Dear VLF,
In germany there is a distributor that offers such a TV transformer, of course
without the HV cascade, so with an AC output. I have ordered such one (not this
one of my picture i sent). There was a stamp, dated 1988, out of the DDR (GDR)
;-) It costs just abt 7 EURO and the secondary is put in epoxy.
So, generally, the TV transformers have been used at higher frequencies in the
TV so there will be no HV problems. There will just be thermal problems if the
output current gets to high for a longer time.
Remember that the ratio between voltage/frequency has it's limit for a given
core, so the high output voltage (anyway if with or without a load) will not be
the problem. Thus, the question is not "How many amperes can the transformer
stand" but "what is the maximum voltage per turn that can be applied". This
depends on the core dimensions and the core material so one has to check this
by applying a new primary winding, say 8 turns and watch the output voltage or
the DC current of the PA. Change the frequency and check where saturation
occurs. If you want a optimal winding for 8,97 kHz, the core gets into
saturation at say 8 kHz. If it occurs at 4 kHz, just half the number of turns...
The output transformer of my 300W PA used in the first experiments gets into
saturation at about 8 kHz. I have to apply 17,94 kHz since there is a frequency
divider in the circuit to achieve an exact 50% duty cycle for the FETs. When
driving the PA at 8,97 kHz, it takes 0,15A DC current at 13,8V. Once i have
done local tests with the VFO on 8,97 kHz and have forgotten to double the f
for the PA, so i drove the transformer with 3,985 kHz (without a load). In this
case, the DC current was > 3A since the core was in saturation!
One way to achieve the maximum output voltage is to adjust the air gap between
the core ferrite core parts, e.g. with a piece of paper. With that you can
adjust the L and resonate the antenna, or come closer to resonance. This also
changes the frequency where saturation occurs (to lower frequencies) so you
have to adjust the primary winding after resonating or just apply a higher
input voltage ;-)
Wish you good luck with the TV transformers.
In the last days here on the reflector was much traffic on the VLF theme. This
is good entertainment. So, no one here still needs his TV any more, right? Just
take the transformer out of it, if it is old enough ;-)
73, Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von [email protected]
Gesendet: So 30.05.2010 14:18
An: [email protected]
Betreff: Re: LF: 8.97kc - TV transformer info needed
I need to look around for an old tube TV for the same thing
Just wondered how well a neon sign transformer would work.
back in the days I used a pair of HV transformers
back to back as a modulation transformer which appeared to work as
well as one designed for that. so it passed good audio up to 3Khz at
least
I also use a standard transformer in a LF converter which works
well up to 200 Khz or so. The point being would an iron core transformer
work in low power tests at elf ?
Bob K3DJC
On Sat, 29 May 2010 21:48:48 -0500 "Mike.WE0H" <[email protected]> writes:
> Found my Zenith black & white TV chassis tonight. Got a flyback
> transformer that looks like Litz wound with bees wax for potting
> compound. No diodes, lots of room for the primary winding that I
> need to
> wind yet. Should be good to go now for some 8kc experiments.
>
> Thank you everyone for the advice on the transformers today. Now I
> have
> the correct one on my bench.
>
> Mike
> WE0H
>
>
>
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