Tab Electronics Guide to Understanding Electricity ... - Sciences Club

206 Chapter Seven
“starving” the zener diode from its holding current when the current
flow through R L
was maximum. The nearest standard value below 3.98
ohms is 3.9 ohms, which is the best choice. By using any of the familiar
power equations, the power dissipated by R1 comes out to be about
1 watt. A 2-watt resistor should be used to provide a good safety margin.
The worst-case power dissipation condition for ZD occurs when there
is no current flow through R L
. If all current flow through R L
ceases, the
full 502 milliamps must flow through ZD. Actually, the maximum current
flow through ZD could be as high as 513 milliamps because you
chose a 3.9-ohm resistor for R1 instead of the calculated 3.98 ohms. The
power dissipated by ZD is the voltage across it (12 volts), multiplied by
the current flow through it (the worst case is 513 milliamps). This is the
familiar power equation P IE. The answer is 6.15 watts. Therefore, ZD
would need to be a 12-volt, 10-watt zener with an appropriate heatsink.
Another option would be to use two 5-watt, 6-volt zeners in series. The
latter option eliminates the need for a heatsink, but care must be exercised
to assure plenty of “air space” around the zener diodes for adequate
convection cooling.
As the previous design example illustrates, zener-regulated power
supplies are not extremely efficient because the zener diode wastes a
significant amount of power when the current flow through the load
is small. For this reason, zener-regulated power supplies are typically
restricted to low-power applications. However, zener diodes are commonly
used as voltage references in high-power circuits, as is illustrated
later in this chapter.
Varactor Diodes
Going back to diode fundamentals, you might recall that when a diode is
reverse-biased, a “depletion region” of current carriers is formed around
the junction area. This depletion region acts as an insulator resulting
in the restriction of any appreciable current flow. A side effect of this
depletion region is to look like the dielectric of a capacitor, with
the anode and cathode ends of the diode acting like capacitor plates. As the
reverse-bias voltage across a capacitor is varied, the depletion region will
also vary in size. This gives the effect of varying the distance between the
plates of a capacitor, which varies the capacitance value. A diode that is
specifically designed to take advantage of this capacitive effect is called a
varactor diode. In essence, a varactor is a voltage-controlled capacitor.