Assessing unknown Ferrite Toroids

Ok so you have had a good day at the Rally and you have come back with a carrier bag full of ferrite toroids that "just might be useful for something". How do you find out what they are? Very few seem to be stamped with any indication of their properties, and some are 'painted' garish colours which presumably means something.

First of all there are basically two types of toroidal core, one is a sintered iron dust and is usefull at higher frequencies, and the other is one of two grades of ferrite material. The ferrite has much higher permiability (more Henries per turn) than the iron dust and is usable at much lower frequencies. Ferrite will thus make fine wide-band transformers, and baluns with bandwidths of at least a decade of frequency (e.g 3 to 30 MHz). There are a range of 'mixes' of the ferrites leading to a wide range of cores with different parameters. Several firms make ferrite material and cores and it is well worth searching for their web sites. This will not help if you do not know the properties of your Bargain cores.

The way to get some insight into the material of your cores is to measure a parameter often given as 'A' which is defined as the inductance per turn (sometimes it has a subscript 10 or 100 in which case it is the inductance of that many turns on the core. It is quite difficult to measure small inductances so wind 8 turns on the core. If you have a handheld DVM with an inductance range just put it across the coil and read the answer. Remember that you cannot have fractional turns on a toroid. every time the wire goes through the central hole it is one turn. Next you need to remember that inductance increases as the SQUARE of the turns. So if you measured 100uH, 4 turns would be 25uH, and 2 turns 6.25uH, so A is 1.56 (uH for 1 turn). You can now compare this with figures quoted for various grades of ferrite in the data sheets, and those used in published designs.

What do you do if you do not have an inductance range on your meter. The next best thing is to put a known value of capacitor with the coil to make a tuned circuit and find the resonant frequency. Lots of people will tell you that you cant 'GDO' a toroid, which is only partly true. If you put a loose wire closed-loop as a single turn through the core you can couple a GDO to that and measure the parallel resonance. You don't need to use the parallel resonance. If you feed a signal generator to the coil and a known capacitor in series you will get a minimum impedance at series resonance. So connect a large resistor 10K to 100K (it depends how sensitive your detector is.) between your signal source and the series LC circuit and connect the other end to the ground of the source. Connect an oscilloscope or an ac milliviltmeter across the LC and swing the generator for a dip. Then use the resonance formula you learned for the RAE, trying not to get confused with the micros, millis and picos. You can even 'ring' a parallel tuned circuit with a square wave and measure the frequency of the ringing on an oscilloscope. I do recommend an inductance meter. Hand held LCR meters can be bought for a very resonable price and help to 'demystify' inductors for you. In fact, like me, you will wonder however you managed without one for so long.

What can you actually use your cores for? Without the full data it is difficult to be sure, but a good guideline is that the very high permiability cores are good for transformers and chokes, but not good for filter inductors. The lower permiability ferrite cores or the iron dust cores will work well as filter inductors. It may be a 'suck-it-and-see' decision, if the filter core gets hot with RF power applied then it is probably not the right material.

Inductors are FUN, they are one of the last of the "make it yourself" radio components.

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