On Wed, 08 Jul 2009 17:34:45 -0700, Mycelium
wrote:

On Wed, 08 Jul 2009 18:30:57 +1000, Tony wrote:



Yes MOVs eventually wear out, but they are cheap and readily available with high ratings.

If the device you are replacing looks like a big diode, it is a
Tranzorb. If it looks like an old TV varistor, it's an MOV, which is
what it is a variant of.

If indeed it looks like a diode, you would be better off sourcing the
suitable Transzorb and doing the service only one time. With the MOVs,
you only get a hundred or so spike suppressions before they are
operationally different than when they were made.

http://search.digikey.com/scripts/Dk...ords=transzorb

Even better:

http://search.digikey.com/scripts/Dk...ords=transzorb

That is the purpose these devices were made for.

We used them all the time across our FETs and IGBTs right at the lead
frame or real close to it on the PCB. They are great for suppressing
polarity switching transients with exacting precision.

The other benefit is that you do not have to go back into the assembly
and keep repeating the process of replacing the MOVs, which would fail in
this application.

Fair enough. But how about apples for apples? the transzorbs in your link are rated at
only 500W. Sure the common MOVs degrade when hit with their rated 100 kW or so, but if you
hit them with only 500 W (like the transzorbs) they will last virtually forever. The
physical volume of active material is simply vastly larger in a MOV than in a transzorb.
Transzorbs are great if used within their ratings, but MOVs can have vastly higher
ratings, and can also provide good lifetime when de-rated in lower energy circuits, so
they shouldn't be dismissed without proper consideration.

Cheers, Tony