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The resistor limits current drawn by the Zeners to a safe value. When the acts as a normal diode. The Zener and Avalanche breakdown both occur in diode under reverse bias. March 2009 Small-signal modeling is a common analysis technique in which is used to approximate the behavior of containing with.

This is called the small-signal model. This is 15 mA for the Zener diode plus 2 mA for the load, i. Using the zener regulator circuit above calculate: a.

3.9V Zener Diode

A Zener diode is a special type of rectifying diode that can handle breakdown due to reverse breakdown voltage without failing completely. Here we will discuss the concept of using diodes to regulate voltage drop and how the Zener diode operates in reverse-bias mode to regulate voltage in a circuit. Because this is an exponential function, current rises quite rapidly for modest increases in voltage drop. Another way of considering this is to say that voltage dropped across a forward-biased diode changes little for large variations in diode current. Forward biased Si reference: a single diode, 0. Conversely, a decrease in power supply voltage would result in an almost equal decrease in resistor voltage drop, with just a little decrease in diode voltage drop. In a word, we could summarize this behavior by saying that the diode is regulating the voltage drop at approximately 0. The Use of Voltage Regulation Voltage regulation is a useful diode property to exploit. Suppose we were building some kind of circuit which could not tolerate variations in power supply voltage, but needed to be powered by a chemical battery, whose voltage changes zener diode equivalent circuit its lifetime. We could form a circuit as shown above and connect the circuit requiring steady voltage across the diode, where it would receive an unchanging 0. This would certainly work, but most practical circuits of any kind require a power supply voltage in excess of 0. One way we could increase our voltage regulation point would be to connect multiple diodes in series so that their individual forward voltage drops of 0. For instance, in our example above bif we had ten diodes in series, the regulated voltage would be ten times 0. We know that diode forward voltage is a fairly constant figure under a wide range of conditions, but so is reverse breakdown voltage. Zener diode equivalent circuit voltage is typically much, much greater than forward voltage. This is shown in the figure below a. However, it is possible to build a special type of diode that can handle breakdown without failing completely. This type of diode is called a Zener diode, and its symbol is shown in the figure above b. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called Zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that Zener voltage point. However, this stability and accuracy is generally good enough for the Zener diode to be used as a voltage regulator device in common power supply circuit in Figure. So long as the power supply voltage remains above the Zener voltage 12. Like any semiconductor device, the zener diode is sensitive to temperature. Interestingly enough, when Zener diodes fail due to excessive power dissipation, they usually fail shorted rather than open. A diode failed in this manner is readily detected: it drops almost zero voltage when biased either way, like a piece of wire. Figure b a Zener Voltage regulator with 1000 Ω resistor. If excessive power dissipation is detrimental, then why not design the circuit for the least amount of dissipation possible. Why not just size the resistor for a very high value of resistance, thus severely limiting current and keeping power dissipation figures very low. Take this circuit, for example, with a 100 kΩ resistor instead of a 1 kΩ resistor. Less power dissipation means lower operating temperatures for both the diode and the resistor, and also less zener diode equivalent circuit energy in the system, right. A higher resistance value does reduce power dissipation levels in the circuit, but it unfortunately introduces another problem. Remember that the purpose of a regulator circuit is to provide a stable voltage for another circuit. Consider our first regulator circuit, this time with a 500 Ω load connected in parallel with the Zener diode in Figure. Zener regulator with 1000 Ω series resistor and 500 Ω load. What it is supposed to do is maintain 12. However, as we will see, it cannot accomplish this task. Figure Zener non-regulator with 100 KΩ series resistor with 500 Ω load. This load current would have to go through the series dropping resistor as it did before, but with a new much larger. The situation is easier to comprehend if we temporarily remove the Zener diode from the circuit and analyze the behavior of the two resistors alone in Figure. Both the 100 kΩ dropping resistor and the 500 Ω load resistance are in series with each other, giving a total circuit resistance of 100. With a total voltage of 45 volts and a total resistance of 100. Figuring voltage drops across both resistorswe arrive at 44. Thus, the diode ceases to regulate voltage. The analytical technique of removing a Zener diode from a circuit and seeing whether or not enough voltage is present to make it conduct is a sound one. Remember that Zener diodes work by limiting voltage to some maximum level; they cannot make up for a lack of voltage. If the load resistance is too low, it will draw too much current, dropping too much voltage across the series dropping resistor, leaving insufficient voltage across zener diode equivalent circuit Zener diode to make it conduct. When the Zener diode stops conducting current, it can no longer regulate voltage, and the load voltage will fall below the regulation point. Our regulator circuit with the 100 kΩ dropping resistor must be good for some value of load resistance, though. To find this acceptable load resistance value, we can use a table to calculate resistance in the two-resistor series circuit no diodeinserting the known values of total voltage and dropping resistor resistance, and calculating for an expected load voltage of 12. Any load resistance smaller than 38. With the diode in place, the load voltage will be regulated to a maximum of 12. With the original value of 1 kΩ for the dropping resistor, our regulator circuit was able to adequately regulate voltage even for a load resistance as low as 500 Ω. What we see is a tradeoff between power dissipation and acceptable load resistance. The higher-value dropping resistor gave us less power dissipation, at the expense of raising the acceptable minimum load resistance value. If we wish to regulate voltage for low-value load resistances, the circuit must be prepared to handle higher power dissipation. Zener diodes regulate voltage by acting as complementary loads, drawing more or less current as necessary to ensure a constant voltage drop across the load. Despite this fundamental inefficiency of design, Zener zener diode equivalent circuit regulator circuits are widely employed due to their sheer simplicity. In high-power applications where the inefficiencies would be unacceptable, other voltage-regulating techniques are applied. Zener diodes are manufactured in standard voltage ratings listed in Table. The wattage corresponds to die and package size and is the power that the diode may dissipate without damage. The circuit of Figure has two Zeners connected series opposing to symmetrically clip a waveform at nearly the Zener voltage. The resistor limits current drawn by the Zeners to a safe value. This causes the Zeners to clip at about 10 V. The back-to-back diodes zener diode equivalent circuit both peaks. For a positive half-cycle, the top Zener is reverse biased, breaking down at the Zener voltage of 10 V. The lower Zener drops approximately 0. Thus, a more accurate clipping level is 10+0. Similar negative half-cycle clipping occurs a -10. Figure shows the clipping level at a little over ±10 V. Zener diode clipper: v 1 input is clipped at waveform v 2.

The Use of Voltage Regulation Voltage regulation is a useful diode property to exploit. Since the actual voltage curve is not ideally vertical, a change in zener current Δ Iz produces a small change in zener voltage Δ Iz , as illustrated. This could destroy all the chips being powered. From the bottom of the knee, the zener breakdown voltage Vz remains essentially constant although it increases slightly as the zener current Iz, increases. Any variations in current drawn by the load must be minimised as these will change the current through the Zener diode and cause slight voltage variations. A zener diode is a silicon pn junction device that is designed for operation in the reverse breakdown region. Avalanche Breakdown The mechanism of avalanche breakdown occurs because of the reverse saturation current.