Ferriteless Toroids

In the club talk on electronic kits such as the MKARS80 by Steve Drury, G6ALU, on Tuesday, 20/9/11, it was said that, owing to the poor availability of Ferrite Toroids in India, constructors had taken to winding toroids on tap washers. A question was asked: “How does the inductance of this compare with the same number of turns wound as a normal solenoid”? It was suggested that, as the magnetic field was in a continuous closed ring in a toroid, its inductance should be greater than that of a solenoid. The questioner was left with a lingering doubt about this on several grounds, so it was decided to do some experiments. Instead of a tap washer, a rubber grommet was employed as this was much nearer to the shape of typical ferrite toroids. A suitable grommet was selected and its dimensions carefully measured as shown in figure 1. All dimensions are in mm. A photograph of the grommet prior to winding wire onto it is shown in figure 2.

From these dimensions, the “core”, or winding area was calculated to be 49.53mm2 which is equivalent to a circular area with a diameter of 7.04mm. A wooden dowel was turned down to this diameter to act as a coil former, and drilled with two small holes to anchor a solenoid winding. This was to be used later for comparison of inductances.
In order to prevent the winding compressing the slot in the grommet, (which normally engages with the hole in a chassis), two plastic sleeves stripped from PVC covered wire were pressed into it. The toroid was then wound with 0.68mm (bare), 0.73mm (enamel) wire, (thought to be 22SWG). 21 turns were required to completely fill the central region. A photograph of the completed toroid is shown in figure 3, which also clearly shows the yellow plastic sleeve filling the slot. Measurement of inductance was made using an “Atlas Peak LCR meter” at a frequency of 200kHz. This meter only reads to the nearest 0.1?H, but repeated measurements often yield readings differing by 0.1?H, which can be useful in indicating whether the actual inductance is slightly higher or lower than the figure indicated. The inductance of the “Ferriteless Toroid” was estimated at just over 0.3?H.
The wooden coil former for the solenoid comparison inductor was also wound with 21 turns as shown in figure 4.

One mechanical disadvantage of a solenoid compared to a toroid is the need to anchor the ends of the winding, otherwise, when the winding tension is released, the winding tends to unwind like a spring, resulting in slightly fewer turns of greater diameter. This tendency to unwind is much less prevalent with a toroid. By drilling holes through the centre of the wooden former and anchoring the wire ends through them, the wire at each end of the main coil represents half a turn, because it bisects the magnetic field emerging from the end of the solenoid. Thus the winding consisted of 20 complete turns plus two half turns. The inductance was again measured several times with the LCR meter and is estimated at just over 0.7?H, i.e. about twice that of the toroid.
In conclusion, it appears that winding an inductor as a toroid on non ferro-magnetic material is not an effective way of constructing an inductance compared to winding contiguous turns as a solenoid.



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