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Re: what am I really achieving here ?
Posted by Jon Risch on April 4, 2000 at 18:48:29:
As I once designed some surge suppressors professionally, I looked into the whole gamut of surge protection devices. MOV's, TransZorbs (a high peak current Zener type device), large wattage conventional zeners, gas tubes, Triac/Thyristor combo's, etc.
They all have their strengths and weaknesses.
What bothered me the most about the gas tube devices I tested and looked into, was that once they triggered, they tended to send a burst of oscillatory RF energy into the protected component, usually as a result of some filter capacitors used in conjunction with the gas tubes. While this could be minimized by the use of an RC snubber network across the device, it could never be eliminated altogether.
They also usually required a current limiting resistor to avoid self-destruction, as once they triggered into conduction, they were nearly a dead short, and would easily burn themselves up (or melt the leads) without some form of current limiting. The current limit resistor would cause the clamp voltage to go quite a bit higher than any of the other devices, often reaching levels that were high enough to pose a potential for damage of the components hooked up to the surge suppressor.
These results are for medium to large surges or spikes. Smaller short spikes were not nearly as problematic, and this is what the vast majority of AC line disturbances consist of. However, most equipment can handle those fairly well, it is the medium to really big ones that we need the surge protection.
As for MOV's, I found that the mistaken attempt by many to absolutely minimize the let-thru voltages was the main culprit for the wear-out mechanism and the adverse affect on sound systems/components. Use of the 130 VAC rated MOV's was the problem, and use of the slightly higher 150 VAC rated device avoided most of the wear-out and the adverse sonic affects. I briefly bring this up in my DIY AC Power Line Filter & Surge Suppresor note, a section of which I copy below:
For US AC lines :Harris V150LA20A,B or C from $.78 to $1.02
This is a 150 VAC rated part, capable of over 80 joules of transient
energy absorption, and peak currents over 6500 amps. The maximum
rated clamping voltage is 390V vs. 340V for a 130VAC rated part.
This is a minor difference, and the higher voltage point keeps the
MOV from conducting regularly on high line peak voltages, avoiding
the worst of the wearout mechanism that MOV's are prone to, and
preventing the degradation of the sound.
For 240 VAC use, I recommend the V275LA40A,B or C from $.97 to $1.85
which is capable of over 140 joules of transient energy absorption,
and peak currents over 6500 amps. Maximum rated clamping voltage is
710 volts @ 100 amps.
Both are available from Allied or Newark, and the manufacturers part
# is also the stock number.
Siemens and Panasonic also make excellent and reliable varistors,
but beware other brands! There are many cheap and unreliable MOV's
being offered, that can barely take rated surges once! The Harris
and other two recommended brands can take repeated overloads and
abuse and still keep functioning. MOV's generally either fail by
permanently short circuiting or exploding! In severe overload tests,
brands other than GE/Harris, Siemens or Panasonic tended to explode.
Once a MOV fails in the short circuit mode, the circuit breaker will
trip, and will not reset. This indicates the need for replacement
of the device.
END OF EXCERPT
for the full note.
If the MOV is kept from partial conduction as much as possible then the non-linear capacitance is not exhibited as much, and the slight comporession fo the AC waveform peak is avoided too, so the adverse sonic affects are minimal. The better brands are also more conssitent in their trigger voltage, and a lot more capable of handling overloads and surviving, as well as resisting the wear-out mechanism better.
I have had 150 VAC rated Seimens and Panasonic MOV's operating for over 15 years continuous on the AC line with no signs of degradation or wear-out. I did loose one whole unit to a near miss by lightning, and had to replace the front end MOV's, but the suppressor did it's job and I had no damage to any of my video components. Later I found out that three of my neighbors lost their TV's, and several other appliances that night!
The most common mistake made with any surge suppression device is allowing the device connection loop area to get large, as this causes excessive surge clamping device inductance, and limits the devices ability to respond to fast spikes or surges, and also allows higher let-thru voltages to occur. The leads need to be short and close to the entry point of the AC power, with either low inductance layout traces or twisted leads up to the point of device connection.