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[Towertalk] Gluing broken ferrite

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Subject: [Towertalk] Gluing broken ferrite
From: (Chuck Counselman)
Date: Sat, 11 Jan 2003 10:46:47 -0500
Someone mentioned that he had discarded the pieces of a ferrite 
toroid that had broken in half (due to overheating); I suggested 
"glu[ing] them back together, using rubber bands to clamp them 
tightly while the glue set, to minimize the gap"; and he asked 
off-list, "Does that really work?"

To answer this question I've done another experiment.

A while ago I received a shipment of FT-240-77 or -43 toroids, of 
which a few had been broken by rough handling.  I no longer have the 
paperwork showing which type (77 or 43) they were.  Anyway, I used 
all of the intact toroids for RFI-suppression common-mode chokes on 
computer, telephone, and other non-RF-power-carrying cables in my 
house; and I saved the pieces of the broken toroids, planning to glue 
them back together when I got a round tuit.

In response to last night's question, I glued one of the broken 
toroids together.  This toroid had broken in half pretty cleanly.  (I 
used epoxy and two rubber bands; and this morning I was pleased to 
find that the rubber bands came right off.  Apparently epoxy does not 
bond to rubber bands -- which by itself is useful knowledge.)

A few years ago I had measured the impedance of a five-turn winding 
on each of two FT-240-77 toroids and one FT-240-43 toroid; and I had 
saved the results; so this morning I wound five turns on the glued 
toroid and measured it.  Here are the results of measuring all four 
toroids.  Where only the magnitude and not the angle of the impedance 
is given, I judged the phase-angle measurement to be unreliable 
because the magnitude of the impedance was so high.  Within my 
estimated uncertainties of 10 to 20 degrees, all of these phases were 
zero.  In other words, these chokes looked more like resistors than 
inductors -- as expected for these ferrites at these frequencies. 
(Type 43, and especially type 77 ferrite, is suitable for 
RFI-suppression chokes but not for inductors at these frequencies.)

Broken & glued toroid        Never-broken FT-240-77 #1
Freq.| |Z|  |                Freq.| |Z|  |
(MHz)|(ohms)|                (MHz)|(ohms)|
-----+------+                -----+------+
  3.5 |  320                   3.5 |  320
14.0 |  183                  14.0 |  225
28.0 |  138 angle 27 deg     28.0 |  232 angle 13 deg

Never-broken FT-240-43       Never-broken FT-240-77 #2
Freq.| |Z|  |                Freq.| |Z|  |
(MHz)|(ohms)|                (MHz)|(ohms)|
-----+------+                -----+------+
  3.5 |  332                   3.5 |  365
14.0 |  235                  14.0 |  244
28.0 |  203 angle 18 deg     28.0 |  246 angle 13

At f = 3.5 MHz, the broken & glued toroid is indistinguishable from a 
never-broken one.  Note that type-77 and -43 toroids are not 
precision components.  Substantial variation from sample to samples 
is the norm.

At 14 MHz there is a hint, and and 28 MHz there is a stronger 
indication, that the "air gap" (actually, two glue-filled gaps) in 
the broken & glued toroid has reduced its effectiveness.

Based on these results, I would advise going ahead and gluing broken 
ferrite cores back together.  If you have an impedance bridge, I 
advise using it to check any glued core.  Depending on both the 
ferrite type and the radio frequency, the air (glue) gap may or may 
not matter.

Probably the best types of glue for ferrite are superglue and epoxy. 
Superglue has the possible advantage of fast curing.  I don't know 
whether superglue is as strong or resists water and heat as well as 
epoxy.  I would not use Ambroid(R) or Duco(R) cement or "hot" glue if 
the ferrite might get hot or wet.  I would also not use paper or wood 

73 de Chuck, W1HIS

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