THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

DEVELOPING AN AC CURRENT GENERATOR

by Ernesto Gradin & Aubrey Kagan

Start ý Features ý Magnetics ý Primary and Secondary Turns ý Primary and Secondary Winding ý Primary Inductance ý Hardware ý Firmware ý Analog to Digital Conversion ý User Interface ý Sources and PDF

PRIMARY TURNS

The maximum allowable flux density before saturation of an iron core is 12,000 to 15,000 gauss (8000 to 10,000 gauss is a safe value). Iron cores can be used up to a frequency of 400 Hz. For higher frequencies, a ferrite core must be used. In this case, the flux density should be reduced to no more than 2500 to 3000 gauss to prevent saturation of the core.

Magnetic theory predicts the following relationship for the primary of the transformer:

E = 4.44 ý f ý N_P ý S ý B ý 10^-8

Where E is the RMS voltage, f is the frequency, N_P is the number of turns on the primary, S is the cross-sectional area of the core, and B is the flux density in the core (in gauss).

You can rearrange this to find the number of turns:

In our setup, E = 14 V_RMS. This is derived from the supplies being ± 24 V. Allowing for 4-V "headroom" for the power amplifier, this gives V_PP of 40 V:

The frequency (f) is nominally 60 Hz. The cross-sectional area (S) is equal to H ý W = 2.15 ý 1.60 = 3.44 cm². B is our maximum flux density of 8000 gauss. Substituting these values:

This is the number of turns required to maintain the core below saturation at the maximum input voltage.

SECONDARY TURNS

The next step is to calculate the number of turns on the secondary (N_S). Based on Ohmýs Law (see Figure 2):

V_OUT = I_OUT ý (R_S + R_i)

Because we chose the output to be 10 A, R_S equals 25 milliohms and R_i equals 25 milliohms:

V_OUT = 10 ý 0.05 = 0.5 VAC

The turns ratio is the proportion of the input voltage to the output voltage:

Therefore, the number of turns on the secondary (N_S) is:

PREVIOUS • NEXT