Engineering Expert Witness Blog

     Last time we introduced the Zener diode and the fact that it performs as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage.  Let’s see how it works.

Figure 1

     In Figure 1 we have an unregulated power supply circuit introduced in a previous article in this series.  We learned that this power supply’s major shortcoming is that its output voltage, V_Output, is unregulated, in other words, it’s not constant.  It varies with changes in the direct current supply voltage, V_DC.

     It also varies with changes in, R_Total, which is the total internal resistance of components connected to it.  R_Total changes when components are turned on and off by microprocessor and digital logic chips. When V_Output is not constant, those chips can malfunction, causing the device to operate erratically or not at all.

     But we can easily address this problem by adding a Zener diode voltage regulator between the unregulated power supply and the external supply circuit.  See the green portion of Figure 2.

Figure 2

     Our power supply now consists of a Zener diode and a limiting resistor, R_Limiting.  The limiting resistor does as its name implies, it limits the amount of electric current, I_Z, flowing through the Zener diode.  Without this limiting resistor, I_Z could get high enough to damage the diode, resulting in system failure.

     Next time we’ll 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.

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