Lightning protection circuit

Abstract

A lightning protection circuit including a number of series capacitors and shunt inductors, the capacitors having a breakdown voltage level sufficient to withstand lightning and an equivalent reactance adjusted to keep a voltage across the series capacitors less than their respective breakdown voltage levels.

1 . A lightning protection circuit comprising: 
 a plurality of series capacitors and shunt inductors, wherein the plurality of the capacitors have a total breakdown voltage level sufficient to withstand lightning and have an equivalent reactance adjusted to keep a voltage across the plurality of series capacitors less than their respective breakdown voltage levels.    
   
   
       2 . The lightning protection circuit of  claim 1  wherein the capacitors and inductors comprise a first input impedance to lightning and a second input impedance to a signal having a lower frequency than the lightning, the first input impedance being higher than the second input impedance.  
   
   
       3 . The lightning protection circuit of  claim 2  further comprising a plurality of snubbing diodes disposed across the second input impedance.  
   
   
       4 . A lightning protection circuit for a modem coupled to a twisted pair transmission line comprising: 
 an analog front end circuit coupled to the twisted pair transmission line;    an isolation transformer disposed in the analog front end circuit; and    a high pass filter coupled to the isolation transformer including high breakdown parts, wherein a primary of the transformer is coupled through the high pass filter to each conductor of the twisted pair transmission line and a secondary of the transformer has each terminal coupled to the analog front end.    
   
   
       5 . The lightning protection circuit of  claim 4  wherein the high pass filter comprises: 
 a first high breakdown capacitor having a first terminal coupled to a first terminal of the twisted pair transmission line;    a first inductor having a first terminal coupled to a second terminal of the first capacitor;    a second high breakdown capacitor having a first terminal coupled to a second terminal of the first inductor;    a second inductor having a first terminal coupled to a second terminal of the second capacitor and the analog front end;    a third high breakdown capacitor having a first terminal coupled to a second terminal of the second inductor and the analog front end; and    a fourth high breakdown capacitor having a first terminal coupled to a second terminal of the first inductor and a second terminal coupled to a second terminal of the twisted pair transmission line.    
   
   
       6 . The lightning protection circuit of  claim 5  wherein each of the first, second, third, and fourth capacitor have a capacitance of approximately 0.056 micro farad, each of the first and second inductors have an inductance of approximately 150 micro henries, and the breakdown levels of the first and fourth capacitors are higher than the breakdown levels of the second and third capacitors.  
   
   
       7 . The lightning protection circuit of  claim 4  wherein the isolation transformer is included in a plain old telephone system splitter.  
   
   
       8 . The lightning protection circuit of  claim 4  wherein the analog front end includes a converter configured to convert a differential input signal to a single-ended input signal, and convert a single-ended output signal to a differential output signal  
   
   
       9 . The lightning protection circuit of  claim 4  wherein the converter includes a hybrid splitter.  
   
   
       10 . The lightning protection circuit of  claim 4  wherein the analog front end includes an amplifier having automatic gain control.  
   
   
       11 . The lightning protection circuit of  claim 10  wherein the automatic gain control comprises a variable attenuator configured to attenuate the single-ended input signal.  
   
   
       12 . The lightning protection circuit of  claim 11  wherein the variable attenuator comprises a voltage controlled resistor.  
   
   
       13 . The lightning protection circuit of  claim 12  wherein the voltage controlled resistor comprises a field effect transistor.  
   
   
       14 . A lightning protection circuit comprising: 
 a pair of series capacitors and a shunt inductor, wherein the pair of series capacitors have a total breakdown voltage level sufficient to withstand lightning and have an equivalent reactance adjusted to keep a voltage across the pair of series capacitors less than their respective breakdown voltage levels.    
   
   
       15 . The lightning protection circuit of  claim 14  wherein the shunt inductor is a secondary of an isolation transformer.  
   
   
       16 . The lightning protection circuit of  claim 14  wherein the pair of capacitors and the inductor comprise a first input impedance to lightning and a second input impedance to a signal having a lower frequency than the lightning, the first input impedance being higher than the second input impedance.  
   
   
       17 . The lightning protection circuit of  claim 16  further comprising a plurality of snubbing diodes disposed across the second input impedance.