Electronics > Projects, Designs, and Technical Stuff

When to use MOV, TVS or GDT?

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T3sl4co1l:
The MOV is a passive device, in that it acts like a zener diode, or a wonky resistor.  Normally its voltage drop is zero, because the GDT's leakage current is very nearly zero.

The GDT is an active device, in that it acts like a switch, triggered by a high voltage (in excess of the rated voltage).  When this happens, as long as the current remains above the holding current, the voltage drop will be very low, perhaps 20V or so.

Say the MOV is 300V and the GDT is 500V.  Say we apply anything up to 500V: nothing happens, because the MOV voltage drop is zero, the GDT voltage drop equals the input, and breakdown does not occur.

Say we apply something more than 500V, from a low impedance surge generator: now the GDT breaks down, and goes from dropping 500V+ to ~20V, and suddenly 480V+ is applied to the MOV, which now draws a large current from the surge generator's source impedance.  Say the surge peak voltage was 1kV in this case, and say the MOV breakdown voltage is 600V.  That drops 400V across the surge generator's source impedance, and if that is 2Ω, the current is 400V / 2Ω = 200A.  This is in the MOV's surge area (where it may be dropping even more voltage, like 700-800V), and more than enough current to keep the GDT turned on at a low voltage drop.

When the surge current disappears, the MOV voltage drop falls as well, but so does its current flow.  As this drops to, say 1mA, this may be below the GDT's holding current, and its voltage drop rises.  As the voltage drop rises and current falls, the situation returns to normal.  This may be delayed by line current (one reason why they test with surges timed to the peaks and valleys of the mains), which can keep the MOV and GDT conducting for longer; at worst, they remain conducting for about a quarter cycle (~5ms), as the line voltage crosses through zero and the GDT turns off for sure.  (In that case, it's probably that the MOV has failed shorted, and mains fault current flows through the pair, blowing the fuse in front of the protection circuit.)

The time required for these changes is fractional microseconds.  Typical surge pulses have a rise time of a few µs, and a pulse width of 10s of µs.  The GDT may take a microsecond or more to break down, when the applied voltage is low and slow; or it may take mere ns when the voltage is high and fast (say, ESD, which delivers many kV in a few ns).  The MOV's response is largely instantaneous, delayed only by its capacitance, which is relatively large (~nF usually).

Tim

MagicSmoker:

--- Quote from: T3sl4co1l on April 29, 2019, 08:53:26 pm ---The MOV is a passive device, in that it acts like a zener diode, or a wonky resistor.  Normally its voltage drop is zero, because the GDT's leakage current is very nearly zero.

The GDT is an active device, in that it acts like a switch, triggered by a high voltage (in excess of the rated voltage).  When this happens, as long as the current remains above the holding current, the voltage drop will be very low, perhaps 20V or so.
...

--- End quote ---

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.  That said, it's more accurate to model both components as capacitors in their pre-breakdown state, with the MOV handily beating the GDT in raw capacitance so that when presented with a high dV/dt transient it may very well breakdown last, even if it has a lower breakdown voltage than the GDT.

That's my intuitive sense of how things would behave, but I confess I haven't actually tested this out even in SPICE, much less the real world (that said, the neon bulb model in LTSpice is surprisingly good at mimicking the behavior of spark gaps and GDTs when the various parameters are tweaked).

coppercone2:
when you feel like getting into a fight with cost people

wraper:

--- 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.

--- Quote ---That's my intuitive sense of how things would behave
--- End quote ---
Many things are completely counter-intuitive. For example, for flight stability rockets need to have center of mass towards it's top, not bottom as you might think.

Diablo2813:
Thank you all for this explanation. That's much clearer now.

Do you think this kind of association could be suited to protect from 4k surge voltage ? rising 1us and falling 100us.

NB:
At the moment I'm using a gas discharge tube simulation model from TDK EPCOS. It's available on demand. Looks like its working well for now.

Thanks, thanks again,
Benjamin.

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