Document version 1.1 Page | 18
Appendix.
Winding guide:
There is no special trick to winding a toroidal core, simply wind the wire in any direction and any way you like.
However there are some recommendation how to do, it and you need to know what counts as a turn and what
does not.
In addition, the way you apply the turns will affect the inductance slightly. In my calculations for this design, I
assume that you spread out the turns as much as you can.
How to spread the windings on the core.
Look at the picture below. It shows two inductors, both with 10 turns of wire but the lower one has all the
turns compressed. The top Inductor is the normal way of winding and the inductance as measured is 0.46uH.
The lower one has a much higher measured inductance of 0.77uH.
There is nothing wrong with the lower inductor if you wanted 0.77uH in your circuit. However, most designers
- me included - has standardized on winding with turns spread out on the core and calculate the number of
turns based on this assumption.
If you happen to not follow this and compress the turns or wind it in a messy way it will not be the end of the
world in this low pass filter, it will give more attenuation of unwanted signals with is good, but will also give
slightly more insertion loss which is bad.
What to count as one turn.
The general rule how to count the cumbers of turns can be summarized as follows: every time the wire passes
in to the middle hole it counts as one turn
, se picture below.
