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Dear Mal, LF group,
----- Original Message -----
Sent: Tuesday, May 17, 2005 12:24
AM
Subject: LF: CORES
I have been winding some transformers
recently and have been getting some odd results.
All cores have a prim of 9 turns and 27 turns
sec.
1. 3C85 58 mm diam Prim inductance 167uh Sec 744
uh
2. Same material abt same size but E core
Prim 292 uh Sec 2810 uh no gap.
3. Material unknown out of a smps similar
size Prim 21 uh Sec 178 uh no gap
4. TV lopt core again abt same size Prim 79 uh
Sec 865 uh no gap
The figures
for the primary inductance seem quite reasonable, except no. 3 has rather
low AL value for a ferrite transformer core, if that is what it is - perhaps it
is iron dust? But with 3:1 turns ratio, the secondary inductance should be close
to 9 times the primary - definitely not so in the case of 1, so something
odd there. As to whether that is a good number of turns, it depends what DC
voltage and power output level you are looking to operate at.
The Prim wire is litz 2mm and sec 1mm copper enam
wire in all cases.
1 and 2 seem to have a big difference considering
the same material, 3 must have a low u.
Has anyone else tried comparisons of
cores.
If cores are selected at random and not checked
there would be a big variation in results even using the same turns
ratio.
Selecting components at random will usually give
random results ... but provided the transformer has enough inductance, the right
turns ratio, and does not saturate or overheat, a very wide range of
constructions will actually work. These cores are probably capable of
handling between several hundred watts to a couple of kilowatts with optimum
winding design, but even with non-optimum design they will still work OK but be
less efficient and more likely to overheat.
The above are intended as output matching
transformers in the FET amps like the YXM and MRF class D/E
design.
I normally use 1 in my amps but have tried 3 and
in spite of the big difference in values it does seem to work and I have not
noticed any difference in RF output, which seems strange.
73 de Mal/G3KEV
If you could take a selection of ideal, loss-free,
inductors, capacitors, switching devices and transformers, connect them together
in a circuit, then apply DC and 137kHz drive at one end and take output at the
other end via a 137kHz low-pass or band-pass filter, you would have a 100%
efficient switching mode PA stage. It would not matter what the component values
were, or even how they were connected together, since the only way energy could
leave the system would be as a 137kHz sine wave. Of course the real world is not
quite like this; many possible combinations of components would result in
impractically large or small voltages and currents, and real components do have
losses, get hot, go bang etc. But even so, provided the circuit is halfway
sensible and the components have reasonably low loss at the voltage and
current levels they operate at, the majority of the DC input power will end up
as RF power in the load, simply because there is nowhere else for it to go. This
makes switching PA stages very tolerant of variations in some components. It is
quite easy to end up with a PA circuit that "works" in so far as it
generates substantial RF output, but on connecting an oscilloscope you find the
most peculiar waveforms, with more harmonics than fundamental and
wild voltage and current spikes. But because the output usually has a quite good
low-pass filter, a nice sine wave is delivered to the load. However, a nasty
waveform usually means at least some of the components are under a lot of
stress, so it is always worth checking the PA has reasonable-looking waveforms,
even if it otherwise seems to be working.
Cheers, Jim Moritz
73 de M0BMU
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