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TM123
GREAT-Terry
How about aiding the whole system with active clamp circuit like LTC4366? By ...
Surge protection—Stop fried electronics
Mike Fahrion, B&B Electronics
8/8/2012 1:57 PM EDT
This ground connection is crucial for proper operation, providing a shunt path for excess energy as well as a ground reference at the same potential as the host. In most cases, this means the ground connection should be made from the surge suppressor directly to the chassis of the host device. The impedance of the ground connection is critical. The voltage presented to the data port is equal to the clamping voltage of the surge suppression device plus the voltage drop in the suppressor's ground path to the node being protected. Any voltage drop in the ground connection will effectively increase the clamping voltage seen at the data port. Transient currents can be very large, with magnitudes measured in thousands of amps. At these current levels, the DC voltage drop (I X R) can be very large.
For example, 6 feet of 18AWG wire has approximately .039 Ω resistance. Although this initially appears to be a good ground connection, calculating I X R with a 3000 amp transient yields a voltage of 117 volts across the ground wire – enough to destroy any data port. To make matters worse, at the frequencies illustrated in Figures 1-3, the inductance of the ground wire causes an additional voltage drop that can be much higher than the I X R voltage drop. To minimize this voltage drop the ground connection should be made with heavy gauge wire and kept as short as possible. If the cable must be longer than one meter, braided cable intended for grounding purposes must be used.
Selecting a Surge Suppression Device
Two basic types of surge suppression products are available as illustrated in Figure 4. In either case, the system designer should consider the clamping voltage of the unit as well as its physical attributes, such as connector type and method of making the grounding connection.
Single Stage Devices
The most common device uses a single TVS or MOV for each protected line. This type of unit is usually small and inexpensive. If a proper ground connection is made, they should offer protection against most transients. A disadvantage of this device is that if a large transient damages one or more of the components there often is no indication that the unit has failed, leaving the node unprotected against future transients.
Three Stage Devices
More advanced units use three components on each protected line to handle much larger surge currents and to provide internal self protection, reducing the risk of undetected failures. The first stage is a gas discharge tube; this stage can shunt very large currents, but is slow reacting and requires a relatively large voltage before conduction begins. The second stage is series impedance; this stage limits the current flowing into the final stage of the circuit. Finally, a TVS device clamps at a voltage acceptable for the data port and maintains the clamp until the gas discharge tube begins conduction.
Conclusion
Protecting a system against transients with surge suppression requires attention to the selection as well as the installation of the suppression device. Any surge suppressor can be rendered ineffective if proper grounding techniques are not carefully followed. Confronting these issues early in the system design can reduce potentially time and money consuming problems before they occur.
About the Author
Mike Fahrion, director of product management at B&B Electronics is an expert in data communications with 20 years of design and application experience. He oversees development of the company’s rugged M2M connectivity solutions for wireless and wired networks based on serial, Ethernet, wireless and USB communication technology. Fahrion has particular experience in reliable connectivity solutions for devices deployed at the “edge” of networks in remote, harsh or uncontrolled environments. He is a speaker and widely published author. He holds a BSEE from Iowa State University.
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jonnydoin
8/9/2012 5:23 AM EDT
The article mentions 8/20 surge pulses as 8ms/20ms. That's wrong. The time scale is in microseconds. A pulse of 8kV with 20ms decay is a *very* large energy. TVSes are capable of absorbing power up to 400~5000W, depending on the model, but a 20ms pulse would require a much larger device.
- Jonny
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Mike Fahrion
8/9/2012 12:05 PM EDT
Great catch Jonny - the mu symbol in the text got lost in the web translation. We'll get that corrected.
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cmathas
8/9/2012 12:48 PM EDT
Sorry, I didn't catch the change in the translation. It is fixed now. Thanks for letting us know.
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David Ashton
8/12/2012 8:10 PM EDT
While you're fixing stuff Carolyn, on page 2 2nd para is: "6 feet of 18AWG wire has approximately .039 W resistance." W should of course be the ohms symbol - which is Capital W in Symbol Font.
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TonyDave
8/10/2012 4:24 AM EDT
Hi,
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.
Stokersson
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EREBUS
8/11/2012 4:59 PM EDT
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.
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I_B_GREEN
8/12/2012 5:45 PM EDT
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.
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TM123
8/12/2012 6:18 PM EDT
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.
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David Ashton
8/12/2012 8:06 PM EDT
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.
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GREAT-Terry
8/14/2012 4:54 AM EDT
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,
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TM123
8/15/2012 6:15 PM EDT
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.
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GREAT-Terry
8/15/2012 11:15 PM EDT
How about aiding the whole system with active clamp circuit like LTC4366? By using this kind of clamping device, the TVS can be smaller I think.
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TM123
8/21/2012 7:59 PM EDT
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!
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