**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.