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For a simple electronic voltage regulator using Zener diode, I use the following circuit. Zener Shunt Regulator

For deciding Rs and RL, I need to know Izmax and Izmin of the zener diode 1N4736A used. From this data sheet,

There are different current specifications available. Maximum working current is given as 133mA. Again test current at working voltage is 37 mA. Somewhere, I found if zener works continuously at maximum working current, it will get damaged. Also, somewhere minimum zener current is taken as 10% of maximum zener current whereas somewhere it is taken as the knee current (1mA) at which zener impedance is tested. Which is the correct choice?

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    \$\begingroup\$ Maximum is due to power dissipation. If you exceed the maximum, the Zener may fail. But minimum, well, there isn't really a minimum. As the current through the Zener decreases, the voltage will also decrease. It is up to you to decide what range of voltage variation you can tolerate. In some cases, you might see 3.3V using a 3.9V Zener diode if you use a small current like 100uA or 10uA. Maybe you can edit your question to give some idea of what you are doing or what your acceptable variation is. \$\endgroup\$
    – user57037
    Commented Mar 21, 2020 at 0:42

3 Answers 3

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A Zener diode voltage regulator is highly dependent on the load current. When designing such a circuit, you have to know the current (or range of currents) you will be drawing from this regulated voltage.

For proper regulation, you want the Zener diode current to be between a certain minimum value \$I_{Z,min}\$ (see the datasheet for a working curve, let's assume 5 mA here) so that the Zener voltage has developed. But you also want the Zener current to be below its maximum working current \$I_{Z,max}\$ (let's say 133 mA) to be able to dissipate all the heat.

Suppose your circuit will be drawing a current \$I_L\$ between \$I_{L,min} = 10\$ and \$I_{L,max} = 50 mA\$. The difference in current (50-10 = 40 mA) has to go through the Zener diode. So, if you know the current range of your load circuit, you can size the Zener diode, which has to be able to source at least \$I_{Z,min}+(I_{L,max}-I_{L,min}) = 5+(50-10) = 45 mA < I_{Z,max} = 133 mA\$.

If you know these values, you can also size \$R_S\$. The current \$I_S\$ through \$R_S\$ should equal the maximum load current plus the minimum Zener current, thus \$I_S = I_{L,max} + I_{Z,min} = 50 + 5 = 55 mA\$ (and use Ohm's law to find the resistance value).

In most practical cases, \$R_L\$ is not needed and is shown as a "model load." You would thus simply replace \$R_L\$ by your circuit.

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  • \$\begingroup\$ Thanks for the explanation. This is actually for demonstration purpose of working of the circuit from minimum possible load resistance to no load condition. \$\endgroup\$
    – Jhead
    Commented Mar 21, 2020 at 15:36
  • \$\begingroup\$ @Jhead -- What is your target audience? The answer to this question will indicate how theoretical our answer should be. Thanks for helping us to help you. \$\endgroup\$ Commented Mar 23, 2020 at 13:22
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Jack Ganssle, in his newsletters, The Embedded Muse 229 and 230, rants and laments about how Datasheets should be more helpful.

In 229, he says

Datasheet Rant

It's all about the code - unfortunately. As mentioned above it's hard to get concrete information about the Kinetis' L-series power consumption. But that's a common problem today -- datasheets increasingly lack the electrical information a design engineer needs to both select a part and properly design it in.

In 230, he says

One reader, who works for a major semiconductor vendor, responded with a challenge: What should a datahseet contain? With tongue in cheek I replied: "a complete transistor-level schematic of the part!" Actually, I appreciate how hard it is to really document these complex devices.

But it's frustrating when electrical parameters are not fully characterized. Often only typical values are given, not max and min. Some vendors treat these components as logic devices, when the truth is there are a lot of electronic issues involved (voltages, currents, etc).

Just look at the competition. In addition to your Nexperia (NXP) zener datasheet, I found this On Semiconductor one, and this Vishay one. Only the On Semi zener datasheet lists voltage at 37mA test current of Min=6.46, Typical=6.8, Max=7.14 -- Thank you, On Semiconductor!

It is very expensive to fully characterize devices! In some ways, this raises the bar, the barrier to entry, reducing the number of competitors a company has, because it just takes a lot of work, and money, and time...

I would love it for the "Maker" community, or a big IEEE club, to start a non-profit organization to characterize common simple devices like these for the masses, because it's a statistical study, and it requires a decent sample size. We can tackle this together, don't you think? Instead of crowdfunding with KickStarter, we could have CrowdCharacterization, CrowdPCB, CrowdPlacement, and CrowdGrunt (final assembly).

Here's my point-by-point response to your question:

For a simple electronic voltage regulator using Zener diode, I use the following circuit. (Zener Shunt Regulator)

This is a building-block circuit, and if you look more closely, even though the datastheet says "Voltage Regulator" the "Applications" section says "voltage stabilization" -- it's not terribly accurate. For accuracy, you would want to use a voltage reference -- unloaded -- and use that with some kind of amplifier (NPN, FET, or Op Amp) to deliver power at a specified voltage.

For deciding Rs and RL, I need to know Izmax and Izmin of the 1N4736A zener diode.

I recommend looking around and studying how zeners are used in other circuits. As a hobbyist myself, I got an old copy of "The Art of Electronics" which has a good section on power electronics and linear regulators, which is how you are trying to use this zener, but for which this zener has not been intended. If your application is particularly low-power and not picky about the incoming voltage, it might work. But you still might want to get 100 (or 1,000) and plot the results, and do the statistical study, if this will eventually go into a real product.

From this [data sheet], There are different current specifications available. Maximum working current is given as 133mA. Test current at working voltage is 37 mA. (Somewhere, I found) The Zener will get damaged if it works continuously at maximum working current.

The engineer often has to handle pulses and short durations of high power, and the maximum figure is for those usages. It's only a fuzzy idea, really, of what the device is capable of, and part of eliminating parts that will never work for the application "because the max is simply too low." You are the one responsible for making sure the part will work for your application.

For calculating the minimum zener current, one source says that it is 10% of the maximum current figure, and another source says it is the knee current (1mA) at which the impedance is tested. Which is the correct choice?

Manufacturers will often test their parts different from other manufacturers. This makes it impossible to properly compare parts, at least while shopping. You are the one who has to test the part to see how it behaves. You have my sympathy. And as you have seen above, my heart is that some people group that's big enough band together and enable some of this for the little guy.

(from a comment) This is actually for demonstration purpose of working of the circuit from minimum possible load resistance (Max. Load) to no load condition. (Min. Load)

Before you teach somebody else (except, perhaps, a children's beginners electonics club) you probably want to put a lot into both study and testing of this circuit and more sophistocated (and useful) zener / voltage reference circuits. You don't want to be embarrased when your zener goes up in smoke (unless you've staged it to "teach" them) or when they ask questions you can not answer.

I will leave you with a circuit from The Art of Electronics, second edition, page 319. This is an "Active power zener" a kind of active clamp. The idea is, if the incoming power supply generates too much voltage, make a short-circuit hard and fast, so as to blow a fuse or throw a circuit breaker and prevent the slower destruction of the circuit that is past this, and possibly prevent fire hazard as well. Overvoltage = short-circuit. Simple and dependable, and protective of the rest of the circuit.

Active Power Zener circuit

You can see that, for this virtual component, even though it is supposed to "turn on" at 6.8 volts, it's more like it "slides on", and is already sinking more than three amps at its nominal 6.8 volts. Now, the curve depends a lot on what transistor I plug in -- a 2N3055 can probably handle more power dissipation, but has a significantly lower beta (gain) -- making it probably a better fit for an active clamp at 6.8 volts. But I would probably choose the power transistor first, and do a binary search on the zener.

By binary search, I mean continually dividing the search space in half each time, which allows me to quickly find the answer. Best of luck to you.

By the way, if you're doing a demo, they always love to see "the magic smoke". First show them what not to do. It's the best way to instantly have their attention. My (awesome) high school teacher used to ask, "can water explode". Then he would take his class out back, drop some sodium or potassium in some water, and make a small crater. The kids loved his class!

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Why not operate a Zener diode at its load current? Then its voltage is spec'd.

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