Problem 64-6    Source:   Problem elaborated by the author's of the site.

Look at the circuit shown in Figure 64-06.1. Assume that R_S = 150 Ω, V_Z = 12 V and the input voltage is constant value V_i = 30 V. We know that the maximum power that the zener supports is 1 W and the minimum zener current is 5 mA. If the load is purely resistive, determine the maximum and minimum load that the circuit can support.

Solution of the Problem 64-6   

Note that in this problem we have the current through the zener variable, as the current in the load is also variable. Consulting the theoretical part we see that we are facing   CASE 2. Then we will use the equations eq. 64-10 and eq. 64-11 . Initially, we must calculate the maximum current that the zener supports, as we know its power and working voltage. Let's use eq. 64-01, shown below.

To find the load variation limits, we must calculate the maximum and minimum currents on the load. In this case, as the value of R_S = 150 Ω was given, then we have that R_S = R _Smin = R_Smax. Using this data and performing an algebraic transformation in eq. 64-10, we obtain:

Replacing the variables with their respective numerical values, we find:

Carrying out the calculation, we have:

Therefore, knowing the minimum value of the current that can flow through the load, we can find the maximum value of the load, or:

Now working algebraically with eq. 64-11 let's find the value of the maximum current that can flow through the load. Like this:

Replacing the variables with their respective numerical values, we find:

Carrying out the calculation, we have:

Therefore, knowing the maximum value of the current that can flow through the load, we can find the minimum value of the load, or:

In summary, we can write that to solve the problem the relationship below must be satisfied.