Electronics > Projects, Designs, and Technical Stuff
When to use MOV, TVS or GDT?
T3sl4co1l:
But yes, that's the basic idea. Surge currents will push it into the arc range, not the glow range. Both have a negative resistance (switching) characteristic, going from high resistance (dark discharge, low leakage) to low (glow or arc discharge).
Tim
Diablo2813:
--- Quote from: wraper on May 02, 2019, 04:25:19 pm ---That's neon bulb, not GDT suppressor.
Here is GDT (page 4): https://www.mouser.com/pdfdocs/bourns_gdt_white_paper.pdf
--- End quote ---
--- Quote from: T3sl4co1l on May 02, 2019, 07:58:25 pm ---But yes, that's the basic idea. Surge currents will push it into the arc range, not the glow range. Both have a negative resistance (switching) characteristic, going from high resistance (dark discharge, low leakage) to low (glow or arc discharge).
Tim
--- End quote ---
Thanks.
To summarize,
General information :
MOV and GDT are both able to short a surge when a specific breakdown voltage is reached.
GDT acts like a switch and do not significantly dissipate. It also do not limit the current and induce a risk to burn the lines.
A low DC voltage across the GDT may prevent it from returning to his normal state. Using it alone to the ground isn't an option.
GDT switch delay is faster as dV/dt rise and can be few us for lower dV/dt.
In the other hand, the GDT have zero leakage current and is much more durable than a MOV.
MOV acts like zener and does dissipate. It's able to limit current using his "zener" voltage only.
It doesnt care much about DC voltage. But it also have a low but not zero leakage current.
MOV switch delay is low and constant.
The MOV have a lower life expectancy and is likely to burn over the uses and induce a short-circuit.
A serial fuse should be use in any case.
Serial association :
Now, if i understand right, the only synergies i see are :
from adding a serial GDT to a MOV
* To modify global breakdown value so the MOV get to support fewer energy.
* To prevent leakage current from the MOV (but that's not something that important ?).
and from adding a serial MOV to a GDT is :
* To secure the return of the GDT to his blocking state
In my opinion
So i actually don't see much interest in adding a serial GDT in my application. (4kV surge voltage)
Some SMT MOV even got decent sizing close to GDT ones. I would rather add another MOV than a GDT.
wraper:
--- Quote from: Diablo2813 on May 03, 2019, 08:43:51 am ---Some SMT MOV even got decent sizing close to GDT ones.
--- End quote ---
Then look at their ratings and think again. Small MOV do explode quite often when actually suppressing surge.
T3sl4co1l:
You got it :)
Leakage is important for some standards, like IEC 60950-1 which prohibits MOVs directly from line to ground (for most equipment), however a MOV+GDT is allowed. This is for ground leakage reasons -- on equipment where the ground leakage could potentially be exposed to the user (an insulation fault in a double-insulated product, or via enclosure / connections if the ground pin gets lifted), those MOVs would be a big problem just to begin with, but also as they age and decay, leakage current increases and also breakdown voltage may fall (or one outright shorts due to a large enough surge, or enough hits), and that's where the GDT comes in, blocking that leakage until surge voltage is applied.
SMTs, are simply rated what they are rated. If that's adequate for your application, great!
I wouldn't recommend them for mains (IEC 61000-4-5, 2kV and 2Ω, say), but they may be adequate for ring wave, and are definitely adequate for ESD and EFT purposes. Indeed, you can get tiny (0603 and such) chips for ESD and minor surge purposes -- they handle more energy than TVS diodes of the same size, though again with the same rather lax voltage drop, characteristic of any MOV.
Note that low voltage MOVs have somewhat poor energy ratings, partly because not much material thickness is required. MOVs below 80V also use a different formulation, with less voltage drop per thickness but much higher internal resistance --> higher peak voltage drop. In fact, manufacturers usually recommend that, if you need a 60 or 70V MOV, just go ahead and choose the 80V part, because it will actually have lower clamping voltage. (40V parts I think are about a wash, usually, while lower voltages are fine.)
But yeah, if we're talking mains voltages here, whatever is fine. A typical 20mm, 300V MOV can gobble up over a hundred joules in its final heroic act, and can do it repeatedly for smaller surges. If you only need ring wave that's not too strenuous, maybe a 7 or 10mm part is fine, or even an SMT?
The other downside to SMTs, they seem to be more expensive. Go figure?
Tim
Diablo2813:
In the end, i voted for a MOV+GDT serial association.
I'm willing to pm my setup if anyone is able to check it out.
That would be very helpful.
Thanks all of you,
Benjamin.
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