Re: restore bad ferrites


From: John Wendler (
Date: Mon Nov 21 1994 - 08:36:12 EST

> There is another way of restoring toroid cores, that could be almost
> as exciting as spoiling spouse's supper. You will require a HIGH
> current A.C. source, such as power from the 60 Hz. A.C. line. A means
> of adjusting the current is necessary as well.
> An appropriate setup would be a VARIAC (a variable Autotransformer)
> going into a step-down transformer (something like 6.3v A.C. out).
> Wind as many turns of heavy gage wire through the core to be restored
> as you can fit, and connect to the low-voltage output of the step-down
> transformer. To get a nice high current flowing thru the wire, the
> connections must be very low-resistance.
> To restore the core, turn up the VARIAC , and very SLOWLY turn it
> down again. By slowly, take about twenty seconds to turn it down.
> The idea here is to get the core magnetically saturated on the
> peaks of the 60 Hz. waveform. Since the core will alternately be
> saturated first one way (on +ve peaks), then the other way (on -ve
> peaks), if you slowly decrease the current, the core will be left
> with random magnetization in the end.....its restored!
> Larger cores will require more current to restore. The really small
> memory cores (you can't fit them onto a resistor lead) require half an
> amp to saturate. I'd roughly guess that a small FT37 or T50 type
> core that is common in ham usage would require tens of amps to
> adequately saturate, and restore.
> Glen Leinweber VE3DNL
This is a nice variant of the "ring down" technique that I mentioned
in a previous post. The critical quantity that needs to be maximized
is not current alone, but ampere-turns. Ampere-turns is, as might
be expected, N*I. If, for example, a core requires 10 Ampere-turns to
saturate, then this can be reached with 10 A through 1 turn, 5 A through
2 turns, or 100 mA through 100 turns. Hope this saves someone from
threading some #8 gauge wire 8-)
73 de N5CQU

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