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LF: Combining LF amps revisited

To: [email protected]
Subject: LF: Combining LF amps revisited
From: Chris Wilson <[email protected]>
Date: Sat, 13 May 2017 11:25:52 +0100
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Reply-to: [email protected]
Sender: [email protected]
Hello rsgb,

  I finally had the right combination of time and assistance to get my
  huge Farnell linear variable voltage and current power supply
  upstairs into my shack in the last 3 days. I have always had a
  sneaking worry that some of my blown FET issues when running at the
  limit were perhaps due to the SMPS doing odd things when the load
  was suddenly relinquished.

  I decided to try the W1VD Wilkinson combiner again on this PSU and
  was pleased to see no blown FET's! I think it's "softer" in
  transition between load on and off than the big server SMPS which
  probably was never intended to serve hard switched loads. Just
  guessing though.... I managed 60V at near 40 Amps on the two amps,
  but have now hit a new snag. In the combiner the output transformer
  is three stacked FT-240 77 material toroids, wound with 2 mm OD
  enamelled wire (12 AWG) in a 5 turns primary, 7 turns secondary
  configuration, and it gets damned hot. I chickened out at 75 degrees
  Celcius on my infra red thermometer. I am unsure if it's the
  windings or the cores getting hot, it's hard to tell as each heats
  the other of course. The ferrite beads on the Teflon co-ax don't
  even get warm. Only other downside to combining is the drain
  waveforms are a bit more spiky. Jay, W1VD stated this combiner was
  designed to combine two 500 Watt amps, I am trying to combine two of
  his 1kW ones, albeit I will run them at a reduced drain voltage in
  that format

  So, to my questions please. How can I tell if the combiner's
  transformer is heating due to core losses, or wire heating? If it's
  the stacked core what do you suggest i use instead, I like to
  de-rate things as much as possible, even if I go a bit OTT in the
  process :) What about wire type and gauge?

  Andy G4JNT mentioned abandoning the Wilkinson combiner and using a
  single transformer with coupling windings one from each amp's
  output, and a single secondary to the LPF. I don't see an earlier
  post i made asking what the turns ratio needs to be for this, but I
  do occasionally seem to miss posts from Blacksheep. If anyone can
  assist in this and suggest a conservatively sized and type of core
  to wind it on, with suggested wire type and gauge I'd be very

  The amps being combined:

  The combiner:

  The LPF:


Best regards,
 Chris 2E0ILY                        mailto:[email protected]

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