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Can I use this MOV+PTC as a surge protector?

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alphacat

New Member
Hello,

I currently have 2 MOVs in my product, one from Live to Earth and the Other from Neutral to Earth.
The mains can be either ~250V or ~120V.

Now I want to replace each MOV with the following MOV+PTC:

untitled-png.36150


Will it be ok to use this MOV+PTC?

Thanks.
 

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MikeMl

Well-Known Member
Most Helpful Member
Why do you need to clamp the voltage from Neutral to Ground?

Do not put a current limiter in series with the Ground Line. I'm not sure about a current limiter in the Neutral Line?

It seems to me that only the following have any benefit: Current limit in Line. Voltage clamp Line to Neutral. Possibly, Voltage Clamp Neutral to Ground.
 
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alphacat

New Member
I need to clamp the voltage from Neutral to Earth since a lightning strike might also affect the Neutral wire.
Moreover, the Neutral wire's resistance might increase its voltage to high levels.

The PTC is not connected in series with Earth, but in series with the MOV, as follows:

untitled-png.36166


Is this configuration alright?

Moreover, do the electrical specifications of the MOV and the PTC allow me to connect them to the mains (~250V or ~120V) in that way?

Thanks.
 

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MikeMl

Well-Known Member
Most Helpful Member
Your method of connecting these is not correct. They require connections to all three terminals. The load current is supposed to flow from 1 to 2, and the clamp is supposed to happen between 2 and 3. So, for the one installed in the line side; Line to 1, Load to 2 and 3 to Neutral. You do not want to limit the current in either the Neutral or Ground wire, so if you insist on clamping Neutral to Ground, just use a two-terminal MOV or Transorb.
 

alphacat

New Member
Hey,
Thank you for your great answer.

The thing is that i want the PTC to protect the MOV, not the load.
Moreover, the load can draw up to 16Arms, so a ~10ohm PTC is not something i can connect in series with the Load, you see?
 

MikeMl

Well-Known Member
Most Helpful Member
This device is designed for circuits like phone line inputs to a modem where the 10Ω resistance is not an issue. If you put the PTC in series with the MOV, then you might as well as not bother putting in the MOV at all!

Look at Transzorbs. Read the application notes on the Vishay web site.
 
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alphacat

New Member
If you put the PTC in series with the MOV, then you might as well as not bother putting in the MOV at all!

For a surge protector to receive UL certificate, it needs to have thermal fuse protection against protector meltdown (UL1449).

The time it takes to the PTC to start increasing its resistance is longer than the time it takes to the MOV to become a shortage.

Therefore, when a lightning strikes, the MOV shorts the beginning of it to Earth, but if the voltage is still high, then the PTC will protect the MOV and only the Load will get damaged (instead of both Load and MOV getting damaged).

Do these MOV and PTC have the right electrical specifications for such usage?

Thank you very much.
 

alphacat

New Member
@Gayan.
I read many articles about MOVs - from Wiki and other websites/forums.
The problem is with the thermal fuse protection.

@Hayato.
Thank you, a good idea.
Could you please help me with the electrical specifications that such thermal fuse should have?
I'd like it of course to be attached to the MOV's body (for it to sense the MOV's temperature), and the fuse together with the MOV should be connected from Live to Earth (first pair) and from Neutal to Earth (Second pair).
The mains could be either ~120V or ~250V
 

Hayato

Member
You can find thermal fuses that open with 50°C to 280°C.

When a MOV explodes, it explodes. The you have a "heat surge" and it will not cause flames.

The most dangerous thing is when the voltage surge is so high that it arcs inside your apparatus, causing fire.
But when this happens, a thermal fuse, a PTC cannot... Are not useful at all.

For a surge protector, the circuit alone is not the only responsible for a flame or not. One of the most important thing is the casing construction and material. You must use non-flammable materials for that, to get a UL.
 

alphacat

New Member
Thank you mate :)

I found some thermally fuse protected MOVs on Mouser.

What should be the MOV's clamping voltage and Energy rating? (for mains that are either ~120V or ~250V).

For some reason, all the thermally protected ones have a minimum clamping voltage of 710V.
I'm afraid that its too high, isnt it?


I've found the following MOV:
**broken link removed**
Clamping voltage at 150 A (8/20 μs): 710V
Max. operating AC voltage: --------- 275V
Surge current (8/20 μs) 1 time: ----- 20KA
Energy absorption (2 ms) 1 time: ---- 375J
Average power dissipation: ---------- 1W

What do you think?
 
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