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Re: [RFI] Bulk order source for Ferrite beads?

To: rfi@contesting.com
Subject: Re: [RFI] Bulk order source for Ferrite beads?
From: Larry Benko <xxw0qe@comcast.net>
Date: Sun, 3 Feb 2019 13:19:10 -0700
List-post: <mailto:rfi@contesting.com>
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


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