I need to wind an inductor with a value between 47uH - 200uH. I have a 10 mm (in diameter) ferrite core from an RF choke used in an old radio (probably lossy ferrite). The core is 3 cm long (I also have a 5 cm long core too though). I will be using 19SWG magnet wire to wind the coil. How many turns do I need? I will be using the inductor for a ZVS flyback driver (Schematic). My power supply is 24V @ 6A.

  • 3
    \$\begingroup\$ You need at least know magnetic permeability of your ferrite. \$\endgroup\$
    – Vovanium
    Apr 22, 2014 at 16:18
  • \$\begingroup\$ The value doesn't have to be accurate. I'd say it is on the lower side of the ferrite permeability spectrum, though. \$\endgroup\$
    – AvZ
    Apr 22, 2014 at 16:56
  • \$\begingroup\$ So, did you tried any inductor calculators? Did they give inconsistent results? \$\endgroup\$
    – Vovanium
    Apr 22, 2014 at 16:59
  • \$\begingroup\$ ...You may use ferrite rod antenna equations for your task. \$\endgroup\$
    – Vovanium
    Apr 22, 2014 at 17:05
  • \$\begingroup\$ I didn't find any for ferrite cores. If you know any, would you share a link, please? \$\endgroup\$
    – AvZ
    Apr 22, 2014 at 17:06

2 Answers 2


If magnetic permeability of ferrite core is unknown - you have to wind it and measure it.

That may be complicated without oscilloscope or multimeter with inductor measurement.

In addition - I think RF ferrite may not work. Radio ferrites are not designed to store so much energy. It probably oversaturate and not work as desired at high current.

It will be even harder to determine if that happens without oscilloscope.

I think it would be easier to use some inductor with few thick windings from ATX power supply or other DC/DC converter and try how good or bad your converter is by measuring power efficiency.

Power input:

$$P_{in} = I_{in} \times U_{in}$$

Power output:

$$P_{out} = I_{out} \times U_{out}$$


$$n = \frac{P_{out}}{P_{in}}$$

That kind of flyback converter should have 65%-75% efficiency.

However if coil is very badly chosen and you have big power supply - you may blow up transistors because energy must go somewhere when back EMF happens on big transformer. Maybe try with smaller (1A?) power supply first and check transistors temperature.

  • \$\begingroup\$ Thanks, I'll try it with a smaller supply then. By the way, does the length of the core matter? If I have a longer piece, will that prevent saturation? \$\endgroup\$
    – AvZ
    Apr 22, 2014 at 18:25
  • \$\begingroup\$ Generally - mass of core inside your coil matters. Shape matters (closed magnetic circuit or not). Everything matters :) \$\endgroup\$
    – Kamil
    Apr 22, 2014 at 20:45

The circuit below is the one you linked to. This makes the question clearer and would have been good to see from the start.

enter image description here

The inductor is relatively non critical - it is not really part of the oscillator proper but serves to provide a high AC impedance path to feed DC by. If you were using a ferrite core there you could instead just use an air wound inductor. Both will result in you having a substantial RF transmitter. Using a large electrolytic capacitor on the DC input will help somewhat, but ideally you want to use an iron powder cored toroid there. You could use an old iron core transformer with relatively few turns and an airgap to improve current handling and stop the core saturating.

This actual circuit diagram is posted by many people but none seem to refer to the originator. You'll see that most use a toroid there.



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