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Author Topic: Question on ferrites  (Read 6375 times)
WX2S
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« on: February 23, 2013, 04:38:06 AM »

Ferrites are sold as having X ohms impedance at Y MHz. Does that apply to all the wires through them if there is more than one? Or would it be better to use one ferrite per wire? (Application: smoke alarms.)

73,
-WX2S.
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
AC2EU
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« Reply #1 on: February 23, 2013, 07:23:46 AM »

It refers to one turn.
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WX2S
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« Reply #2 on: February 23, 2013, 07:30:39 AM »

Yup, but what I was really asking is this. The smoke detectors in my house have three wires. Say that the ferrite is rated at 100 ohms at 10 MHz. If I put all three wires through the ferrite and give them all one turn, will each of the three see a 100 ohm reactance at 10 MHz, or something less?

73,
- WX2S

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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
WX2S
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« Reply #3 on: February 23, 2013, 10:52:23 AM »

Well, after reading K9YC's RFI FAQ, I'm led to believe that the noise on the detectors is differential mode and therefore ferrites are not going to do a bit of good. So I'm ordering some caps to build delta bridges between the three connectors to the smokes.

Quote from: K9CY
Doorbells, Smoke Alarms, etc. Coupling of RF into these products is often differential mode. When it is, a good RF capacitor across the terminals is usually an effective fix. 0.1 – 0.47 µF will usually do the job. Be sure that the voltage rating of the capacitor is sufficient – study product documentation to see what voltages are used. Most products like this use 12-24 volts for control.

The detectors use 120V hot and neutral wires, plus a signaling line. So I bought some 0.47 uf 1KV caps. I'll let you know how it goes because I've seen others with the same problem (QRO sets off the smoke alarm.)

73,
- WX2S.

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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
W5DXP
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« Reply #4 on: February 23, 2013, 12:51:55 PM »

If I put all three wires through the ferrite and give them all one turn, will each of the three see a 100 ohm reactance at 10 MHz, or something less?

Good question. Choking discriminates against  common-mode signals. When the signals are differential, as they are for two-wire AC power and two-wire RF transmission lines, the choke doesn't have much effect on the differential signals. So you would have to know what types of currents exist in those three wires in order to know what effect they have on the signals in the three wires.
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W6EM
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« Reply #5 on: February 24, 2013, 08:40:30 AM »

Your question describes what would be a simple current transformer with three windings.  If the alternating current flowing to and from the smoke detector device is balanced, meaning the net magnetic flux produced in the core would cancel when all winding currents sum to zero, then it should appear as three series inductances in parallel to a common mode RF signal on all three.

As to the inductor values, when you have multiple windings, magnetically coupled to each other, there is mutual inductance from one to the other, as well as each self inductance.  And, with three in parallel, the common mode reactance would be roughly one third that of one winding plus the mutual reactance.

Try a snap-on split core with high relative permeability ferrite like a mix 43 or 31 and several turns.  There are some other ferrite compounds with higher permeabilities that might work even better, if you can find a source.  Fair-rite has a list of its distributors on its site.  Mouser carries quite a few of them.
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KB3HG
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« Reply #6 on: February 25, 2013, 07:52:45 PM »

Real dumb question here, did you contact the smoke detectors manufacturer? A real observation here, when it comes to life safety devices, when in doubt through it out. Should you have a fire and the Fire Marshall finds something not normal it could void your insurance.  I know this ferrite question came from somewhere else but smoke heads are cheap. Someones life is not worth it.
contact the manufacturer.
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WB6BYU
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« Reply #7 on: February 25, 2013, 07:59:14 PM »

Quote from: W6EM

...when you have multiple windings, magnetically coupled to each other, there is mutual inductance from one to the other, as well as each self inductance.  And, with three in parallel, the common mode reactance would be roughly one third that of one winding plus the mutual reactance.




When the conductors are closely coupled (as they usually are when passed
through a common ferrite core) then the common mode inductance is the
same as that of ONE winding, regardless of the number of wires.
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N1UK
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« Reply #8 on: February 27, 2013, 01:36:40 PM »

Quote
The detectors use 120V hot and neutral wires, plus a signaling line. So I bought some 0.47 uf 1KV caps

Be careful because capacitors that go across the mains have to be a special type, X or Y class capacitors.

Here is some introductory reading

http://www.vishay.com/docs/28153/anaccaps.pdf

You don't want to start a fire.


Mark N1UK
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W6EM
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« Reply #9 on: March 01, 2013, 05:10:30 PM »

Quote from: W6EM

...when you have multiple windings, magnetically coupled to each other, there is mutual inductance from one to the other, as well as each self inductance.  And, with three in parallel, the common mode reactance would be roughly one third that of one winding plus the mutual reactance.




When the conductors are closely coupled (as they usually are when passed
through a common ferrite core) then the common mode inductance is the
same as that of ONE winding, regardless of the number of wires.


Interesting postulate.  Please give me a technical reference, and not from an amateur radio publication.
« Last Edit: March 01, 2013, 05:12:41 PM by W6EM » Logged
W6EM
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Posts: 882




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« Reply #10 on: March 01, 2013, 05:25:00 PM »

Quote
The detectors use 120V hot and neutral wires, plus a signaling line. So I bought some 0.47 uf 1KV caps

Be careful because capacitors that go across the mains have to be a special type, X or Y class capacitors.

Here is some introductory reading

http://www.vishay.com/docs/28153/anaccaps.pdf

You don't want to start a fire.


Mark N1UK


Intersting read, but primarily of concern for thin film dielectric capacitors.  A good non-polar dielectric like ceramic or mica with WVDC rating at 3 or more times the RMS AC value should work fine.  600V ceramics should be fine.
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W5DXP
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« Reply #11 on: March 02, 2013, 06:50:04 AM »

Interesting postulate.  Please give me a technical reference, and not from an amateur radio publication.

We probably don't need a technical reference. Common mode current is, by definition, the current common to both (two) wires. How could the toroid possibly tell the difference between one amp of common mode current flowing in each of two wires vs two amps of common mode current flowing in one wire? Ideally, the induced flux in the toroid is identical in both cases no matter how many wires are involved through which the total common mode current divides.
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WB6BYU
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« Reply #12 on: March 02, 2013, 12:17:38 PM »

Quote from: W6EM

Interesting postulate.  Please give me a technical reference, and not from an amateur radio publication.




Hard to find it in a non-ham publication, because any Engineering text would
leave it as an exercise for the reader.

But try this:  Terman* gives the inductance of two coupled coils in parallel as

Ltot = (L1 * L2 - M * M ) / ( L1 + L2 -/+ 2M )

      (we use the minus sign in the last term because the fields are aiding)

where L1 and L2 are the coils and M is the mutual inductance.  In the case
in question, L1 = L2, and M = k * the inductance of either winding, where k
is the coupling coefficient.

Unfortunately the formula reduces to 0 / 0 when k = 1 and M = L, but
if you solve for the limit as k -> 1 (or M -> L) you'll find that this reduces
to L * ( 1 + k ) / 2, so when k = 1, Ltot = L.

Or you can simply solve the original equation for M = 0.90 L and for M = 0.99 L
and draw your own conclusions about how close Ltot will be to L.

* Terman, Fredrick Emmons, Radio Engineers' Handbook, First Edition,
McGraw-Hill, 1943, pp 64-65
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WX2S
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« Reply #13 on: March 02, 2013, 03:49:19 PM »

Quote
The detectors use 120V hot and neutral wires, plus a signaling line. So I bought some 0.47 uf 1KV caps

Be careful because capacitors that go across the mains have to be a special type, X or Y class capacitors.

Here is some introductory reading

http://www.vishay.com/docs/28153/anaccaps.pdf

You don't want to start a fire.


Mark N1UK
Well, the caps didn't work anyway. I spent a couple of hours today making up assemblies and stuffing them into the wall boxes. When my amp is key-down for more than about 30 seconds, the detectors go off.  Sad

- WX2S
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
W6EM
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Posts: 882




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« Reply #14 on: March 03, 2013, 09:44:50 AM »

Interesting postulate.  Please give me a technical reference, and not from an amateur radio publication.

We probably don't need a technical reference. Common mode current is, by definition, the current common to both (two) wires. How could the toroid possibly tell the difference between one amp of common mode current flowing in each of two wires vs two amps of common mode current flowing in one wire? Ideally, the induced flux in the toroid is identical in both cases no matter how many wires are involved through which the total common mode current divides.

True, Cecil, but it wasn't a flux question, per se.  Although mutual inductance is involved in the end result of what the inductance is for three paralleled windings.  In fact, as you know, you can increase magnetic field intensity to a point beyond which flux density ceases to increase.
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