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I have been looking at circuit protection using TVS and Zener diodes.

I have seen the following symbols used to represent TVS diodes in circuit diagrams:

schematic

simulate this circuit – Schematic created using CircuitLab

I guess the first question is whether there is a meaningful difference between TVS and Zener diodes, and the answer seems to be, "Their characteristics are similar, but their design and test specs, and intended applications, are different: Zeners are for specific and potentially continuous voltage regulation. TVS diodes are less precise about voltage and designed rather to shunt (and survive) large power transients."

My impression so far is that of the symbols above:

  1. Should be assumed to refer to a Zener diode (unless notes indicate otherwise).
  2. Unambiguously indicates a TVS diode.
  3. Unambiguously indicates a TVS diode.
  4. Probably refers to a pair of Zener diodes, but could refer to a single TVS diode.

Are these reasonable assumptions?

I imagine that the only time one would consistently run into trouble is when using a TVS diode instead of a pair of Zener diodes. E.g., using a TVS diode, with its imprecise breakdown voltage, when the circuit calls for a "waveform clipper" would produce terrible results. On the other hand, using a Zener when a TVS was intended one would likely either never notice the difference if large power transients aren't part of routine operation, or else one would probably notice the difference quite quickly as the Zener was fried?

Or is the correct answer to this ambiguity simply, "Yes, they're ambiguous. And until you're sure which diode to use you're not ready to build the circuit."

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The reason the same symbol is sometimes used for TVS diodes (Transorbs) and Zeners, is a Transorb has a lot in common with a Zener. An ideal Zener and an Ideal TVS-diode would be indistinguishable in their characteristics. This leads to ... laziness in library management (or ignorance) and the same symbol is used.

Regulator Zeners and TVS-Zener diodes differ in aspects of their construction to facilitate either higher continuous rating or high pulse capability.

Zener TVS devices are constructed with large area silicon p-n junctions designed to operate in avalanche and handle much higher currents than their cousins, Zener voltage regulator diodes

Only uni-directional TVS diodes are created at wafer level. The bidirectional TVS diodes you can buy are just two such dies packaged in series.

Examples of symbols for some TVS devices:

enter image description here

enter image description hereenter image description here

From your images

  1. Zener diode unless the part number calls up a TVS
  2. TVS
  3. TVS
  4. Back-to-back Zeners unless the part number calls up two unidirectional TVS

http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

Using a TVS diode instead of, or in addition to a MOV for AC line protection?

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  • \$\begingroup\$ ... and so a "bidirectional" diode is both theoretically and practically equivalent to two uni-directional diodes of the same specification? Or is there some significance to drawing the back-to-back diodes in contact vs connected by a small wire? \$\endgroup\$ – feetwet May 11 '16 at 21:07
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    \$\begingroup\$ The only drawback in using two unidirectional parts is 1) additional BOM part, 2) larger board area 3) additional stray inductance due to the additional trace. Considering TVS-Diodes are quite often used in areas with high dv/dt transients, this could be an issue. \$\endgroup\$ – JonRB May 11 '16 at 21:15
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From a practical point of view (and not so much looking at the symbols), the biggest difference is the pulse handling capability. The underlying effect is the same for all - they are designed for a fairly precise avalanche breakdown (strictly speaking, the Zener effect is for low-ish breakdown voltages, the avalanche breakdown is for higher voltage ratings). However, as you say already, TVS diodes can handle energy pulses far better than Zeners designed for voltage stabilization.

Concerning the symbols: There are also unipolar TVS (surge suppression) diodes, and the symbol is the same as for (voltage stabilization) Zeners. Sometimes, only the application (in the context of the surrounding schematic) or a Mfg./Part number will tell you the whole story...

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  • \$\begingroup\$ Oh yeah, I neglected to consider uni-directional vs. bi-directional diodes. Though again, my understanding is that both Zeners and TVSs have reliable bi-directional characteristics, so is that another point of intentional ambiguity? Or would you expect symbols #1 and #4 to be explicitly employing unipolar diodes? (And if so what is the distinction between those and bipolar variants?) \$\endgroup\$ – feetwet May 11 '16 at 19:54
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    \$\begingroup\$ @feetwet, #1 is unipolar. #2, 3, and 4 or bipoloar. #1 will allow current in the reverse direction with only about 0.7 V drop. The others will need a voltage to reach the breakdown voltage to conduct in either direction. \$\endgroup\$ – The Photon May 11 '16 at 20:30
  • \$\begingroup\$ @feetwet Hmmm... I would not necessarily call it intentional ambiguity. Think about how many different types of resistors are out there, and it's always the same symbol. A schematic always is just that: A schematic (read: abstract, simplified) representation of the real thing. For readability, things are left out. Much like sheet music, where the same melody looks identical on paper, but sounds a lot different when played by a guitar, piano or horn. \$\endgroup\$ – zebonaut May 11 '16 at 22:29

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