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Transient V Suppression/MOV's

Banksy

Jun 17, 2020
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Jun 17, 2020
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Hi everyone :D
I'm fitting a new Access Control Panel With a MagLock, keyboard Etc. The manufacturers tell me that the main panel will supply power to my MagLock but that I need to put a Metal Oxide Varistor across the power rails and I'm really struggling with the math (As a Newbie who's also rubbish at theory and math !)

The MagLock draws 420mA @ 12V, the power supply provides 14.3 V at 500mA and the panel Supplies 12V @ 500mA for the MagLock. PLEASE can any one suggest a suitable MOV value? Hopefullly with a part Number/Supplier?

Any help is very much appreciated.

Thanks everyone :D

Banksy.
 

Harald Kapp

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Nov 17, 2011
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Current draw is irrelevant for this application. The MOV is put in parallel to the power supply to protect from overvoltage.
for 12 V and 14.3 V a 15 V (DC rating) MOV should work.
See here for a selection of suitable ones.

Here's some more detail on selecting MOVs.

The manufacturers tell me that ... I need to put a Metal Oxide Varistor across the power rails
Thus the manufacturer saved a few cents at your expense :(
 

Harald Kapp

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Nov 17, 2011
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Plus: if it is not already there add a fuse between the power supply and the MOV. The MOV will become low-resistive when activated (this is the protective function). It will then draw a lot of current, dissipate a lot of power and burn out i current is not interrupted. This is where the fuse comes into play. Of course you'll have to replace teh fuse after such an event, but this is cheaper than replacing the whole board.
 
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