several different ways to build one.
It is basically a untuned "crystal radio detector". The idea is that the RF is too fast for the scope so we detect the RF and get the audio to look at.
I think a lower frequency version can be made to detect the presents of AC that is to fast for the meter.
 

Signalglow

New Member
Thank you, that is very interesting,
I would then have to up the capacitors yes, i think i will try it out, i would just have to figure out where to get a cheap probe i can modify, or i can of course do this probe circuit in a different form.
 

ronsimpson

Well-Known Member
Most Helpful Member
I think you can just use a coax or twisted wires to a meter. Nothing fancy. The output will be DC to audio. The point to turn fast AC into a DC level you can measure.
 

dr pepper

Well-Known Member
Most Helpful Member
The coil will appear as a coil ie inductive below resonance and capacitive above.
The readings on yout lcr will be what you expect, ie calculate from the dims & turns below resonance, and will be way off probably capacitive above resonance.
So your meter if its highest test tone is higher than the srf then you'll be able to get some idea what the freq is.
 

Signalglow

New Member
The coil will appear as a coil ie inductive below resonance and capacitive above.
The readings on yout lcr will be what you expect, ie calculate from the dims & turns below resonance, and will be way off probably capacitive above resonance.
So your meter if its highest test tone is higher than the srf then you'll be able to get some idea what the freq is.
Okay thank you, i get it now, makes sense.
 

unclejed613

Well-Known Member
Most Helpful Member

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