AVR-Chat

Kathy Quinlan wrote:
> Russell Shaw wrote:
> 
>>Kathy Quinlan wrote:
>>
>>>Hi all,
>>>
>>>I need to find Bmax for a core set I plan to use, I am using a ETD34 with 3C90 
>>>ferites, but I can not find the Bmax :(
>>>
>>>Anyone have any idea ??
>>>
>>>I have spent all night looking lol, (found lots of good data on other parts, 
>>>but no this one)
>>
>>0.33T.
>>
> Thanks Mate,
> 
> I have a few more Q's:
> 
> When winding a Transformer, if I wind the turns to one end, and that is 
> enough, is it wrong to bring the end of the wire back down the transformer 
> parallel to the core center leg ? or should I wind half to the end, then wind 
> the other half back to the beginning ?

Assuming there's more than ~5 turns, it won't matter. It's more important
to insulate it from any high voltage secondaries.

> The transformer whistles badly, it is no where near the switching Freq 
> (~52Khz) is this due to the way I wound it ?

No. The feedback loop needs slope compensation if it's the cycle-by-cycle
regulation type. If it's a slow averaging integrator type loop, it's a
matter of rethinking the integrator gain and damping coefficients.
If you have a high voltage secondary, the switching frequency may
be too high.

> Where can I buy materials to gap the cores, it says no metallic, so I guess 
> anything I can get in sizes of say 1,2,5,10,20 mils would be fine :)

If you need lots, a tapes wholesaler might be a good place because they wind
the tape onto metre-long roles before cutting it. Get some offcuts off them
that haven't had the sticky stuff put on (assuming they don't import it already
coated). The tape place i last went to had yellow mylar transformer tape.

Other places would be plastics wholesalers, but it'll be hard to get small
quantities unless they have offcuts.

Another option (which i've done) is poke around the stationary section at
K-mart or Officeworks with a micrometre;)

What i do is just keep all kinds of plastic sheet such as cellophane packaging
wrappers, christmas wrappers, that really thin cellophane off pre-coated PCB blanks,
anti-static bags, etc.

> At least it does start, and sort of works, the output is a bit low, but I used 
> just 1n4007 diodes instead of shotkey for the rectifier and the ones in the 
> circuits around the primary (they were specked at controlled avalanche ones 
> (1n5062 IIRC) But I can not find any in stock and no where is really open till 
> Tuesday lol.

1N4007 is *far* too slow. 1N5819 is a good general purpose shottky from Jaycar and DSE.
For a temporary fix <200mA, you could use 1N914.