Re: xfrmr question, was: Re: That other HV List

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

I think you can use this formula for ferrite cores. It won't be totally 
accurate because it assumes a sinusoidal voltage and most switched-mode 
power supplies run with a square wave. So:

For E use the peak value of the fundamental component of whatever waveform 
you're feeding the transformer. If you can't be bothered with Fourier 
analysis then just multiply the amplitude of the square wave by 1.5 :)

Also the saturation flux of ferrite is not the same as transformer iron. I 
found this explanation of the equation on 
http://www.tortech-dot-com.au/toranalysis.html

"In terms of real-world transformer design, Faraday's low becomes:
e = 4.44NBfA x 10^8
where the voltage in a winding is proportional to the number of turns (N), 
the flux density 'fl gauss (B), the Ac
-frequency-/ (I), and the core cross-sectional area in square centimeters (A)."

So B is in gauss. (There are 10,000 gauss in one Tesla) You want to aim for 
a value that's about 75% of the saturation flux density for the material. 
So for your ferrite core, B would be about 0.75*0.51*10000= 3825.

I think the 60,000 figure for transformer iron might be wrong, given that 
it has a Bsat of between 12000 and 17000 gauss depending on the type. Or 
'B' might be in different units than gauss, in which case the 3825 figure 
for ferrite is obviously B.S.

To give you a ballpark idea: Ordinary sized ferrite cores (0.5 in2 cross 
section) running at about 30kHz usually generate about 5 volts per turn.

Steve C.


>the "roll your own transformer"
>web page <http://members.tripod-dot-com/~schematics/xform/xformer1.htm> has
>the formula for Primary Turns:
>
>N = (E X 10^8) / (4.44 X F X A X B)  Where F = frequency   A = area in sq.
>inches.   B = magnetic flux assume 60,000
>
>This is clearly for silicon-steel transformer core material. If I am using
>a ferrite core with these parameters what is the formula?
>         type 77 ferrite
>         permiability    u = 1800
>         saturation flux Bs = 510 mT (at H=1194 A/m)
>         residual flux   Br = 170 mT
>         coercive force  Hc = 16 A/m