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I am confused about the difference between rectifier and Zener diodes. I've already read all the answers at https://www.quora.com/What-is-the-difference-between-a-diode-and-a-Zener-diode, but still have some significant doubts.

When I search for diodes on aliexpress there are always differences in how the parameters of rectifier and Zener diodes are specified (by providers that sell them.) For example, some shops specify only amperage and voltage for rectifier diodes (eg 5A/100V.) When it comes to Zener diodes, they specify only watts and voltage (for example, 0.5W 10V.) Why?

As I understand the principle differences are (1) Zener diodes can conduct current in the opposite dirrection without damage when Vz is reached and (2) the maximum reverse voltage of a rectifier diode (before it will burn out or conduct and burn out anyway) is much higher. I assume that Zener diodes conduct only the specified/rated voltage in contrast to rectifiers.

JRE
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Timur Fayzrakhmanov
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3 Answers3

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I'll try to answer this simply, I'm no expert myself so it's likely some of the comments will correct me little

Although they are both called diodes they have very different uses.

Rectifier diodes are used mainly for only allowing current/voltage to flow in one direction. As mentioned in the comments above, the specification values for rectifier diodes refer to the maximum current they can pass in the forward direction and the maximum voltage that can be applied in reverse before the diode begins to breakdown.

enter image description here

Here is an example of a rectifier diode in use, in this application it is known as a flyback diode (google for more information). In this configuration, any high voltage spikes cause by switching off the relay (or any inductive load) pass through the rectifier diode back to Vcc, protecting the transistor. As long as the reverse voltage rating of the diode is higher than Vcc it will 'block' Vcc from passing through it.

Zener diodes work a little differently, they conduct in reverse and can then recover unlike rectifier diodes. The voltage specification of a zener diode is it's breakdown voltage, this is the voltage that the zener will pass.

enter image description here

In this circuit they have used a 5.1V zener diode, if you were to use a multimeter across Vout then you would measure 5.1V, all the other voltage is dropped across the other components in the circuit, in this case a 1K resistor. A 1W zener will safely dissipate 1W before burning up.

In the circuit example above, as it is absorbing 6.9V dropped across the resistor (12-5.1) then this means the circuit current is 7mA (6.9/1000 [I = V/R]) and so the zener will dissipate 35mW (0.007*5.1 [P=I*V]) Zener diodes are used to regulate voltages in certain applications

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    In the circuit above, the Zener isn't absorbing 6.9 volts, that's what's being dropped across the resistor. The current through the Zener will be the same as the current through the resistor, and that'll be: $$ I_Z = \frac{12V-5.1V}{1000\Omega} \approx \text { 7 milliamperes}$$. The Zener, then, will only be dissipating $ 7mA \times 5.1V = \text{ 35 milliwatts}$. – EM Fields Apr 04 '16 at 14:49
  • Part 1. Sorry, but since a Zener is a shunt regulator, all a load will do is direct current away from the Zener and, if it's a heavy enough load, increase the resistor's dissipation. For example, if we set up your circuit so there's 100 mA through the Zener, R1 will need to be 69 ohms, it'll dissipate 690 milliwatts, and the Zener will dissipate 510 milliwatts. Then, lets say you connect a 100 ohm resistor across the zener. Since the Zener voltage will stay at 5.1 volts, that means 50 milliamperes wlll now be shunted away from the Zener and into the 100 ohm load. – EM Fields Apr 04 '16 at 15:07
  • So your maximum load current is always going to be less than the current across R1? Also is there a minimum value for Iz? –  Apr 04 '16 at 15:18
  • Part 2. Now, since there's still 12V coming out of the supply and there's still 5.1 volts being dropped across the Zener, the supply won't know anything about what's going on out there and it'll just keep on cranking out 12 volts at 100mA into R1. On the other end of R1, though, since there'll only be 100mA available for everything, if the Zener is the only thing there it'll suck it all up but if something else comes along and wants some of that 100mA, the Zener will let it go until it goes out of regulation. – EM Fields Apr 04 '16 at 15:20
  • I'm guessing this is why zeners are only really used as regulators for lower current loads? –  Apr 04 '16 at 15:21
  • The current through - not across - R1 must always be higher than the load current because of the current needed to keep the Zener in regulation. There's no minimum Zener current per se, but the current which will guarantee the Zener will meet its specs is the "Zener test current" or $ I_{ZT} $ on the data sheet. – EM Fields Apr 04 '16 at 15:38
  • Lower current only??? Check this out – EM Fields Apr 04 '16 at 16:18
  • If you're running a zener like that then you're going to need a giant resistor before it surely? As in 10W sort of size? –  Apr 04 '16 at 16:27
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The rectifier diode and the Zener diode are both types of semiconductor diodes, but they serve different functions in electronic circuits.

Rectifier Diode

Function: Rectifier diodes are primarily used for converting alternating current (AC) to direct current (DC) in power supply applications. They allow current to flow in one direction only, essentially rectifying the AC signal.

Forward-Bias Operation: In a forward-biased state, a rectifier diode conducts current easily, allowing current to flow from the anode to the cathode.

Reverse-Bias Operation: In a reverse-biased state, a rectifier diode blocks the flow of current, acting as an open circuit.

Zener Diode

Function: Zener diodes are designed to operate in the reverse breakdown region, maintaining a constant voltage across their terminals. They are often used for voltage regulation and voltage reference in electronic circuits.

Reverse-Bias Operation: Zener diodes are used in their reverse-biased state, and when the voltage across them exceeds a specific value (the Zener voltage or breakdown voltage), they start conducting in the reverse direction.

Voltage Regulation: Zener diodes provide a stable voltage output, making them suitable for applications where a constant voltage is required.

Differences

Operating Region: The key difference lies in the operating region. Rectifier diodes operate in the forward-biased state for rectifying AC signals, while Zener diodes operate in the reverse-biased breakdown region for voltage regulation.

Functionality: Rectifier diodes convert AC to DC, whereas Zener diodes regulate voltage by maintaining a constant voltage drop across their terminals.

Breakdown: Rectifier diodes are not designed for intentional breakdown, while Zener diodes are specifically designed to operate in the breakdown region.

You can find more information about the differences between the Zener diode and rectifier diode in this post: https://medium.com/@chris_53665/differences-between-zener-diode-and-rectifier-diode-413208e9bbf0

Tim Williams
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SZComponents
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    The characteristics are not exclusive, either; the combination is usually called an avalanche rectifier, rated for some minimum breakdown voltage and energy. Probably the most common highly-robust example is those used in automotive alternator rectifiers, breaking down at 40V or so to clamp load dump in case of a sudden load change with a disconnected or faulty battery. – Tim Williams Nov 30 '23 at 10:22
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Fundamentally, they are optimized for different operating modes.

Both types of diodes you mention here are a junction between two differently doped types of semiconductor. The doping profiles are chosen to improve specific characteristics. For a rectifier diode, you want good high-current handling performance, low forward voltage, low reverse leakage, and high reverse breakdown voltage. For a Zener diode, on the other hand, you don't care about forward voltage and reverse leakage becomes a moot point, but you need a low and predictable reverse breakdown voltage that remains somewhat constant over a range of current.

Semiconductor physics involves a lot of math and physics (I've got a few textbooks on this subject sitting on my shelf), but your main knobs are the spatial distribution of the impurities you doped with, which impurities you use, and what substrate the diode has.

mapplejacks
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