Tuesday Apr. 9, 2013
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In our last lecture we looked at how you could protect structures from lightning strikes.  A direct or nearby lightning strike is likely to induce large transients the signal cables and electrical wiring that connect to sensitive electronics inside a building and even in the electronic circuits themselves. Today, in a relatively short lecture, we will briefly examine some of the ways of protecting sensitive electronics from these overvoltages.  We will only give this topic cursory coverage; it is covered in much greater detail in a book by Ronald B. Standler Protection of Electronic Circuits from Overvoltages recently published by Dover Publications.



Just as in the last lecture we will consider my rental home.  What sort of an over voltage might you expect on the incoming power line?  Uman addresses that question in his book "The Art and Science of Lightning Protection."  Apparently the insulation levels on the AC wiring in a typical home are 5,000 to 10,000 volts.  This includes insulation on the wiring, in wall outlets, lamp fixtures, switches, and that sort of thing.  So this is an upper limit, anything larger than this will breakdown the insulation and short to ground.  In one respect the household wiring itself acts as a Surge Protection Device (SPD also formerly known as a Transient Voltage Surge Supressor).    Transients on power lines are usually much lower, a few 100s of volts.  In the upper 1% of cases overvoltages might range from 300 volts to a few 1000s of volts (the latter coming from a nearby strike).

One of the first protection considerations is the concept of topological shielding.  This consists of nested enclosures that might each offer some degree of shielding.  Incoming transients are reduced at the entrance to each successive enclosure.  The idea is shown on the figure reproduced below (source: "Applications of Advances  in Lightning Research to Lightning Protection," M.A. Uman, Ch. 5 in The Earth's Electrical Environment, National Academy Press, 1986. available online)



In a home the first line of defense could be surge protection outside the home at the circuit breaker box where electrical service arrives at the home.  Once in the home plugging sensitive electronic equipment into a surge protector would offer additional protection.  The electronics equipment itself will often have surge protection installed.

What might the transient protection look like.  In the most general terms it will probably consist of a blocking impedance followed by a shunt impedance.



The blocking impedance should present a high impedance to the transient signal to prevent it from reaching the electronics.  The shunt has a low impedance and will divert the transient signal to ground.

For the signal of interest, Z1 should appear as a low impedance.  You might use an inductor if the incoming signal is low frequency (60 Hz power for example) and you want to block high frequency transients.  If the incoming signal is high frequency, a capacitor would block low frequency transients.

The shunt resistance should appear as a high impedance for the signal of interest, a capacitor if the incoming signal is low frequency.

I have heard it is a good idea to provide an alternate path to an incoming signal that encounters a blocking impedance (though the blocking impedance may also cause the transient signal to be reflected rather than diverted).  I.e. something like the following:



The first component is often a gas discharge tube.



A gas discharge tube or "spark gap" is an example of a "crowbar" device.  It creates a short circuit (like putting a crowbar across the signal leads) once it exceeds a certain voltage threshold.  The figure below (from Uman's lightning protection book) shows the operating characteristics of a typical gas discharge tube.

The spark gap depicted here quickly becomes conducting once a voltage threshold of about 600 volts is crossed.  Spark gaps can carry large currents and are bipolar.  They turn on relatively slowly however (~1 μs).  Note that once created the arc discharge can be maintained even at low current levels.  A device like this is sometimes difficult to "turn off."

The role of the gas discharge tube is not to carry or divert all of the transient signal to ground.  Rather the transient signal is reflected back in the direction it came from.  A short discussion of reflection at the end of a short-circuited line and a line with infinite impedance (open circuit) has been added to the end of today's notes. 

MOV in the figure stands for metal oxide varistor.  A varistor is a voltage-controlled resistor.  The operating characteristics are shown below (again from Uman's book).  Varistors are clamping devices which means they hold or limit the voltage to a particular value.


In the figure above, the varistor becomes active when the voltage across it reaches perhaps 180 volts (prior to that it has a large impedance).  The voltage is then held at about that value until the current through the varistor reaches perhaps 100 A (currents higher than that would presumably destroy the device).  A 200 v clamping voltage would be appropriate for a 110 volt AC power line.  MOVs turn on very quickly (nanoseconds) and are bipolar.  They do however have a relatively high capacitance and are not able to divert overvoltages for a sustained period of time.

The MOV could be followed by another shunt impedance such as a zener diode which would clamp the incoming signal to an even lower voltage.  This is sketched below.  Each device adds some additional attenuation of the transient signal.


The figure below contrasts the operation of an ordinary diode and a zener diode.


The left figure shows a conventional diode.  The diode begins to conduct when the forward bias voltage reaches about 0.65 volts.  Very little current flows through the diode when back biased.  The zener diode at right operates in the same way when forward biased.   When back biased the diode doesn't conduct much current until the voltage reaches a breakdown or zener voltage (Vz).  Then voltage will be maintained at Vz over a wide range of currents.  In addition to use as a surge protective device, zener diodes are also used as voltage regulators.  Note that a zener diode is not a bipolar device, it offers surge protection against only one polarity of overvoltage.

Many of the devices we have been discussing can be found inside a commercial surge protector (in this case an Inmac 8215 Surge Protector)






A gas discharge tube can be seen near the center left edge of the printed circuit board (white ceramic case).  A total of 6 MOVs can be seen.  They are green, circular, and might otherwise be mistaken for capacitors.  There is room for two inductors on the circuit board (the two circles printed on the right side of the circuit board).  The inductors are apparently not included in this model.


Finally a few notes from the chapter on medical issues and personal lightning safety in Uman's lightning protection book.

Lightning deaths and injuries are probably underreported.  There are about 100 people killed in the US every year by lightning and 1000 people are injured.  This is more than for any other storm related phenomena except for floods.

The primary causes of death from lightning are cardiopulmonary arrest and damage to the central nervous system.  I had always thought that someone struck by lightning would be seriously burned (internally and externally).  This is apparently not the case, the duration of the lightning current is too short.  Sometimes a person's clothing catches on fire, however, and that can cause serious burns.

A lightning strike can cause eye damage and hearing loss (one or both eardrums is(are) often ruptured).  Psychological effects (anxiety, fatigue, chronic headaches or other pain, personality changes and depression) are apparently a significant and long lasting result of a lightning strike.

Telephone injury is the most common type of indoors injury associated with lightning.


I would encourage you to read "Updated Recommendations for Lightning Safety - 1998" by R.L. Holle, R.E. Lopez and C. Zimmerman if you ever find yourself in a situation where you might be responsible for providing lightning safety recommendations or warning to a group of people.

You should also be aware of the National Lightning Safety Institute , an organization that is dedicated to providing accurate lightning safety information and interested in lightning safety education.

Lastly I'll mention the The 30/30 RuleBasically if there is less than 30 seconds between a lightning flash and the sound of the thunder, that lightning discharge is close enough to present a risk to you.  You should be under cover.  You should wait 30 minutes after the last lightning discharge from a thunderstorm before concluding that the storm no longer presents a lightning hazard to you.



We had time at the end of the class period to begin a short section on lightning spectroscopy.  That has been moved over to the Apr. 11 notes so that it will all be together in one place.



Here's a short look at what happens to a voltage pulse when it arrives at the end of a transmission line.  We look first at the case of an open circuit (infinite terminating impedance) at the end of the line.



The top of the figure shows an incoming voltage pulse with amplitude Vi arriving at the end of the line.  The current associated with this pulse is Vi/Zo where Zo is the characteristic impedance of the line. 

Because of the infinite terminating impedance, there can be no current flow across the end of the line.  A reflected voltage pulse, Vr,  with the same polarity and amplitude as in the incoming pulse is produced.  The reflected current is -Vi/Zo.

Thus at the open end of the line you would measure a voltage V equal to twice the amplitude of the incoming pulse (the superposition of the incoming and reflected pulses) and zero current.



This figure shows the same voltage pulse, Vi, arriving at the end of a shorted line.  Now the requirement is that the voltage at the end of the line be zero.  The reflected voltage pulse has opposite polarity but the same amplitude as the incoming pulse.  The reflected current is equal to the incoming current.

The measured voltage at the end of the line is zero.  The current at the end of the line has an amplitude that is twice the incoming current.