Coils/Inductors do have a "resonance frequency"?! - Page 1
Forum section: electronics / beginners

author: Axel1973
author_rank: Regular Contributor
timestamp: June 18, 2012, 07:50:07 pm
content: Last days i tried to learn something about coils/inductors from the interwebs. didnt got very far yet. The Math stuff on Inductors on wikipedia makes me switch to overload. however..on youTubes some "low tech" Radio freak said that every "coil"/inductor would have a certain resonance frequency. Is that true?!I know that LC Cirtuits do have resonances. But a simple handwound coil without a C do too?!I would expect a coil to be conductive to DC and low freq AC while becoming more resistive when AC freq increases. If freq gets even more higher i would expect the coil to get even more resitive but never more conductive at higher freq. So i would not expect any "resonance" frequency anywhere without an additional capacitor.Am i correct or do i miss something here?Im always willing to learn. Thanks for any usefull/informative feedback!greetsAxel

author: deephaven
author_rank: Frequent Contributor
timestamp: June 18, 2012, 07:57:11 pm
content: There will always be a bit of stray capacitance around, distributed between adjacent turns on the inductor's winding etc. Whenever you have two pieces of metal, you will have a capacitor. So your 'self-resonance' is just the inductor forming a tuned circuit with those stray bits of capacitance.

author: gregariz
author_rank: Frequent Contributor
timestamp: June 18, 2012, 07:58:28 pm
content: Every inductor has some capacitance between the indiviual turns of the inductor. ie this 'distributed' capacitance is why all coils have a resonance (Self Resonant Freq or SRF in the data sheets).

author: T4P
author_rank: Super Contributor
timestamp: June 18, 2012, 08:12:02 pm
content: 2 pieces of metal = BAAM you got a capacitor same for mosfets

author: Axel1973
author_rank: Regular Contributor
timestamp: June 18, 2012, 08:58:00 pm
content: well yeah.i knew about this "small" capacitance even though i dont know about its values/amount. But is it realy significant on freqs <1Ghz?And does it realy means that at resonance freq there will be a "peak voltage level" that will again drop if freq will be increased above res-freq?thanks for feedback.yoursAxel

author: deephaven
author_rank: Frequent Contributor
timestamp: June 18, 2012, 09:32:30 pm
content: Yes, a couple of pF is significant at much lower frequencies than 1 GHz, so you will get that voltage peak at the resonant frequency. Inductor manufacturers will often quote their inductors' self resonant frequency.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 18, 2012, 09:39:14 pm
content: Every inductor has a self resonant frequency. That is the frequency where the parasitic capacitance forms a resonance. At this frequency the impedance is maximum, and above that frequency the device acts as a capacitor, not an inductor. Self resonant frequency varies quite a bit from inductor to inductor, and for a large inductor with many turns it could be quite low in frequency.

author: w2aew
author_rank: Super Contributor
timestamp: June 18, 2012, 10:36:26 pm
content: As others have said, it's the parasitic interwinding capacitance and/or lead-lead capacitance that leads to the self-resonant frequency. In many cases it's only a few pF, more or less, but that can make a difference. The SRF will be equal to 1/(2*pi*SQRT[L*C]), where C is the equivalent parasitic capacitance seen across the inductor.Capacitors have a self-resonant frequency too - due to the parasitic inductance of the leads/termination/construction.Even resistors can start having capacitive or inductive behaviors at high frequencies due to the parasitics of the packaging and construction.

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2012, 10:59:14 pm
content: A typical Tesla coil works at around 300khz and the secondary coil LC circuit uses only a metal object sitting on top to create the capacitor.The 15pf or so from that object provides enough capacitance to bring the secondary resonance to the same frequency as the primary.So yeah, a few picofarads can make a huge difference

author: codeboy2k
author_rank: Super Contributor
timestamp: June 19, 2012, 01:14:40 am
content: Quote from: w2aew on June 18, 2012, 10:36:26 pmI'm being a pedant here, but the inductor has a small resistance over it's entire length as well, and the capacitance is across the series LR.So the Self Resonant Frequency (SRF) in radians/second will be:where L is the inductance, C is the parasitic inter-winding capacitance and R is the resistance of the inductor wire.and in Hz will be :

author: westfw
author_rank: Super Contributor
timestamp: June 19, 2012, 04:35:25 am
content: I would say that one NORMALLY designs their circuits and picks their inductors in ways such that the self-resonant frequency is NOT particularly relevant. (Just like one designs transistor circuits in a way that the actual gain of the transistor is not relevant.)

author: Axel1973
author_rank: Regular Contributor
timestamp: June 19, 2012, 06:39:38 am
content: WOW!Now thas was quiet a bunch!That little discussion is realy interesting and realy cleared things up for me a lot!THANKS TO ALL FOR ALL THE FEEDBACK! You helped me a lot!greetsAxel

author: szhighstar
author_rank: Contributor
timestamp: July 01, 2012, 07:59:10 am
content: yes, difference SRF value for defference magnetics parts, we use WK3260 to test SRF if SRF is lower than 3M, we use 4287A to test if SRF is higher.SRF is complex,it depends on core material and winding technical, even how to stay leader of wire, relation inductor and capacitance and DCR.