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, ## 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 R. In fact, the voltage across _{Limiting}R is the difference between the voltages at each of its ends._{Limiting} Applying this knowledge to our circuit, the voltage on one end is V. According to Ohm’s Law the equation which allows us to solve for _{Zener}I is written as:_{PS}
V – _{Unregulated}V) ÷ _{Zener}R_{Limiting} And if we have a situation where V, 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:_{Zener }( V) = 0_{Zener }And if this is true, then the following is also true:
R= 0_{Limiting} In other words, this equation tells us that if V, then the current _{Zener}I will cease to flow from_{PS} the unregulated portion of the circuit towards the Zener diode and the external supply circuit. Put another way, in order for I to flow and the circuit to work, _{PS}V must be greater than _{Unregulated}V._{Zener} Next week we’ll continue our discussion and see why the resistor ____________________________________________ |