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== Signal Input Protection ==
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== signal input protection ==
  
 
Alas, people outside the electronics world live in places that have "low humidity" and "carpeted floors".
 
Alas, people outside the electronics world live in places that have "low humidity" and "carpeted floors".
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A variety of signal input protection circuits:
 
A variety of signal input protection circuits:
  
* resistor(s)
 
  
* zener diodes
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resistor
  
* 2 clamping diodes to plus and minus power rails
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zener
  
* 2 Schottky clamping diodes to plus and minus power rails
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2 clamping diodes to plus and minus power rails
  
* 2 Schottky clamping diodes to plus and minus power rails, and a resistor. (All 3 components directly connected to the microcontroller input pin; the other end of the resistor connected to the outside world).
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2 Schottky clamping diodes to plus and minus power rails
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2 Schottky clamping diodes to plus and minus power rails, and a resistor. (All 3 components directly connected to the microcontroller input pin; the other end of the resistor connected to the outside world).
 
[http://www.piclist.com/techref/mimedecode.asp?url=piclist%5C2006%5C03%5C06%5C111156a%2Etxt&part=3]
 
[http://www.piclist.com/techref/mimedecode.asp?url=piclist%5C2006%5C03%5C06%5C111156a%2Etxt&part=3]
  
* transistor buffer ([http://www.piclist.com/techref/postbot.asp?by=time&id=piclist\2007\03\22\101919a Matt Pobursky])
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transistor buffer ([http://www.piclist.com/techref/postbot.asp?by=time&id=piclist\2007\03\22\101919a Matt Pobursky])
  
 
...
 
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[[Reverse Protection Diodes]] (protects against reversed GND and +power)
 
[[Reverse Protection Diodes]] (protects against reversed GND and +power)
 
rectifier -- not only protects against reversed GND and +power, but also allows the device to work either way.
 
  
 
fuses
 
fuses
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... or some combination.
 
... or some combination.
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[http://sound.westhost.com/impedanc.htm#spkz "Speaker Protection Systems"]
  
  
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-- Tjaart van der Walt 1999
 
-- Tjaart van der Walt 1999
 
[http://www.piclist.com/techref/postbot.asp?by=time&id=piclist\1999\05\01\151255a.txt]
 
[http://www.piclist.com/techref/postbot.asp?by=time&id=piclist\1999\05\01\151255a.txt]
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"DESIGNING ELECTRONIC EQUIPMENT FOR ESD IMMUNITY: Annotated Bibliography
 
"DESIGNING ELECTRONIC EQUIPMENT FOR ESD IMMUNITY: Annotated Bibliography
 
http://www.dbicorporation.com/esd-anno.htm
 
http://www.dbicorporation.com/esd-anno.htm
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"AP-125 Designing Microcontroller Systems for Electrically Noisy Environments"
 
"AP-125 Designing Microcontroller Systems for Electrically Noisy Environments"
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----
 
----
 
=== Positive Temperature Coefficient fuses (PTC Fuse) ===
 
 
Also known as '''Polymeric Positive Temperature Coefficient fuses (PPTC fuse)'''. Brand names include '''PolySwitch''', '''Everfuse''', '''Polyfuse''', and '''Multifuse'''.
 
 
PTC fuses may be used with SCRs in crowbar circuits for overvoltage protection.
 
  
 
Russell McMahon says
 
Russell McMahon says
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* 4 page overview and selector guide http://www.circuitprotection.com/06Databook/PSWFundamentals(80-83).pdf
 
* 4 page overview and selector guide http://www.circuitprotection.com/06Databook/PSWFundamentals(80-83).pdf
  
Be careful with these devices, they have a long term failure mechanism which means that after some cycles they do not return to their low impedance state. I cycled a device for 1.1 million cycles and all seemed well. However we still get devices back from the field that have failed (6K at room temp rather than 10 ohms). I expect that some are more susceptible.
 
 
== Input Protection by Circuit Type ==
 
 
( next largely stubs, some covered by material above )
 
 
=== Blocking Diode ===
 
 
=== Zener Diode ===
 
 
=== Crowbar Circuits ===
 
 
Not sure where the term comes from, in some cases it is like throwing a crowbar in a machine, perhaps that is it.  Generally a crowbar circuit detects a fault and responds by creating a short of the input voltage.  In some cases removal of the input will reset the circuit, in other cases it may go on to blow fuses. 
 
 
[http://en.wikipedia.org/wiki/Crowbar_circuit Crowbar (circuit) From Wikipedia, the free encyclopedia]
 
 
=== Filters ===
 
 
=== Buffer Amplifiers ===
 
 
 
== Input Protection by what is being Protected ==
 
 
=== Analog to Digital Converter Input Protection ===
 
 
Input protection for ADC has its own set of issues.  The first in input voltage.  Often this needs to be within the supply limits of the ADC itself.  The ADC may have protection diodes to V+ and Gnd but excess voltage can easily push currents into these diodes that will damage or destroy the chip.  First level protection can be as simple as a diode.  You can then assume some over voltage on the chip and see what current will result.  Some references suggest that a few ma should be ok, check you data sheet perhaps it will have some info.  Raise the resistance and get more protection, or use two resistors and add your own diodes after the first diode.  A problem with added resistance is that it can effect the accuracy of you converter, so once again check the data sheet.  To avoid the problem of input resistance you can use a buffer amplifier.  Protect the amplifier, if necessary, with its own input protection then use its output to drive the ADC.  If the supply to the amplifier is within the limits of the permissible input to the ADC then it is unlikely that it will overdrive the ADC.  A simple amplifier might be an emitter follower.  I prefer an operational amplifier.  With any amplifier, but particularly most op amps there may be a problem with the output swinging across the whole power supply range, particularly to the positive rail.  A way around this is to use the positive reference pin available on most ADCs ( including those on PIC and other microcontrollers ).  Lower that input to the range of the amplifier where it has low output impedance.  This preserves the full digital range of the ADC.  If you do not do it you should consider not using the top of the range.
 
 
Filtering the high frequency response of an ADC input can also be useful if there are large variations at a frequency higher that you want to, or  the ADC can resolve.
 
 
 
 
 
=== Power Supply Input Protection ===
 
 
=== Speaker Protection ===
 
 
 
=== Semiconductors and Static Discharge ===
 
 
Static discharge can damage a wide range of semiconductors, through inputs or any other connection.  More discussion would be nice.
 
 
[http://sound.westhost.com/impedanc.htm#spkz "Speaker Protection Systems"]
 
  
== Further Reading ==
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== further reading ==
  
* [[Printed Circuit Boards#References]] discusses EMC guidelines
 
 
* [http://www.ce-mag.com/99ARG/Bjorklof&Joha124.html CE magazine: "Immunity Testing: Practical Aspects of Basic Standards"] by Dag Björklöf and Lars-Olov Johansson
 
* [http://www.ce-mag.com/99ARG/Bjorklof&Joha124.html CE magazine: "Immunity Testing: Practical Aspects of Basic Standards"] by Dag Björklöf and Lars-Olov Johansson
* The "super reverse protection diode emulator" idea is detailed in [http://www.geofex.com/Article_Folders/mosswitch/mosswitch.htm "Advanced Power Switching and Polarity Protection for Effects"] and [http://www.irf.com/technical-info/designtp/dt94-8.pdf IRF: "Reverse battery protection with HEXFETs doubles battery life"]; a BJT version that works almost as well is described in [http://www.geofex.com/Article_Folders/cheapgoodprot.htm "A cheap - and good - polarity protector"].
 
* [http://electronics.stackexchange.com/questions/14609/reverse-polarity-protection-on-model-rc-aircraft "Reverse polarity protection on model RC aircraft"] discussion
 

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