[time-nuts] Need help with transformer core

Alexander Pummer alexpcs at ieee.org
Mon Sep 1 23:42:15 EDT 2014

forward converter [not fly -back]
look for pot-cores material 3C80, 3C81, 3C85 or equivalent, you will 
need  a cca 21 to 28mm dia pot-core if you run it at 80 to 120kHz, 
driver ic TL495 , which is a PWM push pull controller
estimate the efficiency for 60% for the input power, look for very good 
and large 10Megohm resistors you wil nee four of them 3700V/01mA = 37 
Meg ohm, the must be temperature stable and must be able to carry 1000V 
each, you need a high quality resistor for the bottom part of the 
feedback divider same sort as the 10Megohms  value cca 57k use 56k + an 
1k pot.
Philips has fast high voltage rectifiers you need some with very short 
recovery time
how to design a transformer:

a more elegant way is a resonant converter, less noise, less 
complicated  high voltage transformer, and the recovery time is not so 
critical,  but that is for more experienced DIY people

On 9/1/2014 3:54 PM, Chris wrote:
>> 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
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