A zener diode is a type of diode that acts as a voltage regulator when installed by itself or with other semiconductor devices. This device is unique because it's primarily used to conduct current under reverse bias conditions. (See Part 1 in last month's issue for description of reverse bias.) As we know, standard diodes most often conduct current in the forward bias condition; as such, they can be damaged or destroyed if the reverse voltage or bias is exceeded.

Another name for the zener diode is avalanche diode. This name is applied because the zener diode usually operates in reverse breakdown.

The basic symbol used to designate a zener diode is shown in Fig. 1. Note that this symbol differs from that of the standard diode: the normally vertical cathode line is slightly bent at each end.

How does a zener diode work?

To better understand how a zener diode works, let's refer to Fig. 2. Here we see an operating characteristic curve for a typical zener diode. Relatively speaking, a zener diode's forward and reverse characteristics are similar to those of a standard diode. There are some very important differences, however.

Forward direction source voltage. When a source voltage is applied to a zener diode in the forward direction (blue portion of curve in Fig. 2), the result is a breakover voltage and a forward current.

Reverse direction source voltage. When the source voltage is applied in the reverse direction (red portion of curve in Fig. 2), the current remains very low until the reverse voltage reaches reverse breakdown, or what is commonly called zener breakdown. At this breakdown point, a zener diode will conduct heavily, or avalanche. This is the important difference between a standard diode and a zener diode: when a zener diode conducts, it may continue to conduct for some time, and at considerable current, without damage in the reverse direction.

What's important about this characteristic? It's the voltage regulation: the voltage drop across a zener diode remains almost constant, despite the very large current fluctuations.

How is voltage regulation maintained?

The zener diode acts as a constant voltage source because of resistance changes taking place within the PN junction. (See Part 1 in last month's issue for description of PN junction.)

Let's look at Fig. 3 for a better understanding. Here we see a reverse operating characteristic curve. When a source voltage is applied in the reverse direction, the PN junction's resistance stays high; thus a reverse current only in the microampere range will be produced. As the reverse voltage is increased, however, the PN junction reaches a critical voltage and the zener diode begins avalanching.

What's happening at this particular point? Well, once the avalanche voltage is reached, the PN junction's normally high resistance drops to a low value. The current then increases rapidly, but generally is limited by a circuit resistor or resistance [R.sub.L], as shown in Fig. 4. The breakdown current usually will not destroy the zener diode; however, it may be destructive if it becomes extremely excessive or if the zener diode's heat dissipating capabilities are exceeded.

What are pertinent ratings?

As we mentioned earlier, zener diodes are designed to have a specific breakdown voltage rating, usually a close approximation of the necessary circuit control voltages. Examples include 4.7, 5.1, 6.2, and 9.1V.

Zener diodes are manufactured to certain breakdown voltage tolerances, such as [+ or -] 20%, [+ or -] 10%, or [+ or -] 5%. For high precision applications, they are available in the [+ or -] 1% range.

Zener diodes are available in wattage ratings of one millionth of a watt to 50W; popular ratings are 1W and lower. These power dissipation ratings are usually given for a specific operating temperature, which is usually an ambient of 25 [degrees] C.

Speaking of ambient temperature, this temperature will directly affect a zener diode's breakdown voltage. As such, manufacturers usually list a zener voltage temperature coefficient, which identifies the percentage of change in zener voltage per degree of temperature change. Typically this change is about 0.1% per degree Centigrade.

How are zener diodes tested?

Either a zener diode provides voltage regulation, or it fails. If it does fail, it obviously must be replaced so that the circuit can operate properly.

Sometimes, however, a zener diode may fail only in certain situations. This type of failure is called an intermittent. To check for an intermittent, the zener diode must be tested while it's in operation. Here, an oscilloscope is the best troubleshooting tool because it displays the diode's dynamic operating characteristics.

Looking at Fig. 5, we see an oscillo-scope display, with the horizontal axis representing voltage across the zener diode and the vertical axis representing current. A good test display pattern is shown on this oscilloscope display.