There is a bit more to it than just shunting off surges/spikes.
The article has addressed the Ohmic requirements, but what about capacitive and inductive effects.
Depending on cable lengths, there will be some considerable capacitive coupling between conductors for the duration of the surge/spike.
Also multiple conductors (presumably in the same cable/conduit/duct) will suffer from inductive coupling similar to a transformer - after all we are dealing with a changing Voltage/Current waveform i.e. ac - but only for restricted cycles (hopefully only one).
Some care also needs to be taken to prevent these effect through the use of shielded cables and earthed at both ends, to transform a simple shielded cable into a faraday cage in an effort to reduce these other effects.
This raises a can of worms because we now have the probability of earth loops. To protect we must make some compromises.
I know I would rather deal with noise reduction rather than replace everything because it has been fried!
GREAT-Terry, A Bidirectional TVS is matched to provide performance in both directions - equally. A pair of Unidirectional TVS will provide protection but will not necessarily be matched to balance the intrusion levels either side of your supply or common rails.
David Ashton, fair comment on the MOV front - our particular application is inside a pressure vessel and it takes a bit over a day to pump out the gas (SF6), purge and put in walkways to access the electronics. MOVs do work but are not appropriate in our situation.
Good. Is there any difference between putting 2 unidirectional TVS and a single bidirectional TVS? I usually use 2 uniderectional but frankly have no idea if a single bidirectional has poorer. Agree also to add air core inductor in series with the resistor,
Point taken about MOVs, but they are capable of absorbing a LOT of energy, even if they fail shorted. I used to live in Zimbabwe, which has bad lightning during the summer. A company I dealt with there used big MOVs in their mains surge arrestors and they worked a treat. Better to replace a $ 35 mains surge protector than a TV/Video installation.
Same company did some tests and found that a knot in the 3-core mains supply cable provided a lot of protection as well - added inductance. I used to knot all my cables after that and never had a problem with my mains inputs.
I have found that allowing the entire voltage supply (including 0 Volts) to float enables the circuit to float around any surge/spike.
In this case, there are a number of capacitors connecting various voltages, inputs and outputs to the real ground.
A device I use lots of is the good old Pi Filter (or capacitor input filter) on every power supply rail, input and output, along with extra series inductors - everywhere.
To date, I have protected uProcessors, and low voltage electronics (5 Volt, 12 Volt) successfully against voltage discharges around the 8 MV mark (we have electronics inside an Ion Accelerator).
Warning - do not use MOVs as when they fail, they fail short which will require replacement. Use protective devices that are self recovering so the protective components do not become part of the problem.
Only thing missing is an inductor in place of or in series with the resistor. Allows for much more energy withstand. fulcrums energy from (referenced in voltage)shunt protection downstream to GDT via inductor voltage rise due to di/dt. Care must be taken to not saturate core, air gap recommended.
Personnally, I like my circuits fried with a touch of tobasco sauce.
Seriously, you need to put protection on all of your inputs or pay the consequences. Regardless of surge or spike, over voltage destroys semiconductors in femtoseconds.
Anyone remember the old UV erasable proms? I could destroy one from five feet away just from the static charge I would build up walking over the tile floor.
Zaap! It's done.
Surge? "An uninterrupted voltage increase that lasts more than a few seconds is usually called a "voltage surge" rather than a spike."
Nice article about Transient protection, what about the surges. Try 150% for 1/2S (with a profile). Now thats a surge.