3 Oct 2012

On-line LF toroid transformer design tool?

I have some 42mm diameter 3C90 toroids and want to use these in output transformers in 3 applications:

(1) in the output of a 137kHz (up to) 25W transmitter
(2) in the output of an 8.97kHz (up to) 25W transmitter
(3) as an impedance transformer for a TX loop antenna at 8.97, 137 and 500kHz.

I was looking for an on-line calculator to help me work out secondary turns needed, but could not find one.  Andy G4JNT helped with this input:
"The magic equation is Vrms = 4.44.F.N.A.B    all in SI units.   
rearranged  Nmin = V / (4.44 . F . A . B)
Al is irrelevant for transformers.
Use a Bmax of 0.1 Tesla for Ferrites, allowing a decent safety margin.
Your A  (of 25 mm^2)   = 25*10^-6    ,   F = 137000, 
25W in 50 ohms is 35V"
To aid calculations in future I have produced a small spreadsheet to work out the secondary turns from the input data (freq, cross sectional area and RF power out).

As an aside, I use http://www.66pacific.com/calculators/toroid_calc.aspx very often to work out the turns needed for the common HF toroids such as T37-x and T50-x.


Anonymous said...

"Al is irrelevant for transformers"

Huh? Al allows you to calculate the inductance of a winding. Look at the basic transformer equations and you'll see there's a direct relationship between primary and secondary inductance, just as there is with the number of turns on a winding.

Try the mini-Ring Core Calculator from DL5SWB:


Look at the Help screens in mini-Ring, some good explanations there.

Roger G3XBM said...

Thanks for the feedback. My understanding is the statement is generally true for transformers unless the turns required becomes very small. See a more recent blog entry quoting a posting by M0BMU on the RSGB LF group yesterday.