[time-nuts] Need help with transformer core

[Chris]

> Thanks for the inputs everyone,
>
> One of the direct replies got me the data I needed!
>
> Alex, I'd like to by it that way, but A 24VDC input 3700VDC output at 4ma
> does not seem to be available!
>
> Cheers,
>
> Corby

Hi Corby,

The way I would approach the problem would be not ask where to get a 
given pot core, but how do I generate 3700v @ 4mA, starting from 
scratch. I would limit the secondary winding count and use a voltage 
multiplier, Cockroft-Walton style to bring the volts up to the required 
level. 22w is not an insignificant power level, so I would use a switch 
mode regulator chip, driving a pair of small power mosfets, with a 
switching frequency 100KHz or higher to keep the magnetics, winding 
turns count and the multiplier (Use polypropylene) caps small. Lower 
voltage at the secondary also makes it easier in terms of rectifier 
diode selection. All the info is in the chip manufacturers application 
notes. and suggest Unitrode as a starting point. Have done a few of 
these in the past and they are pretty strightforward. Even the magnetics 
are covered by the core manufacturers data books. A pair of small E 
cores sounds sounds about right and will have matching moulded bobbin 
options with tags. Pot cores are a pita in comparison and awkward to 
wind / terminate using foil, which you really need when a primary 
winding may only have 4 to 8 turns, yet be carrying amps.

The other point is that I would never use any switcher, however good 
it's claimed to be, to drive sensitive analog electronics. While many of 
the cheap switcher modules are fine with stable line and load, the 
transient response ids often dreadful and they take some time 
(milliseconds) to recover with step function change at either. Such 
instability can damage driven electronics as well. While it's common to 
use a switcher for initial conversion, that would always be followed by 
monolithic linear regulators, which typically have output noise levels 
of mV. For example, say you have an input voltage of 18-32v and a 
required output of 12v at 1A, an initial switch mode regulator to 
convert to 15v, then even a 7812 or similar to lose the final 3 volts...

Regards,

Chris