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For a 120Vac line protection element, comparing this TVS diode:

tvs characteristics tvs iv

With this varistor:

var characteristics var max clamp

The immediate differences I see are:

  • The varistor is marginally cheaper
  • The varistor specifies a maximum working voltage much, much smaller than its clamp voltage. There seems to be no maximum working voltage on the diode, only the breakdown voltage.
  • The diode is well-specified, and includes many graphs of its I/V characteristics. The varistor only has one graph, of clamp voltage against surge current.

In a simple application of a parallel AC line protection element, why would I choose a varistor over a bidirectional TVS diode?

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    \$\begingroup\$ Consult UL here, but I seem to recall silicon TVS devices are not permitted for mains protection across the board. \$\endgroup\$ – Matt Young Sep 30 '18 at 19:27
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    \$\begingroup\$ @MattYoung Is there an online reference for this? \$\endgroup\$ – Reinderien Sep 30 '18 at 20:48
  • \$\begingroup\$ Not that I have readily available, like I said, if you're working on a design that brings up the question, ask the final authority. \$\endgroup\$ – Matt Young Oct 2 '18 at 0:03
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Varistors are "baked" semiconductors, their properties aren't tightly controlable. Think of them as a mesh of billions of Schottky junctions. Their big pro is they can both short a spike and turn it into heat smoothly because of the high mass of the actual mesh.

In addition, as the varistor heats up, it gets more conducting and this can be used to blow a fuse for the case the overvoltage is not transient but steady. When the overvoltage is gone, the user replaces the fuse and the device can be used again.

You cannot do this with a TVS as it would blow before a fuse could react. You had to limit the current through it some way. Usually, this means you are using a TVS only on signal lines where the current is limited by the existing circuit itself.

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  • \$\begingroup\$ As one who was in that industry for 15 years, that is a very accurate answer. +1 \$\endgroup\$ – user105652 Sep 30 '18 at 22:03
  • \$\begingroup\$ Well, to be fully accurate, one had to write schottky junctions, now that I think of it. \$\endgroup\$ – Janka Sep 30 '18 at 22:10
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    \$\begingroup\$ One important thing about TVS diodes is tiny SMD types with 1.5pF of capacitance could be use to protect cable TV and Ethernet up to 2 GHZ without phase-shift errors (For HDTV). 40mm MOVs had a very high capacitance and could not be used on data lines, just power lines. TVS diodes also had a much tighter clamp voltage, much like a zener diode. \$\endgroup\$ – user105652 Sep 30 '18 at 22:19
  • \$\begingroup\$ So you're saying, a varistor can sustain a surge longer than a TVS? Is there somewhere in the specs I posted that would demonstrate that? \$\endgroup\$ – Reinderien Sep 30 '18 at 23:07
  • \$\begingroup\$ It's mass against allowed temperature. Heat capacity should be similar. Also, a varistor cannot punch-through due to heat, as there isn't one single junction which could fail. \$\endgroup\$ – Janka Sep 30 '18 at 23:08
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The most obvious difference is the continuous Pmax power rating for heat dissipation.

5 watts MOV vs 0.25 watts TVS

Power vs signal vs location affects the disturbances and solutions available. This will not be a tutorial.

Peak current MOV is better for high AC line surges for locations with unbalanced lines so 100A half wave at 60Hz

Consider where one phase might have a fault or short resulting in the other phase possible have a half cycle overvoltage from a delta-Y distribution or a 3 phase transformer. , this then ought to blow a designed-in fuse. This is important in areas of high fault occurrences.

TVS is better for lightning surges 20us impulses so 1500A. But not 10kA. Peak voltages may be reduced with a line filter or a CM choke or CLC PI filter and then TVS V threshold can be lower towards some typical 20% over application voltage.

TVS may not be suitable for AC grid due to overvoltage durations of a half cycle or more but excellent for narrow transients like 20us or signal lines due to lower differential resistance and faster response time.

Best followup is reading Application Notes from each supplier.

Regional differences matter for unit protection vs grid quality in your choices.

A good line filter is best start ( All earthed SMPS have some sort of line filter)

Outside signals vs power lines is another condition where above ground distribution is more exposed to lightning impulses from stray nearby hits. Unlike our residential areas where all pnone CATV and power cables are underground with shielded XLPE coax for power.

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