Up Down Circuit Analysis:

Any circuit has independent variables (such as source voltages and branch resistances) and dependent variables (such as voltage drop across resistors, current and power). Determination of response of dependent variables to variation in independent variable is known as circuit Up Down Circuit Analysis.

Consider a circuit given in Fig. 7.24. A voltage V_S of 5 V is applied to a diode in series with a load resistance R_L of 500 Ω. There are five dependent variables-voltage across load resistor, V_L, load current I, power dissipation of diode, P_D, power dissipation by load resistor, P_L and total power P_T.

Now we are to determine how each of these dependent variables respond when supply voltage V_S increases/decreases. The diode has a voltage drop of 0.7 V (equal to knee voltage). When the source voltage is increased, the voltage drop across the diode remains unchanged. Obviously voltage drop across load resistance R_L will increase which will cause increase in load current I_L. An increase in load current I_L will make the diode power P_D and load power P_L to increase. The total power P_T, being the sum of P_D and P_L will also increase.

When the load resistance R_L is increased, the load voltage V_L will remain unchanged owing to fixed voltage drop across diode (being equal to knee voltage V_K or V_D). Obviously, the load current I_L will decrease. The decrease in load current means reduced P_D, P_L and P_T.

If knee voltage increases slightly, the dependent variables V_L, I_L, P_L and P_T will decrease while P_D will increase.

Table 7.1 summarizes the Up Down Circuit Analysis for increase in different independent variables.