An example of the calculation of circuits with nonlinear elements

Determine the current in the circuit and the voltage on the diodes current-voltage characteristics (VAC) of which are presented in Figure 8, if Uin = 2.5 V, Rn = 25 Ohm

Given:


Pic 8

Solution: build a total IVC of diodes and
"Inverted" current-voltage characteristic
load resistor Fig 9.

U = U1 + U2 + I • Rn
xx: U12 = U = 2.5 V
KZ: Ikz = U / Rn = 2.5 / 25 = 100 mA


Fig.9

Answer: U1 = 0.6 V, U2 = 0.7 V, Un = 1.2 V, I = 45 mA


For the circuit in Fig. 10, the zener diode has the parameters:

Ust = 20 V, Ist min = 1 mA, Rdif = 40 Ohm, Ist max = 71 mA.

Determine the current I in the circuit in a graphical way, if In = 20 mA:



Decision:

I = Ist + In

We build the current-voltage characteristics of the zener diode by three points


Figure 11

∆Ist = Ist max - Ist min = 71 - 1 = 70 mA

∆ Ust = ∆Ist • Rdif = 0.07 • 40 = 2.8 V

Ust min = Ust - ∆Ust / 2 = 18.6 V

Ust max = Ust + ∆Ust / 2 = 21.4 V


CVC resistor. We summarize the CVC-ki. Graphically determine the current unbranched part of the circuit in Fig. 11.

I = 55 mA