Click here to purchase the entire book in PDF format. Zener Diodes There's a special type of diode that is supposed to be reversed biased. This device, called a Zener Diode has the characteristics shown in Figure 2.51. Figure 2.51: A graph showing the idealized characteristics of a zener diode. The breakdown voltage of the zener is a predicted value - in addition, when the zener breaks down and lets current through it in the opposite direction, it doesn't go up in smoke. When you buy a zener, it's rated for its breakdown voltage. So if you put the zener in series with a resistor and the zener is reverse biased, if the voltage applied to the circuit is bigger than the rated breakdown voltage, then the zener will ensure that the voltage across it is its rated voltage and the remaining voltage is across the resistor. This is useful in a bunch of applications that we'll see later on. Figure 2.52: A circuit showing how a zener diode can be used to ``regulate'' one voltage to another. In order for this circuit to work, the voltage supply on the left must be a higher voltage than the rated breakdown voltage of the zener. The voltage across the resistor on the right will be the rated breakdown voltage of the zener. The voltage across the resistor on the left will be equal to the level of the voltage supply minus the rated breakdown voltage of the zener diode. Also, we're assuming that R1 is small compared to R2 so that the voltage across R2 without the zener in place would be normally bigger than the breakdown voltage of the zener. So, for example, if the rated breakdown voltage of the zener diode in Figure 2.52 is 5.6V, and the voltage supply is a 9V battery, then the voltage across R2 will be 5.6V (because you can't have a higher voltage than that across the zener) and the voltage across R1 will be 9V - 5.6V = 3.4V. (Remember, we're assuming that the voltage across R2 would be bigger than 5.6V if the zener wasn't there...) Note as well that the graph in Figure 2.51 shows the characteristics of an ideal zener diode. The real-world characteristics suffer from the same trickle problems as normal diodes. For more info on this, a good book to look at is [Madhu, 1985].