Don't worry about trying to replicate inductance values or even worry
about it tahy don't matter, and probably vary all over the place with
temperature. High u ferrites aren't reknowned for temp stability. In
a transformer inductance is not important - provided it ends up than
enough.
Use the equations I posted yesterday to get your number of turns based
on Bmax. Keeping to < 0.1T or perhaps even lower.
Andy
www.g4jnt.com
On 5 April 2011 19:03, Mike Dennison <[email protected]> wrote:
> Many thanks to Andy, Warren and Mal for their helpful info. It
> certainly looks like I have a shorted turn.
>
> I will start by trying to dismantle the existing pot core, but I am
> not optimistic.
>
> Warren's inductance measurements are very useful as I presume I just
> need to replicate those figures with my new core.
>
> I have a couple of 3C85 toroids which could usefully be pressed into
> action.
>
> But if anyone has a spare transformer (or even a Decca chassis minus
> the amps) it would save me some work.
>
> And I am still baffled by the missing 3.3n cap shown on the circuit
> but nowhere to be seen in practice.
>
> Thanks again.
>
> Mike
> ====
>
>> Any large modern SMPSU core will cope with 1kW of sinusoidal 137kHz
>>
>> Just make sure B is kept below ~0.1 Tesla using the equation to
>> determine teh minumum number of turns
>> V(rms) = 4.44.F.N.A.B
>>
>> F Hz, N turns A core x-sectional area in m^2, B Tesla.
>> (the factor 4.44 comes from SQRT(2) * PI )
>>
>> In my 137kHz 700 Watt Tx, I used an ETD49 core with cross sectional
>> area of 200mm^2 made from 3C85 material designed for Bmax = 0.05T.
>> That runs cold. Thes days, even better ferrites are availble, like
>> 3C90. These are less lossy still, but Bmax is no more. TV LOPTs use
>> generally older types of ferrite, even older than 3C8, so its worth
>> going for a modern SMPSU core rather than cutting corners dismantling
>> old TVs. Although as Mal has clearly found, using LOPTs beyond their
>> design frequency, provided B is kept low by using enough turns, even
>> they will run OK. Just don't use too many turns that copper losses
>> begin to rear their head.
>>
>> Andy
>> www.g4jnt.com
>>
>>
>> On 4 April 2011 19:29, mal hamilton <[email protected]> wrote: >
>> Output transformer info. > Faulty output transformer. > Strip old
>> transformer windings and rewind as follows. > Prim 3 x 6 turns Sec 31
>> turns > or > use large LOPT 3C material with same amount of turns as
>> above. > > Wire guage guide Prim 3mm litz per winding and Sec 1 or 2
>> mm enamel > > > de G3KEV >
>>
>
>
>
>
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