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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

MAGNETICS

Figure 2 shows the equivalent circuit of the current generating transformer. The output current from the power amplifier has three components of three different phases. I_L is the current through the inductive component, I_R is the current through the reflected resistive component (from the secondary), and I_C is the current through the capacitance of the circuit. A capacitor can be added to cancel the current required by the inductive component, reducing the current drive requirements of the amplifier.

Figure 2ýNo real electrical component is ideal. The imperfections, as well as impedances reflected from one side of the transformer to the other, are shown in an equivalent circuit. Also indicated is the way the total current splits to pass through individual components.

The internal resistance of the secondary (R_I) is made up of wire resistance, plus the transferred impedance of the primary, plus the resistance of the measurement shunt and the transferred impedance from the secondary of the current measuring transformer. R_L is the load in the unit under test.

ESTABLISH CONSTANTS

To design the transformer, you must introduce values from the actual circuitry that you are going to use. For example, we decided to generate a 10-A output AC current. The load (R_L) is 25 milliohms (a shunt internal to the product), and the wiring resistance of the secondary, so R_i is assumed to be 25 milliohms as well.

In addition, a transformer core must be selected. We had several spare toroidal transformers that we disassembled. The measurements of the transformer core were as follows:

ý Cross-sectional height, H = 2.15 cm

ý Cross-sectional width, W = 1.60 cm

ý Inner diameter, D = 3.93 cm

Prior to the other calculations, the inductance per turn (L_T) must also be known. This can be calculated from the magnetic constants if the core data is available, as shown as follows:

where S is the core cross-sectional area in square centimeters, C_m is the mean circumference of the toroid (2 ý p ý (outside diameterýinside diameter)), and N is the total number of turns. m _eff is the effective permeability of the material. In a toroid, this is essentially the permeability of the material. Once L has been calculated from the above formula, L_T can be calculated as:

Prior to the other calculations, the inductance per turn (L_T) must also be known. This can be calculated from the magnetic constants if the core data is available, as shown above. Alternatively, it can be measured with an LCR bridge. Winding five turns through the core and measuring the inductance gave 40.7 ýH. The inductance is proportional to the square of the number of turns. Therefore, L_T is calculated as 40.7/5² = 1.63 ýH/turn².

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