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When to use MOV, TVS or GDT?
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MagicSmoker:

--- Quote from: wraper on April 29, 2019, 11:41:53 pm ---
--- Quote from: MagicSmoker on April 29, 2019, 10:49:46 pm ---Sorta maybe probably not. The GDT (literally) insulates the MOV from leakage current flow, so that the MOV's breakdown voltage adds to the GDT's.

--- End quote ---
Nope, voltage across MOV is zero due to it's leakage until total voltage reaches GDT breakdown voltage. Then GDT becomes basically short with very low voltage across it and all voltage drop is then across MOV. Nothing adds ups. It would add up if you connected two MOVs in series.
--- End quote ---

Yeah, ok. Treat the MOV as a very high value resistance in its pre-breakdown state that is in series with the even higher resistance of the GDT; all the voltage gets dropped across the latter. In any event, thinking about this - even by putting forth a wrong argument - was illuminating.
T3sl4co1l:
Yes, that will be fine for ring wave.  The GDT may even stay ionized for the duration of the wave, letting the MOV do its job that much better.

Mind that SPICE models of gas discharge elements are notoriously bad, or conversely, notoriously difficult to make accurate.  I don't know offhand which case that one is.  I should check it out...

Tim
MagicSmoker:

--- Quote from: T3sl4co1l on May 01, 2019, 03:27:07 am ---...
Mind that SPICE models of gas discharge elements are notoriously bad, or conversely, notoriously difficult to make accurate.  I don't know offhand which case that one is.  I should check it out...

--- End quote ---

Yes, modelling an arc discharge in an ionized gaseous medium is challenging, but I've managed to get the "neon bulb" model (under Misc) in LTSpice to do a reasonably good job of mimicking the real world behavior of an automotive spark plug and a massive carbon electrode spark gap (Ipk >30kA). I haven't yet ascertained if it gets the negative resistance characteristic right, but I suspect it doesn't because it wants you to specify the voltage across the arc (Vhold) as well as its impedance (Zon) - but at least it lets you set the gas ionization time (Tau). Right click on the symbol to set all of the above as well as the striking voltage (Vstrike) and minimum arc current (Ihold).

For example, to model a spark plug I set Vstrike to 3600, Vhold to 300, Zon to 0.1, Ihold to 1, and Tau to 1u. For a larger spark gap Tau might increase to 100u to 1m, while Zon will generally fall to 10m to 1m. Ihold and Vhold seem to be less critical as far as getting the simulation to match the real world.

Diablo2813:

--- Quote from: MagicSmoker on April 30, 2019, 12:07:00 pm ---
--- Quote from: wraper on April 29, 2019, 11:41:53 pm ---
--- Quote from: MagicSmoker on April 29, 2019, 10:49:46 pm ---Sorta maybe probably not. The GDT (literally) insulates the MOV from leakage current flow, so that the MOV's breakdown voltage adds to the GDT's.

--- End quote ---
Nope, voltage across MOV is zero due to it's leakage until total voltage reaches GDT breakdown voltage. Then GDT becomes basically short with very low voltage across it and all voltage drop is then across MOV. Nothing adds ups. It would add up if you connected two MOVs in series.
--- End quote ---

Yeah, ok. Treat the MOV as a very high value resistance in its pre-breakdown state that is in series with the even higher resistance of the GDT; all the voltage gets dropped across the latter. In any event, thinking about this - even by putting forth a wrong argument - was illuminating.

--- End quote ---

So where are you wrong ? Does the resistance value of the GDT falls when the breakdown voltage is reached ?

Otherwhise, i was looking for a deeper explanation. I found this : .

Point D is relative to breakdown voltage.
So the glow discharge behaviour is what justify GDT one ?

If this is the case, i think the leakage current is involve next D.
Then when the current goes up, voltage falls until the GDT is really opening.
So the difference of voltage between points D and G is critical in order to get a significant voltage across the MOV ?

Thanks
wraper:
That's neon bulb, not GDT suppressor.
Here is GDT (page 4): https://www.mouser.com/pdfdocs/bourns_gdt_white_paper.pdf



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