Hi Andy,
and thanks for thinking to my FETs :-)
Indeed I made 2 trials, the first with the 5T/12T xfmr and then
changed a bit the ratio but I got about the same kind of resoults.
Now I have a new xfmr
7T/19T mounted and will make a new set of measure this evening (if not
too sleepy) or during the weekend.
With concern to the LC, I stopped to wind the coil when the wire
finished.. and was lucky that the 4x 2200pF caps were resonating it
close to 137 kHz so I made (up to now) no adjustment either on the coil
or on the caps. By the way, while carring out the measures (at that low
power level..) I checked where the tx was peaking and seen that a -3dB
curve is about 20 kHz wide. Now when checking with the new xfmr will
recheck (hopefully at higher power) where it is peaking and how broad
is and eventually try a more flat configuration (of course it is easier
to cut turns and add pF.. )
Will keep you posted
73 Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 30-mag-2017 23.50
A: <[email protected]>
Cc: <[email protected]>
Ogg: Re: LF: Re: I: Fw: For today the FETs survived...
I've just looked again at the circuit diagram you sent - on there the
values are different from your statement in the email. It shows
primary 5
turns, secondary 12 turns so a load resistance in the order of 9 ohms
which
is rather low if you are intending a Vdd of 180V - but closer to the
ideal
Rl
The tank components have a reactance of 130 ohms which is too high a Q
is
used with that 9 ohms Rload, You should be aiming for a Q in the region
of
6.
Even with the optimum load R of 13 ohms described last time for 500
Watts
from 180V rail the resulting Q of 10 is a bit too high - you will end
up
with high voltage and critical tuning
Andy G4JNT
On 29 May 2017 at 19:07, Andy Talbot <[email protected]> wrote:
> Yes.
> As you'll see in my original write up, I originally forgot that the
peak
> of the fundamental sine component of a square wave is GREATER than
the peak
> by a factor of 4 / pi and initially my PA delivered a lot more power
(1.6
> times) than it was supposed to.
>
> So if the square wave has a peak value of 1, its fundamental sine
> component has a peak value of 4/pi or around 1.27. The RMS of the
> resulting sine is SQRT(2) less than this giving a Peak square to RMS-
sine
> ratio of 0.9.. If you specifye peak-peak of the square wave, a
further
> factor of 2 applies, leading to the 0.45 ratio described before.
>
> Incidentally, this same ratio appears in that equation for flux in a
> magnetic code, V = 4.44.F.N.A.B
> The magic number 4.44 is actually SQRT(2) * pi and comes about
from
> the same sort of sine to square transform.
>
> Andy
>
> On 29 May 2017 at 18:48, [email protected] <[email protected]>
wrote:
>
>>
>> uhuh... a slightly silly misleading assumption... Vdc are the same
of
>> Vrms before FETs make their work!
>>
>> Thank you Andy for pointing out it!!
>> With this approach calculation changes a bit and probably with the
>> right Xfmr the PA can give higher satisfaction :-)
>>
>> Hopefully the FETs will survive and this time I'm ready to burnout
the
>> antenna hi
>>
>> Will keep you both updated, thank you once more Andy
>>
>> 73 Marco, IK1HSS
>> ----Messaggio originale----
>> Da: [email protected]
>> Data: 28-mag-2017 21.18
>> A: "[email protected]"<[email protected]>,
>> <[email protected]>
>> Cc: <[email protected]>
>> Ogg: LF: Re: I: Fw: For today the FETs survived...
>>
>> First thing I noticed is that your turns ratio on the output
>> transformer
>> doesn't look right.
>> You quote "* ... with primary winding of 15 turns and secondary of
12
>> turns...*"
>>
>> 180V DC in a half bridge is 180V peak-peak square wave.
>> The fundamental sine part of that is 4/pi * 180 = 229V pk-pk
>> so is 229V /[2.SQRT(2)] = 81V RMS
>>
>> To a good approximation RMS(fund) from a half bridge is Vrms(fund) =
>> 0.45VDC
>>
>> For 500 Watts out, Rload = 81 ^ 2 / 500 = 13 ohms
>>
>> So to match to 50 ohms you need a turns ratio of SQRT(50/13) = 1.9:
>> 1 so
>> call it 2:1 Keeping 12 turns on the secondary means you need 6
turns
>> on
>> the primary
>>
>> When operating at reduced voltage, the power out will vary exactly
as
>> the
>> square of the voltage.
>> Recalculating from first principles for a 12V supply:
>>
>> 12V DC = 12V pk-pk = 12 / [2.SQRT(2)] * 4/pi = 5.4V RMS
(fundamental)
>> in 13 ohms should give 5.4^2/13 = 2.2 Watts
>>
>> check using ratio of voltages, squared :
>>
>> (12V/180V) ^ 2 * 500W = 2.2 Watts which is the same as above.
>> QED
>>
>> Your 15:12 ratio result sin a load impedance of (15/12)^2 * 50 = 78
>> ohms
>>
>> At 40V DC == 18V RMS(fund) that will give 18^2/78 = 4.1 watts which
is
>> actually LESS that you are seeing - the 2* discrepancy is odd, but
the
>> low
>> power is in the area of what you measured..
>>
>> Andy G4JNT
>>
>>
>>
>> On 28 May 2017 at 19:34, [email protected] <[email protected]>
>> wrote:
>>
>> > Hi Chris,
>> >
>> > I tried to post this message on the reflector but apparently I had
no
>> > success..
>> > As promised I keep you updated but as you can read in the
>> > attachment the first trials were not enocouraging...
>> > Andy, may I ask you to read my report? your interpretation and
>> > suggestion are welcome!
>> >
>> > 73, Marco IK1HSS
>> >
>> >
>> > -----Original message-----
>> >
>> > From: "[email protected]" [email protected]
>> > Date: Sun, 28 May 2017 17:01:33 +0200
>> > To: [email protected]
>> > Subject: For today the FETs survived...
>> >
>> > Hi LF,
>> >
>> > hope that also the toroids of Chris survived!
>> > My FETs survived, but they are not working as expected :-(
>> > Attached the report on my attempt to duplicate the half bridge of
>> > Andy..
>> > Has anyone suggestions before I try to cook all connecting to the
>> > 180Vdc supply?
>> >
>> > Thank you
>> > 73 Marco IK1HSS
>> >
>> >
>> > --
>> > This message has been scanned by E.F.A. Project and is believed to
be
>> > clean.
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > ---------- Forwarded message ----------
>> > From: "[email protected]" <[email protected]>
>> > To: <[email protected]>
>> > Cc:
>> > Bcc:
>> > Date: Sun, 28 May 2017 17:01:33 +0200 (CEST)
>> > Subject: For today the FETs survived...
>> > Hi LF,
>> >
>> > hope that also the toroids of Chris survived!
>> > My FETs survived, but they are not working as expected :-(
>> > Attached the report on my attempt to duplicate the half bridge of
>> > Andy..
>> > Has anyone suggestions before I try to cook all connecting to the
>> > 180Vdc supply?
>> >
>> > Thank you
>> > 73 Marco IK1HSS
>> >
>> >
>> > --
>> > This message has been scanned by E.F.A. Project and is believed to
be
>> > clean.
>> >
>> >
>> >
>> >
>>
>>
>>
>>
>>
>
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