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Old 07-11-2007, 14:22   #31
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Transient Voltage Surge Suppressors (TVSS)
The Metal Oxide Varister (MOV) type of Voltage Dependent Resistor (VDR) is manufactured from zinc oxide fragments, compressed under very high pressure. MOVs have a finite design life, because the zinc oxide particles weaken after conducting a current.
Although there response time is very swift, MOVs have a non-linear resistance characteristic. In practice, this means that when an MOV is subjected to a large transient current surge, such as a lightning strike, the clamping voltage can increase to a level at which equipment damage could occur. Furthermore, longer duration current surges - as distinct from shorter transient peaks - will cause an MOV to destabilize.

Probably, the ideal device to partner a MOV is the Silicon Avalanche Diode (SAD).
SADs are semiconductors that can respond very rapidly to a transient voltage surge, and can be specified with clamping voltages that range from a few volts to several hundred. However, because their clamping voltage needs to be as close as possible to the peak value of the mains supply, it is necessary to use a number of SADs, so that the transient energy can be dissipated without sacrificing the device. This makes SADs more expensive than their MOV counterparts/partners.
However, silicon avalanche diodes will not degrade in use, provided their rated capacity is not exceeded.
A combination of SADs and MOVs provides the best solution to the opposing requirements of rapid response to transients and large total current capacity.
Because the SADs react very quickly, they prevent the clamping voltage of the device rising as more current is dissipated by the MOVs.
Nevertheless, it is advisable to use more than just a few SAD, to eliminate the potential damage caused by over current before the MOVs take effect.
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Old 07-11-2007, 22:50   #32
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I understand the addition of protection devices also making the design more prone to other forms of failure. But in my opinion, it is far easier to replace a fuse and one easy to diagnose component, than to fault find and replace major components elsewhere in the appliance.
Plus, I think I would rather have supression diodes in the Alt fail, rather than have my Alt produce a spike that destroys an expensive electronic device.
When I was TV servicing, we had a particular TV with a P/S that was very complex and expensive and a pain in the neck to diagnose as everything shuts down when a fault develops anywhere in the set. Problem was, the P/S would often fail rather spectacularly leaving the fuse intact. We used to have a running joke that the mega dollar P/S was there to protect the 20c fuse.
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Old 08-11-2007, 19:08   #33
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"But in my opinion, it is far easier to replace a fuse and one easy to diagnose component, than "
Wheels, personally I agree with you but Stateside it typically will cost you $75-100 to bring any electronic device into a shop for an estimate--not free but applied to repair costs. Factor that against the replacement cost fo the device (i.e. a $300 five year old tv) and folks just don't want to know something "easy" blew, all they know is "time to buy a new one".

What can you say.

Last time I had an old 19" Trinitron repaired (about $110 ten years ago) it lasted another year and then something else blew--apparently among other things, electrolytics age and blow much easier than they used to.

And that's far beyond the point of sale. If your diode and varistor bumps your retail price to $199 and theother guy is selling at $195...you know which one sits on the shelf and which one gets the market share. Sadly, the mass market usually gets what it demands: The cheapest possible goods, and don't bother them about anything except logo and style.
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Old 09-11-2007, 01:40   #34
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Sure I understand that. But when we are talking equipment like a multistage charger that costs an arm and a leg, then a few bucks for a protection circuit is not really an issue.
I have been recently pricing chargers for myself. I have been looking in the 80 to 100A range. All I have looked at range from NZ$2K and go up from there. A few bucks for a protection circuit may add to the over all cost, but woudl add little to the over all difference in competition. My choice in the end will likely not be the cheapest , because I want the item that works best and safest. I am happy to pay for that.
Fortunately, a second hand Heart Interface Inverter/charger came up for sale here and I managed to buy it for $750.00 plus some freight. Great buy. I hope it is a great product.
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Old 09-11-2007, 09:53   #35
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Whoever fixed the spelling on the title of this thread, I thank you.

Steve B.
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