Surge life comparison of two different size of same volts varistor

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Willen

Joined Nov 13, 2015
320
Hi, I am getting two varistors with same voltage but different sizes: 1) 07d471k (small 07 series) 2) 20d471k (big 20 series). In the datasheet both has same "surge life time" but double surge current. The smaller varistor has already 50A surge current capability, which means it can blow fuse easily, then why I need to buy expensive 100A (big) surge varistor? Does the 'surge life' matter here? Ok, I am going to use it for a smart TV.
 

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MisterBill2

Joined Jan 23, 2018
10,516
This is an interesting question. In a protection scheme, the varistor conducts when there is an over-voltage, and may stay conducting after the over-voltage spike ends. Thus a fuse to terminate the current is required. Certainly some means of limiting the current when the varistor starts conducting is required, because only source resistance will limit the current.
We do not know the current rating of the fuse at this point, but I will guess that it is much less than the 50 amps rating of that varistor. Fuses do require a finite time to clear and so there will be some current rise until that time passes. Depending on the current draw of the load and the supply voltage variation tolerance of that load, one option that is simple and easy would be to add a resistance of an ohm or a few ohms in series with the source, to limit the current that can flow when the varistor conducts. Another option would be to use an isolation transformer with a suitable power rating before the varistor, to absolutly limit the power that can be delivered to the varistor.
In any case, the 50 amp varistor should protect the load at least once. The 8/20 microsecond rating implies protecting for up to 8 20 microsecond spikes. The problem being that at 60hZ the varistor would probably stay conducting for one cycle, or at least a half cycle, with one cycle being 16 Milliseconds.
So the 50 amp device should last as long as the 100 amp device in this application. But add a small value of series resistor and also the fast fuse. An electronic fast fuse would be an even better option. The correctly sized isolation transformer will certainly limit the energy, it still needs the fuse, though.
 
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