Engineering Expert Witness Blog

     Let’s continue our discussion with regard to the example circuit discussed last time and see how the Zener diode works in tandem with the limiting resistor to control current flow and hold the output voltage at a constant level.

Figure 1

     To recap our discussion from last week, the unregulated power supply portion of the circuit in Figure 1 generates an unregulated voltage, V_Unregulated.  Then the Zener diode, which acts as a voltage regulator, takes in V_Unregulated and converts it into a steady output voltage, V_Output.  Because these output terminals are connected to the ends of the Zener diode, V_Output  is equal to the voltage put out by it, denoted as V_Zener.

     The Zener diode, an excellent negotiator of current, is essentially involved in a constant trade off, substituting electric current that originates in the unregulated power supply portion of the circuit for voltage, V_Output, that will serve to power the external supply circuit.   In other words, the Zener diode draws as much current, I_Z, through it as it needs, its objective being to keep V_Output at a constant level, and it will continue to provide this constant output, despite the fact that V_Unregulated varies considerably.

     So, where does the current I_Z come from?  From I_PS, that is, the current flowing from the unregulated power supply area, as shown in Figure 1.  

     I_PS flows through the limiting resistor to a junction within the circuit.  At this junction, I_Z splits off from I_PS and continues on to the Zener diode, while current I splits off from I_PS on its way to the total internal resistance, R_Total, in the external supply circuit.  

     What this means is that when you add I_Z and I together, you get I_PS.  Mathematically speaking this is represented as:

I_PS = I_Z + I

     Why solve for I_PS?  We’ll see why this is important when we revisit Ohm’s Law next week and gain a fuller understanding of how I_PS, V_Unregulated, V_Zener, and R_Limiting relate to each other with regard to the Zener diode.  

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