EMRFD Message Archive 2564
Message Date From Subject 2564 2009-01-03 12:58:04 Niels A. Moseley 100uH RF chokes - how to make them? Hi all,
In many circuits a need arises for 100uH (or greater) RF chokes.
They seem to be difficult to obtain and/or relatively expensive.
Is it possible to make them ourselves easily? And if so, does anyone
have any experience making them?
2565 2009-01-03 13:47:12 Thomas S. Knutsen Re: 100uH RF chokes - how to make them? 2009/1/3 Niels A. Moseley <firstname.lastname@example.org>
> Hi all,[Non-text portions of this message have been removed]
> In many circuits a need arises for 100uH (or greater) RF chokes.
> They seem to be difficult to obtain and/or relatively expensive.
> Is it possible to make them ourselves easily? And if so, does anyone
> have any experience making them?
> Niels PE1OIT.
2566 2009-01-03 14:29:35 leon Heller Re: 100uH RF chokes - how to make them? ----- Original Message -----
2567 2009-01-03 15:11:49 KB9BVN Re: 100uH RF chokes - how to make them? Dan's Small Parts has 10 pieces for $1.50
----- Original Message -----
2568 2009-01-03 18:56:16 Pat Bunn Re: 100uH RF chokes - how to make them? Wind one on a ferrite toroid core.
----- Original Message -----
2569 2009-01-03 18:59:51 KB9BVN Re: 100uH RF chokes - how to make them? A T50-43 is 102 uH with about 14 turns.
A T37-43 is about 100 uH with 15 turns.
----- Original Message -----
2570 2009-01-03 20:59:15 email@example.com Re: 100uH RF chokes - how to make them? Anyone ever measure the self-resonant freq. of a 100 uH toroid?
They only function as an r.f. choke below the self-resonant freq., so it is important to know that parameter.
----- Original Message -----
2572 2009-01-04 03:28:15 Niels A. Moseley 100uH RF chokes - how to make them? TNX! Many thanks to all who responded!
I never realized some ferrite cores give 100uH without winding an
unreasonable number of windings. My question seems almost embarrasing
2573 2009-01-04 08:24:49 Ed - K9EW Re: 100uH RF chokes - how to make them? TNX! Hi Neils,
Don't feel embarassed... I now know where to buy 100uH chokes at
$1.50 for 10 pieces (Dan's Small Parts), and also how to make my own
(16t #43 core). I have bookmarked this thread for future reference.
I, for one, am glad you asked that question.
ed - k9ew
2574 2009-01-04 08:24:49 Alan Yates Re: 100uH RF chokes - how to make them? I just tried it with a rather primitive lash-up:
15 turns on a FT50-43, measured 97.3 uH with the Carver LC-Tester.
I whipped up a quick 50 Ohm line test fixture with some plugs and IC
socket pins. The inductor was put inline with the signal generator
through to an uncalibrated log power meter feeding a VOM for "dipping".
With 3p9 in parallel the minima was observed at 8.2 MHz and with 10p
across it 5.3 MHz. Measurements with larger capacitances, 100 pF and
379 pF gave me somewhat odd readings, 1.47MHz and 787 kHz respectively -
experimental error I assume? I didn't do this with much precision at
The first two measurements suggest a distributed capacitance ~ 0.4-0.5
pF which gives an SRF of abut 22-25 MHz. With no capacitance across the
coil I could not locate a dip until 64.2 MHz. I really have my doubts
about that figure, that means a parasitic capacitance of only 63 fF!
All I can say with confidence is that its SRF is beyond 20 MHz. It is
definately capacitive by 106 MHz, where placed in parallel with a 215 nH
fixture it pulled the resonance indicated by my tone dipper down (rather
than up) but attempts to dip it directly with the tone dipper were
frustrating, probably because it has a large resistive component in
series with the reactive ones. Thought: its total impedance magnitude
seems pretty high, even above self-resonance.
Maybe someone with a better setup could try the experiment? A VNA sweep
would be interesting.
On Sat, 3 Jan 2009 firstname.lastname@example.org wrote:
> Anyone ever measure the self-resonant freq. of a 100 uH toroid?
> They only function as an r.f. choke below the self-resonant freq., so
> it is important to know that parameter.
2575 2009-01-04 21:24:56 kerrypwr Re: 100uH RF chokes - how to make them? "A VNA sweep would be interesting".
I did a couple of quick sweeps; one of a 100uH choke and another of a
The sweeps are in my "Kerry VK2TIL" file folder.
2576 2009-01-05 10:26:10 Alan Yates Re: 100uH RF chokes - how to make them? On Mon, 5 Jan 2009, kerrypwr wrote:
> "A VNA sweep would be interesting".Thanks for doing these.
> I did a couple of quick sweeps; one of a 100uH choke and another of a
> 1mH choke.
Do you have a FT50-43 core for comparison purposes? I can send you some
I took the 100u choke sweep and tried to derive a rough model from it:
A 62.13 dB dip at 10.145 MHz with a 122.5 uH inductor is a Q of about
8.17 and indicates a parasitic capacitance of about 2 pF. The
corresponding series resistance is about 956 Ohms! (Corresponding
parallel effective resistance of 63k8 at self-resonance).
I whacked that into LT Spice:
The parallel model matches the notch depth somewhat more closely, but
the 1 and 60 MHz edges are way off, no doubt because the model is based
entirely on the behavour around self-resonance. Did I do something
wrong or do the core properties and other subtle parasitics make that
much of a difference?
How did you arrive at the "measured" value BTW?
2577 2009-01-05 13:40:30 kerrypwr Re: 100uH RF chokes - how to make them? I didn't have much time yesterday but have found some today.
There are more sweeps in my Files;
One is for a toroid wound as yours; another is for a nominal 15uH RF
The interesting one is a sweep of the 15uH & 100uH RF chokes in series.
The toroid behaves quite well in comparison to the "tuned circuit"
responses of the commercial rf chokes.
The RF chokes have lots of turns of fine wire so are quite different
in parasitic C to the 15-turn toroid inductor.
2578 2009-01-06 21:45:06 Steve Swartz Re: 100uH RF chokes - how to make them? Hello all. I hate to stick my foot in my mouth on my first post here
but something called a choke (as opposed to an inductor) is usually
meant to carry DC current as well as act as an inductor. My memory is
that toriods are much more prone to core satuation effects then things
wound over bars. This may be important depending on were in the circiut
the "choke" is going
2579 2009-01-07 12:04:21 kerrypwr Re: 100uH RF chokes - how to make them? A very good point Steve and one that we may have lost sight of in this
I'm not certain that ferrite toroids saturate more easily than rods
do; there may be a marginal difference arising from the rod field
being partly in air but I think that both types will saturate.
Iron-powder should be better but the catch is that lots of turns are
It might be instructive to examine some devices through bias-tees with
2580 2009-01-07 15:07:22 Alan Yates Re: 100uH RF chokes - how to make them? Indeed, in fact I've recently built a current-ramp testing device for
assessing core saturation in inductors:
This was a result of work on switchmode power converters, but I just
used it to measure the approximate saturation point of my 15 turn
FT50-43 core. Significant inductance drop starts at about 200 mA.
That is about 94 A/m. The core ur will change quite a lot with H, so
without measuring B directly I can't really say how many Tesla it can
take, but assuming it can take about 250 mT (a guess) that puts the peak
ur at about 2100 which seems reasonable.
I might have a bit more of a fiddle with measuring magnetic properties,
this discussion has got me quite interested in it. We often just treat
cores as something you wind N turns on and forget about it, but there is
quite a lot to using them properly.
On Wed, 7 Jan 2009, Steve Swartz wrote:
> Hello all. I hate to stick my foot in my mouth on my first post here
> but something called a choke (as opposed to an inductor) is usually
> meant to carry DC current as well as act as an inductor. My memory is
> that toriods are much more prone to core satuation effects then things
> wound over bars. This may be important depending on were in the circiut
> the "choke" is going
2581 2009-01-08 17:08:06 Nick Kennedy Re: 100uH RF chokes - how to make them? Wow, that's very nice. I've also been stewing on methods to determine
the saturation point, but now I have a tested approach to get me
I'm also interested in learning more about magnetics, partly because
it's so durn complicated. I really enjoyed the article on dissecting
a typical PC switching supply in the Nov/Dec 2004 QEX. I'm wanting to
try to build a Royer oscillator, for no particular good reason except
that it's interesting. Saturation of a core is key to its operation.
2582 2009-01-09 06:28:04 Dave Re: 100uH RF chokes - how to make them? 2583 2009-01-09 12:57:25 joop_l Re: 100uH RF chokes - how to make them? 2584 2009-01-09 18:04:32 Glen Leinweber Re: 100uH RF chokes - how to make them? About inductor saturation...
Have noticed that most inductance meters behave
counter-intuitive when the inductors that they're measuring
are driven towards saturation. I'd have expected that
saturation would cause inductance to DECREASE. But
the meter shows that inductance RISES as you move out
from linear operation of the ferrite or iron. Is this actually
true? Yes, I think it is true.
The initial permeability of ferrite is a bit lower than
permeability on the way to saturation. Linear inductors
operate in the initial permeability region. As you start to
saturate by applying more signal, permeability goes up,
until REALLY large signals hit the true saturation level
where relative permeability approaches one.
So inductance starts with small signals at a constant
level, increases some with large signals, then decreases
with still larger signals.
Once you see that inductance is increasing with signal
level, you're into non-linear operation, and inductor Q is
I've tried some severe tests of popular iron-powdered
toroids to see if they saturate and saw NO saturation.
One was added to a 240W Weller soldering gun, as a
5-turn primary. A sense winding was wound onto
the toroid as a secondary winding and viewed on a
'scope - it remained sinusoidal, so no saturation.
A clamp-on current probe whanged full-scale. I'd guess
that magnetizing current was well over 100A.
A ferrite toroid like 43 material in the same fixture
saturated severely and got a little warm
2587 2009-01-10 06:07:59 Dino Papas Re: 100uH RF chokes - how to make them? As an aside, the M3 Electronix LCR meter has an accessory that allows
you to apply a DC bias (up to 30 VDC/100mA) to the inductor in order
to determine the effect of core saturation on the inductor's value.
2588 2009-01-10 07:11:07 Alan Yates Re: 100uH RF chokes - how to make them? On Fri, 9 Jan 2009, joop_l wrote:
> Nice experiment. I recently followed a discussion elsewhere where itIndeed, I wondered about the core remanence. I'll have to do a full B-H
> was mentioned one should realize that there is some remaining
> magnetism using this method where the current always flows in one
test of the core material, but doing that with a pre-made device is a
bit of a job. I suspect the saturation value is a valid "engineering"
measurement as it will read on the safe side and you probably don't want
to be pushing the core right up to the start of severe non-linearity
I tried biasing the core of a commercial choke with an external
permanent magnet. I've added a video of the effect upon the trace.
Pretty much as you expect you can either make it saturate eariler or
sooner depending on the polarity of the bias field. It appears there is
some non-trivial remanence, but to measure it accurately I'll need to
perform a real B-H test.
> The images 10 and 11 here show the effect (Dutch site):Actually it got me thinking about biasing cores in unipolar service to
> On a sweep to 220mT the remaining magnetism is about 50mT.
> I guess for AC (RF?) use there will be some more headroom than your
> method will show. But for your original DC/DC converter purpose it
> might be adequate.
improve the ultimate saturation. No idea if that is a viable idea, and
doing it with a DC current would probably require dual cores so you
could cancel the effect of the primary current, making it kinda
redundant - just use a bigger/gapped core in the first place. Total
volume saving - no idea?
> Another method could be to use an L meter that does AC measurementsI've tried that before in a lash-up. Seems viable if you are careful
> superimposed on a DC current. One article on a quick google:
about the inductance measurement method's excitation amplitude. The RFC
and/or capacitor needed can be nulled-out as part of the experimental
setup, so it should be fairly accurate. Agrees roughly with the voltage
pulse testing. Doesn't really tell you too much about the full B-H
relationship, but it can be carried out with one winding on a commercial
part very quickly, much like the pulse/ramp test.