Thanks Dwayne, this is really informing. I do have some questions to ask.
Thanks in advance.

1) Understood. IMHO, TVS may be better than Zener in this case. We may need
another metal film resistor in series as well. Am I right?

2) What is really the difference with TVS and MOV in the low voltage side? 
I was using bi-direction 1.5KE TVS (+fuse + metal film resistor) mostly 
for DC interface cards and MOV for AC version (+fuse) last time for
EN50020(>EN50178). But I do not quite understand the reason since the 
I was told by the standard expert in our headquarter to do so. The 
standards are really dry to read. :)

3) Good to know this since we are also thinking of using polyswitch for
some sensor application (for Protection Class II).

4) Why do we need to use TVS if we can use cheap MOVs?
Actually for low voltage sensor application, we only use zener and varistors
(MOVs). But there must be some advantages of 

5) Okay.

6) How do you know that the transformer is already class 2 rated? Someone
also tells me that it is possible to use 0207 SMD metal film resistor in
place of a fuse to achieve Protection Class II. Is this correct?

Xiaofan

> 
> Date: Tue, 30 Nov 2004 11:08:26 -0700
> From: Dwayne Reid <dwayner@planet.eon.net>
> 
> I have a couple of suggestions:
> 
> 1) move the zener (TVS) to after the bridge.  Now you only 
> need 1 instead of 2.
> 
> 2) consider the use of a MOV instead of the zener or TVS - 
> works about as well in this application but costs much less.
> 
> 3) Polyswitch is OK as a fuse replacement so long as it is 
> not operated hot  or cycled frequently.  They degrade in much the  manner
as 
> MOVs - they get  "soft" or "mushy" if they are tripped often.  By this, I
mean 
> that the  current trip point seems to go lower and the "closed" 
> resistance seems to  increase.
> 
> In other words, they are great as an emergency protect device 
> that should  never be called upon to trip more than once or twice over the

> lifetime of  the product.  Don't use them if they will be tripped
frequently.
> 
> 4) note that polyswitches have a MUCH longer time constant 
> than fuses.  You  will have to use a larger (more power) voltage clamp if
you 
> want it to  survive a serious over-voltage event.  In other words, if you 
> are clamping  transients above say 35 Vac input nominal, a 48 or 60 Vac
input will 
> probably cause the zeners to overheat and short before the 
> polyswitch has a  chance to open.  You have to go to a much larger device
to 
> ensure that it  can take the dissipation.
> 
> One more reason to consider a MOV instead of a TVS - the cost 
> of a small  TVS 1.5KE device pays for a huge MOV of similar voltage
rating.
> 
> 5) another thought regarding MOVs - they can go before the 
> bridge since  they are inherently bi-directional.  You can also get 
> bi-directional TVS's  as well but they cost more than uni-directional
ones.
> 
> 6) finally - do you even need a fuse on the low voltage side 
> of things?  If  your transformer is already class 2 rated, you might not
even 
> need the fuse.
> 
> We build a lot of product for the HVAC market.  We've chosen 
> to stay with  2A slow-blow fuses on those cards:
> 1) non class-2 transformers cost less than those that are class-2 rated
> 2) a wiring mistake blows the fuse instead of killing the transformer
> 3) the cost of a tiny 35V MOV and fuse clips and fuse is less 
> than the cost of a suitable polyswitch and huge 35V MOV
> 
> However, most of our industrial stuff uses polyswitches 
> instead of fuses on  the low voltage side of things.  We use them by the
thousands and are 
> darned happy with the results.
> 
> dwayne
> 
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