Engineering Expert Witness Blog

     Last time we learned how the Zener diode, an excellent negotiator of current, is involved in a constant trade off, exchanging current for voltage so as to maintain a constant voltage.  It draws as much current through it as is required to maintain a consistent voltage value across its leads, essentially acting as voltage regulator in order to protect sensitive electronic components from power fluctuations. 

     Now let’s revisit our example power supply circuit and see how Ohm’s Law is used to determine the amount of electric current, I_PS, that flows from the unregulated power supply and why this is important to the function of the Zener diode.  See Figure 1.

Figure 1

     If you’ll recall, Ohm’s Law states that current flowing through a resistor is equal to the voltage across the resistor divided by its electrical resistance.  In our example that would be I_PS flowing through to R_Limiting.  In fact, the voltage across R_Limiting is the difference between the voltages at each of its ends.

     Applying this knowledge to our circuit, the voltage on one end is V_Unregulated, while the voltage at the other is V_Zener.  According to Ohm’s Law the equation which allows us to solve for I_PS is written as:

I_PS = (V_Unregulated – V_Zener) ÷ R_Limiting

     And if we have a situation where V_Unregulated equals V_Zener , such as when the voltage of an unregulated power supply like a battery equals the Zener voltage of a Zener diode, then the equation becomes:

(V_Unregulated – V_Zener ) = 0

And if this is true, then the following is also true:

I_PS = 0 ÷ R_Limiting = 0

     In other words, this equation tells us that if V_Unregulated is equal to V_Zener, then the current I_PS will cease to flow from the unregulated portion of the circuit towards the Zener diode and the external supply circuit.  Put another way, in order for I_PS to flow and the circuit to work, V_Unregulated must be greater than V_Zener.

     Next week we’ll continue our discussion and see why the resistor R_Limiting is necessary in order to prevent the circuit from self destructing.

____________________________________________