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LF: RE: Combining LF amps revisited

To: <[email protected]>
Subject: LF: RE: Combining LF amps revisited
From: "Clemens Paul" <[email protected]>
Date: Sat, 13 May 2017 15:34:32 +0200
In-reply-to: <[email protected]>
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Reply-to: [email protected]
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Thread-index: AdLL04EAARk3VuzUTm6t+2ZnudqF0QAGZeLg
Hi Chris,

I have a tool by DJ5IL here which predicts around 60°C for the cores
(excluding heat caused by the winding)
and ca. 480 Gauss flux density.


>-----Original Message-----
>From: [email protected] 
>[mailto:[email protected]] On Behalf Of Chris Wilson
>Sent: Saturday, May 13, 2017 12:26 PM
>To: [email protected]
>Subject: LF: Combining LF amps revisited
>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
>  appreciative.
>  The amps being combined:
>  The combiner:
>  The LPF:
>  Thanks
>Best regards,
> Chris 2E0ILY                        mailto:[email protected]

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