Comparison of TMOV and "normal" MOV - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: July 09, 2021, 05:54:44 am
content: [EDIT: Title now corrected thanks to Berni]Can anyone compare the lifetime expectancy of these two (otherwise pretty equivalent) MOVs?One is a "TMOV", and is said to go open circuit when its lifetime has expired, instead of overheating and blowing the fuse as a "normal" MOV does.Surely the TMOV will just have a shorter lifetime if exposed to the same transients as the "Normal" MOV?....because its internal "fuse" is likely to nuisance trip on exposure to transients?Or would the TMOV always mean a longer lasting product?1......B72214S0351K101 350V MOV ("Normal" MOV)https://www.tdk-electronics.tdk.com/inf/70/db/var/SIOV_Leaded_StandarD.pdf?ref_disty=digikey2......TMOV14RP385E 385V TMOVhttps://www.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_tmov_itmov_datasheet.pdf.pdf

author: Berni
author_rank: Super Contributor
timestamp: July 09, 2021, 06:35:06 am
content: Your thread title appears to be wrong btw.The problem with MOVs is that after enough surges and old age deterioration they can wear out. When this happens they can start conducting at a lower voltage, if this voltage becomes lower than the mains voltage then current will start flowing trough it making it get very hot and potentially catch fire.For this reason certain configurations of MOVs need a fuse to protect a worn out MOV from catching fire by disconnecting it when it does fail. So these TMOVs are meant to be more convenient by integrating this fuse that you would otherwise need to buy separately and place on the PCB in series with a regular MOV.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 06:45:39 pm
content: Thanks, is there a downside to TMOVs?...like they nuisance trip there internal fuse...or are they really just the same as a "normal" MOV but they mean your product doesnt die when the MOV dies?

author: Berni
author_rank: Super Contributor
timestamp: July 11, 2021, 08:10:56 pm
content: Well they are probably more expensive than a regular MOV.Since a MOV is in parallel with the supply doesn't mean that your product will die if the MOV fails. It will keep working even with a failed MOV. The point of the fuse in series is more of a safety concern to prevent a failed MOV from going conductive, sinking a bunch of power into itself before spectacularly bursting into flame and a shower of sparks that might set the product on fire, resulting in potentially significant damage to property and a lawsuit.The only time such a fuse would blow is when a very large overload happens and the MOV has to sink a large amount of current, with the size of the small MOVs typically used in products this often means the MOV is dead after such a large surge anyway. So there is no protection anymore. For this reason extension cords with built in "lighting protection" often have a indicator light to indicate if this fuse is still ok

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2021, 08:11:03 pm
content: I was going to comment in your other thread about your MOV choice.Thermally protected MOVs prevent fires caused by overly abundant MOVs that often don't offer much protection in the first place. A product 'dying' isn't as bad as one that goes up in flames and burns down the house. A TMOV that disconnects itself simply leaves that device unprotected to be damaged by the next transient instead of shorting and burning. Until someone invents a replaceable module that also disconnects the device upon failure, MOVs will be a limited solution to transient protection.The best way for a device to be protected against an overvoltage transient is for it to simply withstand it without resorting to shunt absorption methods. Then you can back that up with a MOV or TVS that shunts extreme transients. In the case of your device, the problem is the lack of space between the operating voltage (~325pkand the inherent withstand voltage (the 450VDC rating of the capacitor, among other things). Your selected 385VAC rated TMOV will allow transients way above 450V before it clamps. A lower voltage MOV that would clamp at 450V would leak too much at normal operating voltages. Here in the US, if you want to make a transient-robust non-PFC SMPS, you use a 400VDC capacitor and a 170VDC bus voltage. You can find a MOV that fits between those.A TMOV is a good, safe choice--but don't' worry about its lifetime. It is a sacrificial protection device and its life will depend on how much abuse it is subject to. When it has had enough, it will simply no longer offer protection. Just make sure that it actually offers protection--IOW, your device won't be damaged before it clamps. It would be pointless to have the TMOV clamp after the capacitor explodes.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 11, 2021, 11:13:03 pm
content: For switchboard applications, plug-in fused MOV assemblies with indicators and monitor sensing are available. Obviously not applicable for pcb integration.The aspect that should receive more attention is maintenance procedures for testing MOV's, especially those that don't have an in-line fuse with a visual indicator, or have internally degraded or gone open-circuit without visual indication. An insulation resistance tester is afaik the most applicable test tool to use if the MOV model is known and the datasheet is available.

author: MarkF
author_rank: Super Contributor
timestamp: July 12, 2021, 12:37:20 am
content: Don't forget about the GMOV's:https://www.bourns.com/docs/product-datasheets/GMOV.pdf