DESIGNER’S CASEBOOK Circuit with Nonlinear Transformer fVOVideS /Solation for D c /bhaSUrementS

the capacitor voltage in the dual prototype. If Im > Is, the wave- form will be completely different (c). When the DC current I in the secondary is zero (upper waveform), then, for i c IIJ, the rate of increase and decrease of i is slow, and depend on the large time constant T = L R I . For II,I c lil cl I m , when the transformer core is saturated, the rate of increase and decrease is fast, and is determined by the small time constant t = VR1.

Note thatthe waveform is symmetrical, and has twocharacter- istic “spikes.” When DC current I flows out of the dol (b), saturation, which occurs when i > I, + mI during the positive part of the cycle, is not always reached (c). Saturation will occur for A linear transformer is frequently used in AC measurements all values of i c -Is + mI during the negative part of the wave,

to isolate a source from the measuring device. But a nonlinear however. As a result, the average value of the current i (and the transformer can be used for the same purpose in DC measure- average value Va of the voltage va) is positive. On the other hand, ments. Cons ide r when the DC current such a nonlinear n n I flows into the dot,

the average value of i will be negative.

Thus, one can determine the direc- tion (polarity) cif the DC current 1 in the

transformer having a the magnetization c h a r a c t e r i s t i c shown in the figure (a). If the DC cur- rent, I, in the secon- dary is zero, and the voltage source, v, in the primary excites the current i, then

I. M. Filanovsky University of Alberta Edmonton, Alberta, Canada, T6G 2E1

V. A. Piskarev Sevemaya Zarya, Leningrad, USSR

(a) the core’s magnetic flux is w = Li when i < I,; and w= Ys + L(i - I,) when i > Is. For i c 0, the flux has symmetrical nega- t

tive values. When the DC current, I, is flowing out of the I

dot-marked termi- nal of the secondary (i.e.,whenI<O),the magnetization curve moves to the right along the i axis for

(ndnl)I. When cur- rent flows into the dot, the magnetization curve moves in the opposite direction.

A simple RL oscillator can be constructed with this trans- former and a comparator (b). Indeed, it is the dual to an ordinary RC-multivibrator. The comparator changes states as the voltage at its positive input swings from +Vamp to -Vamp, where the parameter p = R3/(R3 +R2). Hence, the amplitude of the current in the transformer primary is Im=(Vomp)/RI.

If the circuit’s resistors are selected such that Im < Is (linear operation), the waveform of i will be similar to the waveform of

1

the distance mI = ( c )

transformer secon- dary s imply by measuring the DC

resistor RI. Suipris- ingly, the chiirac- teristic V,(I) is very

va linear (d) for the values of I less than I, (positive values of I correspond to flowing into the dot. negative to the op- posite direction), and the circuit can as well be used for measuring the cur- rent, l.

This particular oscillator uses a 220 Hammond pulse transformer. and a

LF356 operational amplifier with f 1 2 V power supply voltage. The resistor values are as follows: RI = 1 kR, R2= I O kR, and R3 = 6.8 k!2. The multivibrator will oscillate at about 70 Hz, and the Va(I) characteristic will be linear for III < 5 mA. Under these conditions, the slope will be -1.5 V/mA.

Using a fast comparator and a small ferrite core transformer will yield a more versatile circuit. Ultimately, the multivibrator can work at fxquencies to 30-40 kHz, and an excellent,electromagnetically- coupled isolation amplifier can be developed.

y’ I” - R3! voltage V, at the

(b)

(4

36 Circuits & Devices

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