I was rummaging around for a core for a small switching power supply
and came across some 1.00" OD, 0.55" ID, 0.39" high (25x14x10 mm)
toroids. A 50 turn coil measures about 1 mH (by measuring resonance at
200-500 kHz). This appears to indicate a permeability around 600, by
comparing it with various catalogues. But the Q is rather low
(equivalent series resistance is roughly 250 ohms at 500 kHz and 120
ohms at 200 kHz). Doesn't look likely to be good enough for a
switching supply, so they may have to be relegated to EMI jobs (which I
seem to recall they were used for in a piece of equipment I took
apart).

But does anyone have any idea what material they might be ? Doesn't at
first glance appear to be anything similar to what Fair-Rite makes.
There's a good chance it is of Asian origin, I think.

73,
Steve VE3SMA

wrote:
I was rummaging around for a core for a small switching power supply
and came across some 1.00" OD, 0.55" ID, 0.39" high (25x14x10 mm)
toroids. A 50 turn coil measures about 1 mH (by measuring resonance
at 200-500 kHz). This appears to indicate a permeability around 600,
by comparing it with various catalogues. But the Q is rather low
(equivalent series resistance is roughly 250 ohms at 500 kHz and 120
ohms at 200 kHz). Doesn't look likely to be good enough for a
switching supply, so they may have to be relegated to EMI jobs (which
I seem to recall they were used for in a piece of equipment I took
apart).

But does anyone have any idea what material they might be ? Doesn't
at first glance appear to be anything similar to what Fair-Rite
makes. There's a good chance it is of Asian origin, I think.

73, Steve VE3SMA

Steve, the usual reason for the apparent low Q is that it is not being
measured at the frequency that will give highest Q. Try fewer turns and
higher frequencies as a first attempt. I've measured many cores this
way. If you like, I can email you an XL spreadsheet with the all the
cores I've measured so far. Switchmode cores seems to have
permeabilities quite a bit higher than 600.
Alan
VK2ADB

Alan Peake wrote:

Steve, the usual reason for the apparent low Q is that it is not being
measured at the frequency that will give highest Q. Try fewer turns and
higher frequencies as a first attempt. I've measured many cores this
way. If you like, I can email you an XL spreadsheet with the all the
cores I've measured so far. Switchmode cores seems to have
permeabilities quite a bit higher than 600.
Alan
VK2ADB

Alan, are the ferrite toroids used to filter the output of pc power
supplies or the ac input of microwave ovens useful for other applications?

Or is the ferrite material lossy like the small ferrite beads used for emi
reduction on wires?

Regards,

Mike Monett

Alan, are the ferrite toroids used to filter the output of pc power
supplies or the ac input of microwave ovens useful for other applications?

The PC PSU filter toroids seem to be optimized for the 25KHz -85KHz
range so any applications you have for this frequency range could use
them. Similarly, the AC input to the oven is probably 50/60HZ so those
toroids would be better at the lower audio frequencies.

Or is the ferrite material lossy like the small ferrite beads used for emi
reduction on wires?

Not quite sure about beads - ordinary ferrite material has fairly
constant permeability up to a frequency determined by the ferrite mix,
then drops off. Beads tend to have an impedance peak at some desired
frequency range - e.g. 100-200MHz
Alan

Alan Peake wrote:

The PC PSU filter toroids seem to be optimized for the 25KHz -85KHz
range so any applications you have for this frequency range could use
them. Similarly, the AC input to the oven is probably 50/60HZ so those
toroids would be better at the lower audio frequencies.

Not quite sure about beads - ordinary ferrite material has fairly
constant permeability up to a frequency determined by the ferrite mix,
then drops off. Beads tend to have an impedance peak at some desired
frequency range - e.g. 100-200MHz
Alan

Thanks very much - that encourages me to try them and see how well
they perform.

Regards,

Mike Monett

Ferrites have as many vices as they have virtues.

Ferrite salesmen cleverly make virtues out of vices.

By the way, I found another surplus toroid with somewhat better
characteristics....higher permeability and higher Q, as measured around
200 kHz. That has now been incorporated in a small switching power
supply operating at about 30 kHz. It works, though not as well as is
predicted by a SPICE model. Must be some of those vices that Reg
mentions ! My SPICE model does not take into account the variation of
the permeability/inductance with DC current, so this may be at least
part of the difference. One of these years I'll break down and get an
oscilloscope so I can figure out what non-microwave circuits are really
doing, and maybe a signal generator that works below 150 kHz.

It is adequate (barely) for what I need, however, so it has now been
incorporated as a bias supply in my 3.4 GHz transverter. I'm crossing
my fingers that it keeps working over the temperature range it will
encounter in portable operation.

The Q-measurement technique I have been using involves connecting a
signal generator through a 50 ohm attenuator (to set the output
impedance) to a 50-ohm input microwattmeter. The inductor and a
capacitor are connected as a series-tuned resonant circuit and inserted
either in series between the pad and the meter or shunted across the
meter input. The inductance is obtained by finding the resonant
frequency and working backward through the formula, given the known
capacitance. The tuned circuit is then replaced with a resistor which
is adjusted (by substitution) to give the same power on the meter as
the tuned circuit at resonance. This resistance is equal to the
equivalent series resistance of the tuned circuit, from which the Q can
be determined.

As yet the Q results I obtain with the series and shunt connections
tend to be somewhat different, so my techniques certainly have room for
improvement (there are quite obvious stray-coupling issues, even at
LF), but it gives me a rough idea, anyway.

73,
Steve VE3SMA