MOV+PTC as a surge protector.

Discussion in 'General Electronics Chat' started by alphacat, Dec 9, 2009.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    Hello,

    I'd like to connect the following MOV+PTC from Live to Earth and from Neutral to Earth:
    [​IMG]

    The mains could be either ~250V or ~120V.

    Do the electrical specifications of the MOV and PTC allow me to do that?

    The configuration i thought of was:
    [​IMG]

    Thank you.
     
  2. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    anyone please?
     
  3. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    Good question...MOV manufacturers re-printed datasheets are notoriously confusing to say the least. Generally, you select for trip VAC or VDC...say 115 VAC, select 130 VAC unit. For 230 VAC, select 260 VAC unit. Give yourself 12-15% headspace. The sizes depend upon the amount of energy (Joules, watt sec.) that must be dissipated. Better to use MOV's in combination with other devices (resistors) for DC service. I like to use Quencharcs (ITW) for DC arc suppression...this is an RC combination. When in doubt, use a scope and watch the clamping characteristics when you break the circuit.
    I'm not sure why you want to use a PTC with MOV?

    Cheers, DPW [ Spent years making heaters out of op-amps.]
     
    Last edited: Dec 10, 2009
  4. pradeeba

    Active Member

    Feb 27, 2008
    104
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    Have a look at this pdf
     
  5. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
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    Thank you very much :)

    The Max Clamping Voltage of this MOV is 455V @ 25A, Do they refer to AC voltage?

    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).

    Does this PTC have the right electrical specifications to protect the MOV?
     
  6. westom

    Member

    Nov 25, 2009
    52
    5
    Your are thinking in terms of the wrong parameter. Voltage is a dependent variable. Surges are about current - the independent variable. A surge is a constant current source (a phrase from circuit theory).

    A surge current will increase voltage as necessary so that the current will flow. For example, lightning struck a conductive material - a wooden church steeple. Wood is not a good conductor. So voltage increases as necessary so that a surge current obtains earth ground. 20,000 amps times a high voltage is high energy. A destroyed church steeple.

    Franklin installed a lightning rod connected to what provides protection. That better connector to earth means 20,000 amps times near zero voltages. Low energy. No damage.

    Surge protection via MOVs is same. Any PTC that would stop a surge current - the voltage only increases so that current will continue to flow. (Notice the low voltage number for a PTC. Voltage will increase as necessary to exceed that number.)

    No surge protector should be damaged by a direct lightning strike. Protectors are installed to conduct direct lightning strikes and remain functional. Therefore MOV parameters are based in lightning waveforms. Earth MOVs so that the protector remains functional after a direct lightning strike.

    If an MOV fails, that thermal fuse is to disconnect MOVs for something that occurs much later. No PTC will open (respond) fast enough for the typical lightning current (8/20 usec). The term follow-on current applies. A PTC must disconnect a damaged MOV so that a higher energy source with lower voltages (AC electric) does not flow through the MOV - does not threaten human life.
     
  7. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
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    Thank you so much for the detailed and helpful answer!

    According to the UL1449 standard, you must thermally protect the MOV for it not to melt down due to a continuous lightning strike.

    How do you suggest to protect the MOV if not with a PTC?
     
  8. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    Use what the telephone company uses...gas filled ceramic barrelled arrestors to ground.

    Regards, DPW
     
  9. westom

    Member

    Nov 25, 2009
    52
    5

    Do the numbers. Lightning is not the threat. Lightning is too short - too little energy. From Colin Bayliss "Transmission & Distribution Electrical Engineering":
    > Although lightning strikes have impressive voltage and current values

    > (typically hundreds to thousands of kV and 10-100 kA) the energy
    > content of the discharge is relatively low and most of the damage to
    > power plant is caused by 'power follow-through current'.

    Was that statement directly quoted from UL1449? Or what others told you to believe? Numbers must be provided. I don't see any numbers. Only a subjective claim. Subjective claims are often based in myths. Bayliss defined why your PTC backup system is designed. More numbers to understand it.

    First, lightning surges are microseconds. Fuses take tens of milliseconds to react. Damning numbers. Second, a blocked surge current increases voltage into thousand. How does that PTC block thousands of volts? It does not. Damning numbers.

    Two set of numbers. For a UL1449 claim to be valid, numbers such as these must be provided.

    What is a follow-through current that threatens human life? Current with highest energy. And a current that will not create thousands of volts when blocked. AC electric. Again, numbers that explain the whys.

    Meanwhile, fuses are not for protecting electronics. A fuse is typically for protecting human life after damage has occurred.


    Moving on: that PTC is only backup protection. A lesson we were expected to learn from events such as Challenger. Primary O'rings did not work. No problem. We will depend on the backup system? Of course not. Absolute violation. Or why accusations of murder in the third degree should have existed. A responsible engineer never depends on the backup system. If a primary system does not work, the design is defective - no matter how good the backup system is.

    That PTC is only a backup system. Necessary because the primary design might remotely fail. You have assumed PTC is the primary protection system.

    So. The primary system: MOVs must be properly sized to not fail catastrophically. Only acceptable MOV failure is a 10% voltage degradation MOVs must never fail by shorting as Absolute Maximum Parameters make apparent on datasheet page one. Your design must be sufficient to conduct even a direct lightning strike.

    That is the primary protection system. PTC is only a backup system. Never depend only on the backup system - as Challenger should have taught everyone.
     
  10. westom

    Member

    Nov 25, 2009
    52
    5
    Better telco protectors are now semiconductor based. GDTs suffer from degradation. MOVs are not used on telco lines - excessive capacitance.
     
  11. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    Thank you very much again.

    So first i need to pick up a MOV that its voltage rating is 15% over ~250V, and that he will be able to stand a large amount of current - 3KA is considered a good rating, isnt it?

    Only then, i need to pick up a PTC as a backup system.

    Could you please specify the ratings that the MOV and PTC should have?
     
  12. westom

    Member

    Nov 25, 2009
    52
    5

    Typical lightning strike is 20,000 amps. If constructing effective protection for appliances, your protector must be higher. If constructing for surges on signaling wires, that protector may need not be so robust. Also critical is the function of that wire. For example, if a telco wire, then the protector must not be conductive also at radio frequencies.

    For 240 VAC service, a protector is typically 'rated' to be conductive at 400 volts or higher. Conductive is a term better defined from V-I charts. Not from a table of numbers. For example, non-conductive to some is an MOV that conducts 1 ma at 400 volts. For others, that is too much 'near zero' current.

    One way to avoid the 'art' of matching a PTC with the varistor is to purchase an MOV with the PTC already inside; such as TMOV or iTMOV varistors from Littelfuse.com.

    Never forget one important point. Varistors are not protection. From Telebytes primer on surge protection Section 6.4:
    > Conceptually, lightning protection devices are switches to ground. Once a threatening

    > surge is detected, a lightning protection device grounds the incoming signal
    > connection point of the equipment being protected. Thus, redirecting the
    > threatening surge on a path-of-least resistance (impedance) to ground where
    > it is absorbed.

    Why make a protector with higher joules? Because then it absorbs even less energy. That what a better protector circuit does: absorb less energy.
     
  13. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    Thank you very much :)

    I found 2 MOVs (MOV#1 on Mouser and MOV #2 on Digikey).

    What do you think on them please?

    Is their clamping voltage too high?
    I couldnt find any 275Vac rated TMOVs with a clamping voltage lower than 700Vac.

    [​IMG]
     
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  14. westom

    Member

    Nov 25, 2009
    52
    5
    All MOVs conduct current at most every voltage. You must define what current is close enough to zero. MOV leakage current will be what when 240 VAC peaks at its maximum normal peak voltage (375 volts). (240 volt means 265 VAC is also a normal voltage.)

    Do you want it to leak 1 ma? Or 0.1 ma? Obviously, other operational voltages will increase if you select a lower leakage current. I do not have those V-I charts handy. And tables are too subjective. Only good for a ballpark selection. You must select from datasheets leakage currents and other voltages for higher currents based upon what you want to accomplish.

    From V-I charts, that 275 volt MOV may be 900 volts during a large surge current. Or is 1200 volts acceptable during a large surge current? Use curves from charts; not from ballpark tables. Consider that V-I graph when selecting how much current it will conduct at 400 volts and during surges. Those tradeoffs are a judgment call that only you can make. And another part of also selecting how many MOVs must be paralleled.

    A voltage too low (and the higher leakage current at 375 v) will decrease MOV life expectancy. Another factor to add to your judgment call.

    Eventually a more popular MOV will tend to be the one you have selected. IOW how to be comfortable with your design selection.
     
  15. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    Thank you for your great help :)

    Regarding the leakage current, i can handle 1mA when the MOV is off (not conducting).

    The main problem here is the high clamping voltage it has - over 700V.
    I know that its recommended to have a clmaping voltage that is ~1.4 over the nominal voltage (rule of thumb).

    The problem is that i cant find any 275V (nominal) rated MOV with a clamping voltage of around 350V-400V.

    You see the problem?
     
  16. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    CKE (www.deantechnology.com) makes varistors with this clamping voltage; 275 VAC & 350 VDC, clamping voltage 387-473. There are several sizes based on the 2mS Joule energy figure. Maybe someone at CKE can explain what 8/20 usec means? Why don't they just say 0.4 microseconds. You probably should ask for some samples and make your own evaluations.
    ps: I don't think I have ever asked for 8/20 ths. of something in my entire life!

    Regards, DPW [ Always remember that you are not going to live forever.]
     
    Last edited: Dec 14, 2009
  17. westom

    Member

    Nov 25, 2009
    52
    5

    Welcome to what is popular myth verses the reality of surge protection. Viewing a V-I chart from some varistor manufacturer. Their 275 volt MOV is almost 500 volts at 100 ma and something like 1300 volts at maximum current. That is how surge protectors work - except where others did not read V-I charts from manufacturers.

    This is not a problem. Any 240 VAC appliance (even according to 1970 international standards) had to withstand over 1000 volts without damage. 120 VAC appliances had to withstand 600 volts. Electronics today are even more robust.

    So yes, those numbers are misrepresented when myths are promoted. That 275 volt MOV would be inside a protector with a listed let-through voltage of 400 volts. You are correct. A surge of 100 ma is not destructive to any appliance. When a surge current becomes large enough to be destructive, the protector is at voltages well above 500 volts.

    BTW, this is also why effective protectors are located as close to earth ground as possible so that separation between the protector and appliance contributes to protection.
     
  18. westom

    Member

    Nov 25, 2009
    52
    5
    Read all of those specs to appreciate what alphacat is discussing. That 400 volts is when that MOV conducts a 'near zero' 1 ma. When conducting a trivial surge current, its voltage is about 700 volts. When conducting currents it is rated for, that same 275 or 400 volt MOV has a voltage well above 1000 volts.
     
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