Dave,
If you wind 2 chokes with one on #31 material (u=1000-1500) and one on
#61 material (u=125) and desire the common mode impedance to be 1000
ohms the choke wound on the #31 material may need X turns and the one on
the #61 material ~3X turns. The #31 material choke might have a common
mode impedance of 200+j970 and the one wound on #61 material might be
10+j1000. Even though the phase of the impedances are different they
will both reduce the common mode current to approximately the same
value. A common mode current of 200mA is assumed in a legal limit
environment with the choke which is pretty reasonable. The choke wound
on the #61 material will dissipate 200mA^2 * 10 = .4W while the choke on
the #31 material will dissipate 200mA^2 * 200 = 8W. Also the flux level
on the #61 core will be 3 times lower than on the #31 choke since both
chokes will have the same voltage across them in the common mode
direction and flux is inversely proportional to number of turns.
It is very common to need the permeability of a core to be lower than is
required just to get the number of turns high enough to not have the
flux level be too high. OTOH the more lossy core and the resulting more
resistive common mode choking impedance is perfect for receiving RFI
applications where you don't need to worry about a second choke reducing
the overall choking impedance due to the phase of the impedances. This
is especially valuable when you have some unintentional radiator
interfering with a receiver and needs to be reduced by more than 10dB
which might need more than a single choke. The common mode while
different from the differential mode circuit still obeys the same laws
of RF voltage and current. The common mode circuit has a generator
impedance (not 50 ohms) and a load impedance. If you put a common mode
choke in between them the common mode current will be reduced (it is
possible but unlikely that it increases). Common mode impedances
generally are between 20 and 300 ohms. If you are lucky and your 1000
ohm choke is in a circuit with 20 ohm source and load impedances you see
a huge reduction in the common mode current. However that isn't
necessarily true for a circuit with 300 ohm source and load impedances
where multiple chokes might be required.
I spent years designing EMI compliant telecom and other communication
equipment.
73,
Larry, W0QE
On 2/3/2019 11:08 AM, Dave Cole (NK7Z) wrote:
Larry,
Could you expand a bit on that...
I don't see where mix61 is a better choice over mix31 at HF, other
than the curie point is substantially higher for 61, which allows you
to heat it up more.
Yes with a 31 choke one would need higher impedance to keep heat down
below the curie point, but so what, I want higher impedance in my
chokes... Why does that make 61 a better choice than 31?
Mix61 looks worse to me than 31 for HF, pretty much across the
board... Help me understand you point here please...
73s and thanks,
Dave (NK7Z)
https://www.nk7z.net
Award Manger, 30MDG Grid Awards
ARRL Technical Specialist
ARRL Volunteer Examiner
ARRL OOC for Oregon
On 2/3/19 7:06 AM, Larry Benko wrote:
Jim,
I respectfully disagree with your comment about 61 material cores not
being useful at HF. For transmitting applications they are quite
valuable. The low Q impedance provided by 31 materials is the go to
solution for receiving applications but at high power care must be
taken to ensure that the impedance is much higher than needed to keep
the 31 material from overheating. This is made even more difficult
since the common mode impedance in a circuit is unrelated to the
differential mode impedance and calculating what common mode current
might flow through the common mode choke is not easy to do. Of
course you can sneak up on the problem by just making the choke and
slowly increasing the power at all frequencies you use and see if it
gets too hot in your application. Not very scientific but common mode
circuits are elusive to understand and simulate.
Larry, W0QE
On 2/2/2019 3:30 PM, Jim Brown wrote:
I also recommend Arrow, both for cost and service, especially for
small quantity. It helps a lot that shipping is free for an order of
$20 or more -- IF it's a web order for what's in stock. I also got
a very good price on 100 pieces of the 2.4-in o.d. #31 toroids.
Other good vendors are Lodestone Pacific, Dexter Magnetics, and the
usual industrial electronics houses like Newark, Allied, Mouser, etc.
Note also that there is currently a tariff on these parts, which are
made in Fair-Rite's factory in China.
BUT -- #61 cores are NOT of much use for RFI suppression below UHF,
and they are a terrible choice for a common mode choke (often called
a "current balun").
73, Jim K9YC
On 2/2/2019 9:04 AM, Tom WA9WSJ via RFI wrote:
Hi John,
I once bought some big cores from these guys,
https://www.jpmsupply.com
(FT140-61 ferrites). At that time, a little over a year ago, they
were the best prices I could find.
They do have a couple other types. Maybe they can help you.
Good luck!
73 Tom
On Saturday, February 2, 2019, 9:26:14 AM MST, John Altman
<wa4aip@gmail.com> wrote:
Which company is the best source for bulk bead orders?
Tks,
John
WA4AIP
ARRL Charter Life Member
Sent from my iPad PRO
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