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How to Use Zener Diodes
Rod Elliott (ESP)

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About Zeners
Zener diodes are very common for basic voltage regulation tasks. They are used as discrete components, and also within ICs that require a reference voltage. Zener diodes (also sometimes called voltage reference diodes) act like a normal silicon diode in the forward direction, but are designed to break down at a specific voltage when subjected to a reverse voltage.

All diodes will do this, but usually at voltages that are unpredictable and much too high for normal voltage regulation tasks. There are two different effects that are used in Zener diodes ...

Below around 5.5 Volts, the zener effect is predominant, with avalanche breakdown the primary effect at higher voltages. While I have no intention to go into specific details, there is a great deal of information on the Net (See References) for those who want to know more. Because the two effects have opposite thermal characteristics, zener diodes at close to 5.5V usually have very stable performance with respect to temperature.


Using Zener Diodes
For reasons that I don't understand, there is almost no information on the Net on exactly how to use a zener diode. Contrary to what one might expect, there are limitations on the correct usage, and if these are not observed, the performance will be much worse than expected. Figure 1 shows the standard characteristics of a zener, but as with almost all such diagrams omits important information.

Figure 1
Figure 1 - Zener Diode Conduction

So, what's missing? The important part that is easily missed is that the slope of the breakdown section is not a straight line. Zeners have what is called 'dynamic resistance' (or impedance), and this is something that should be considered when designing a circuit using a zener diode.

The actual voltage where breakdown starts is called the knee of the curve, and at this region the voltage is quite unstable. It varies quite dramatically with current, so it is important that the zener is operated above the knee, where the slope is most linear.

Some data sheets will give the figure for dynamic resistance, and this is usually at around 0.25 of the maximum rated current. Dynamic resistance can be as low as a couple of ohms at that current, with zener voltages around 5 - 6V giving the best result. Note that this coincides with the best thermal performance as well.

This is all well and good, but what is dynamic resistance? It is simply the 'apparent' resistance that can be measured by changing the current. This is best explained with an example. Let's assume that the dynamic resistance is quoted as 10 ohms for a particular zener diode. If we vary the current by 10mA, the voltage across the zener will change by ...

V = R * I   = 10Ω * 10mA = 0.1V (or 100mV)

So the voltage across the zener will change by 100mV for a 10mA change in current. While that may not seem like much with a 15V zener for example, it still represents a significant error. For this reason, it is common to feed zeners in regulator circuits from a constant current source, or via a resistor from the regulated output. This minimises the current variation and improves regulation.

Manufacturers' data sheets will often specify the dynamic resistance at both the knee and at a specified current. It is worth noting that while the dynamic resistance of a zener may be as low as 1.5 ohms at 25% of maximum current, it can be well over 500 ohms at the knee, just as the zener starts to break down. The actual figures vary with breakdown voltage, with high voltage zeners having very much higher dynamic resistance (at all parts of the breakdown curve) than low voltage units. Likewise, higher power parts will have a lower dynamic resistance than low power versions (but require more current to reach a stable operating point).

Finally, it is useful to look at how to determine the maximum current for a zener, and establish a rule of thumb for optimising the current for best performance. Zener data sheets usually give the maximum current for various voltages, but it can be worked out very easily ...

I = P / V   where I = current, P = zener power rating, and V = zener voltage rating.

For example, a 27V 2W zener can carry a maximum continuous current of ...

I = 2 / 27 = 0.074A = 74mA (less a small 'fudge factor', and at 25°C)

As noted in the 'transistor assisted zener' app note (AN-007), for optimum zener operation, it is best to keep the current to a maximum of 0.7 of the claimed maximum, so a 27V/2W zener should not be run at more than about 47mA. The ideal is probably about 25% of the maximum (17mA close enough, although reference 3 says 18.5mA), as this minimises wasted energy and ensures that the zener is operating within the most linear part of the curve. If you look at the zener data table below, you will see that the test current is typically between 25% and 36% of the maximum continuous current. The wise reader will figure out that this range has been chosen to show the diode in the best possible light, and is therefore the recommended operating current :-).

While none of this is complex, it does show that there is a bit more to the (not so) humble zener diode than beginners (and many professionals as well) tend to realise. Only by understanding the component you are using can you get the best performance from it. This does not only apply to zeners of course - most (so called) simple components have characteristics of which most people are unaware.

Remember that a zener is much the same as a normal diode, except that it has a defined breakdown voltage that is far lower than any standard rectifier diode. Zeners are always connected with reverse polarity compared to a rectifier diode, so the cathode (the terminal with the band on the case) connects to the most positive point in the circuit.


Zener Clamps
Often, it is necessary to apply a clamp to prevent an AC voltage from exceeding a specified value. Figure 2 shows the two ways you may attempt this. The first is obviously wrong - while it will work as a clamp, the peak output voltage (across the Zeners) will only be 0.65V. Zeners act like normal diodes below their breakdown voltage, so the first figure is identical to a pair of conventional diodes.

Figure 2
Figure 2 - Zener Diode AC Clamp

In the first case, both zener diodes will conduct as conventional diodes, because the zener voltage can never be reached. In the second case, the actual clamped voltage will be 0.65V higher than the zener voltage because of the series diode. 12V zeners will therefore clamp at around 12.65V - R1 is designed to limit the current to a safe value for the zeners, as described above.

The important thing to remember is that zener diodes are identical to standard diodes below their zener voltage - in fact, conventional diodes can be used as zeners. The actual breakdown voltage is usually much higher than is normally useful, and each diode (even from the same manufacturing run) will have a different breakdown voltage.


Zener Diode Data
The following data is a useful quick reference for standard 1W zeners. The basic information is from the Semtech Electronics data sheet for the 1N47xx series of zeners. Note that a 'A' suffix (e.g. 1N4747A) means the tolerance is 5%, and standard tolerance is usually 10%. Zener voltage is measured under thermal equilibrium and DC test conditions, at the test current shown (Izt).

TypeVZ (Nom) IztRzt Rz at ...Current
(mA)
Leakage
uA
Leakage
Voltage
Peak
Current (mA)
Cont.
Current (mA)
1N47283.37610400115011375275
1N47293.66910400110011260252
1N47303.9649.0400110011190234
1N47314.3589.040015011070217
1N47324.7538.05001101970193
1N47335.1497.05501101890178
1N47345.6455.06001102810162
1N47356.2412.07001103730146
1N47366.8373.57001104660133
1N47377.5344.07000.5105605121
1N47388.2314.57000.5106550110
1N47399.1285.07000.5107500100
1N474010257.07000.25107.645491
1N474111238.07000.2558.441483
1N474212219 7000.2559.138076
1N47431319107000.2559.934469
1N47441517147000.25511.430461
1N47451615.5167000.25512.228557
1N47461814207500.25513.725050
1N47472012.5227500.25515.222545
1N47482211.5237500.25516.720541
1N47492410.5257500.25518.219038
1N4750279.5357500.25520.617034
1N4751308.54010000.25522.815030
1N4752337.54510000.25525.113527
1N4753367.05010000.25527.412525
1N4754396.56010000.25529.711523
1N4755436.07015000.25532.711022
1N4756475.58015000.25535.89519
1N4757515.09515000.25538.89018
1N4758564.511020000.25542.68016
1N4759624.012520000.25547.17014
1N4760683.715020000.25551.76513
1N4761753.317520000.25556.06012
1N4762823.020030000.25562.25511
1N4763912.825030000.25569.25010
1N47641002.535030000.25576.0459
Table 1 - Zener Characteristics, 1N4728-1N4764
  1. Izt = zener test current
  2. Rzt = dynamic resistance at the stated test current
  3. Rz = dynamic resistance at the current shown in the next column
  4. Leakage current = current through the zener below the knee of the zener conduction curve, at the voltage shown in the next column
  5. Peak current = maximum non-repetitive short term current (typically < 1ms)
  6. Continuous current = maximum continuous current, assuming that the leads at 10mm from the body are at ambient temperature

Figure 3
Figure 3 - Zener Diode Temperature Derating

Like all semiconductors, zeners must be derated if their temperature is above 25°C. The above graph shows the typical derating curve for zener diodes, and this must be observed for reliability. Like any other semiconductor, if a zener is too hot to touch it is hotter than it should be. Reduce the current, or use the boosted zener arrangement described in AN-007.


References
1   Reverse Biased / Breakdown Discussing the phenomenon when the diode is reverse biased/breakdown. Bill Wilson
2   RadioElectronics.com - Summary of the zener diode
3   Data Sheet Archive - BZX2C16V Micro Commercial Components 2 Watt Zener Diode 3.6 to 200 Volts.

 

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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright © Rod Elliott 30 Jun 2005