Hi,

I have been given a circuit to try and analyse the current flowing in different sections. I have been trying to use the Kirchoff laws but I am not really sure where to start. I have three branches of the circuit all appear to only have the input line and ground in common and all have zener diodes setting their voltage levels. Any suggestions on where to start with this would be really appreciated it has got me very confused.

I have attached the circuit diagram.

 

There are three loads being driven from a 24V battery through R22. Depending on the current flowing through R22, the three loads receive somewhat less than 24V. I think I would try a successive approximation approach, first assume the loads are driven by 24V and calculate the current & voltage drop in R22, then repeat the calculation using 24V minus the voltage drop in R22, then again.

 

Start by picking a symbolic voltage, such as Vx, for the top node. Now assume that Vx is some value that is consistent with each of the three loads behaving "normally" (and to do this you have to consider what "normal" is for the three circuit -- for that assume that Vx is somewhere near 24 V). Now find the current in each of the loads as a function of Vx. Now apply KCL at the top node and solve for Vx.

If you are willing/able to assume that diodes and BJT base-emitter junctions have a fixed voltage drop (of 0.7V, say) when conducting and that zeners hold the nominal voltage when in reverse breakdown, then there is no need to do successive approximations. Just solve for Vx and then find the currents from there. But you definitely need to consider the breakpoints at which the various circuits change their behavior. For example, at what value of Vx does the regulator no longer output 12 V? As Vx drops below that, what is the output value, at least approximately?

To really see what is going on, you might develop a load-line plot of the sum of the currents versus Vx. That will show you in which region each circuit is operating and would let you do things like easily determine what value of R22 will let all the circuits behave "normally".