[Amps] Real time tests to see if an RF transformer is saturating?

[Alan Ibbetson]

Chris,

Buy Fair-rite toroids from mouser.co.uk (post free over, I think, £20 or 
maybe £25), or uk.rs-online.com who are always post free. Both add VAT 
to their catalogue prices.

The pros don't use designators like FT240-78. Those are numbers from the 
USA reseller Amidon. You have to search on www.fair-rite.com for the 
long string of digits (like 5978003801) and type that into the Mouser or 
RS search box. Finding the right part on Fair-rite is a pig. There are 
drop downs under Products on the home page but it's never easy.

If I was in your shoes I'd try type 78 and wind on as many turns in a 
5:7 ratio as will fit. As Manfred says, several strands of thinner wire 
bundled together will be easier to wind and have a bit less copper loss.

Have fun. Almost no radio amateurs of any callsign series understand 
magnetics so you are becoming an expert in the hobby!

73, Alan

On 15/05/2017 13:41, Chris Wilson wrote:
>
>
> Hello Alan,
>
> On Sunday, May 14, 2017,  you wrote:
>
>> Chris,
>
>> Nobody is answering so I'll have a go. If I muck up the arithmetic
>> others will be quick to cut me off at the knees :-)
>
>> I make the max flux density around 160mT (1600 Gauss).
>
>> If you can't be bothered to do the calculation from scratch there's an
>> online calculator here
>
>> http://www.daycounter.com/Calculators/Max-Flux-Density-Calculator.phtml
>
>> The data sheet for type 77 material says it has 300mW/cc loss at 100KHz,
>> the upper freq limit recommended by Fair-rite
>
>> http://www.fair-rite.com/77-material-data-sheet/
>
>> It's probably around 400mW/cc or even more at 136Kz. Your cores have a
>> mag volume of 22.8cc according to the data sheet
>
>> http://www.fair-rite.com/product/toroids-5978003801/
>
>> so 68.4cc for your stack of three. That makes 27.4W dissipation for
>> brick on the key WSPR operation. I'm not really surprised the cores get
>> hot. Fair-rite want you to use type 78 material, which has 1/3 the loss
>> at these frequencies. This was the experience of the guy who's combiner
>> design you copied
>
>> http://www.w1vd.com/137-500-KWTX.html
>
>> where he says
>
>> "At the kilowatt level issues developed with the output transformer. The
>> FT-240-77 cores that worked well in the 500-watt deck began to show
>> signs of 'stress' while testing the prototype 137 kHz kilowatt deck. The
>> core ran noticeably warm to the touch, transition ringing on the drain
>> waveform was difficult to tame and efficiency was less than expected.
>> Additional core losses at the increased power level and excessive
>> leakage reactance were to blame. After a study of the ferrite
>> literature, the best solution appeared to be a switch to 78 ferrite
>> material which was designed specifically for the 100 - 200 kHz frequency
>> range. As luck would have it, 78 cores were available in FT-240 size and
>> readily available through several distributors! A few days later
>> FT-240-78 cores were being tested in the prototype 137 kHz deck. The
>> cores just warm to the touch, ringing was much easier to tame and
>> efficiency was back in the mid 90% range".
>
>> To answer your specific question, a scope on the drain will probably
>> show flat topping if the cores goes into saturation as its inductance
>> collapses. But in your case it seems unlikely you are saturating the
>> core, it's just that the AC loss is too high.
>
>> Mouser and RS in the UK stock the type-78 part: Fair Rite 5978003801. RS
>> is cheaper.
>
>> Hope that helps,
>
>> Alan G3XAQ
>
>>> Date: Sat, 13 May 2017 19:06:59 +0100
>>> From: Chris Wilson <chris at chriswilson.tv>
>>>
>>> Hi. Two off 1kW quasi Class D push pull FET amps on 136khz feeding a
>>> Wilkinson combiner via the amps own output transformers designed for a
>>> 50 ohm load. Combiner built from plans for one to combine two 500 Watt
>>> amps. Combiner's output transformer is three stacked FT-240-77 ferrite
>>> toroids, 5 turns 12 AWG enameled (2mm OD) wire primary. Secondary is 7
>>> turns of the same wire. The combiner feeds a big LPF bank. At full
>>> power the combiner's toroids get hot quite fast with WSPR 2 signal
>>> applied. Needs a small fan to stay within the realms of sensibility,
>>> even then it creeps over 70C if left too long. How can I tell in real
>>> time, with measuring instruments, if it's saturating, and if so what
>>> cores might be more suitable please? Hopefully the schematic of the
>>> combiner is linked. Many thanks! Chris 2E0ILY in the UK.
>>>
>>> http://www.w1vd.com/137-500-500WCombiner.pdf
>
>
>
>
>
> Hi Alan, I cannot thank you enough for the reply on the "AMPS" forum
> to my questions. I did use 78 material for the two amps built to
> W1VD's specs i have built, and guessed he may not have updated his
> pages to reflect this better material for use in the combiner, but i
> thought i wouldn't be to clever and change anything. a subsequent
> correspondence with Jay, W1VD showed he intended the combiner to be
> used with 2 of his 500W amps, so I am pushing the limits presumably
> with whatever material. I see Fair-Rite lit a 98 material which is
> supposedly better agin than 78, but getting info on what sized toroids
> are available in 98 is proving difficult. Also, Fair-Rite USA charge
> insane postage to the UK making small orders very costly.
>
> Even with the combiner gate and drain waveforms seem good, with a
> slightly more spiky edge to the drain ones when combining, so i guess
> the saturation level hasn't been exceeded, but material choice is
> poor, and maybe turns counts are sub optimal for this power level.
>
>
> I will try two approaches, a copy of my existing combiner transformer
> using spare 78 compound FT-240 sized toroids I have to hand, and
> another with the tripled turns count Manfred kindly suggested. The
> problem with LF for a 2E0 is the guys running "down there" are all
> very experienced and their replies are often a bit over my head,
> especially as I foolishly chose to lark about in maths classes, much
> to my regret when I took up amateur radio at over 60 years of age.
>
> My teacher's words never rang so true, "Wilson, one day you'll wish
> you had paid more attention, as maths is often needed to do things
> that interest you, rather than just at a purely academic level that
> you seem to find so unimportant and boring". Damn, he was dead
> right.... :(
>
>
> Much appreciated, and thank you again Alan!!
>
>

-- 

Alan Ibbetson
alan at g3xaq.net