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In Reply to: Panasonic V-850 posted by mikeyb on April 20, 2007 at 10:25:54:
Since when did inductance depend on frequency? If it did, why would both terms appear in the formula for reactance?
Follow Ups:
All ferrous core chokes exhibit varying inductance over frequency, so much so that specifying one without the other is almost meaningless. The formula for XL is obviously based on the actual inductance at the frequency of operation.
Correct me if I'm wrong here, but all the references I found state inductance is linearly proportional to permeability and effective complex permeability decreases with increased frequency for a number of reasons. That 22 mH device should have more inductance at lower frequencies, not less. It's interwinding capacitances should be small enough that it remains an inductor well past the operational limit of near any OPT, save maybe a Plitron.
Thermionically addicted.
Thank you for putting me right. I see I need to revisit RDH4.
For backing me up on this important point.
Well, I agree that the impedance is frequency dependent.I do think it is reasonable to want filtering at frequencies considerably above 120/100 Hz. What I wanted to show is that it might (probably will) require another choke in addition to the usual smoothing choke. The extra choke does little to smooth low frequency ripple, but who cares? That's not why it's there.
-- Dave
So if we add a "RF" choke to the 120hz choke all is well? I mean we can have them both, good 120hz filtering and good RF filtering? Can we just add the RF choke between the rectifiers and the 120Hz choke? ( In a choke input PS)Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
First of all, I want to make clear that I'm not necessarily thinking of an all-out RF choke. Really, I'm just thinking that something that reaches out to a few tens of kHz might be an improvement over the couple of kHz that an ordinary smoothing choke might be good for. It might be that Jeff's low L chokes do that. I don't know, it's just a thought.But, yeah, I suppose that intuitively it makes sense to put the choke as far upstream as possible, i.e. right after the rectifier. I have expressed concern here before that, depending on the following capacitor, such a choke might see fierce current spikes which might result in core saturation. Again, I don't know, I really can't predict what will happen when one plunks an inductor intended for an entirely different application into their power supply.
-- Dave
Right, and I'll spell it out a little more. Any inductor, even one with an ideal core will have winding capacitance(s) that shunt the coil. What this means is that above some frequency the device acts more like a capacitor than an inductor. It's called the self resonance frequency and we might be surprised how low it is with typical off the shelf power supply chokes. One or two kilohertz is not uncommon.The other factor is the core material. The silicon steel cores used in typical power supply chokes is simply not effective at higher frequencies. Once the core 'goes away' you are left with an air cored inductor which Obviously has much less inductance. It's surprising how soon silicon steel disappears. It's also surprising that going to a higher grade grain oriented silicon steel might actually make things worse.
Anyone can observe this on the bench: put a resistor in series with your inductor and drive the combination with a signal generator. Sweep the frequency while looking at the voltage across the choke. It will reach a peak at some frequency and fall off above that. Don't bother calculating XL=2*pi*f*L above that frequency, or even anywhere near it.
So, to the real questions: do we want our ps chokes to work at frequencies higher than 120Hz? How much higher? What kind of choke do we need for that?
-- Dave
> > So, to the real questions: do we want our ps chokes to work at frequencies higher than 120Hz? How much higher? What kind of choke do we need for that? < <Dave asks EXCELLENT questions. I think the answer lies in the particular service placement of a choke. An inductance that's stable up to 500 Hz. rates to be fine in choke I/P or pseudo-choke I/P service. The ripple waveform is or very nearly is a sine wave. Filtering out overtones of the ripple fundamental is not particularly necessary. OTOH, a voltage doubler using LARGE caps. in the stack exhibits a HIGHLY triangular waveform (SMALL conduction angle). The amount of ripple freq. overtones here is substantial and the choke following the doubler stack is the defense against the "hash". The inductance needs to be stable to fairly high freqencies.
Eli D.
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