From: chris150@cox.net   
      
   Hi all and sorry if this is an elementary level question.   
      
      
      
   About 5 years ago I worked in an IT dept. of a school dist. and all the   
   switch rooms at each school used a battery backup system that included a   
   unit which, in the case of a power failure, would pull DC from battery banks   
   and convert it to 120 ac to keep the equipment running I think they called   
   it a ferro type inverter. The unit, not including the batteries, was as   
   large as a fridge.   
      
   I looked at the schematic of this unit and noticed that on one side of the   
   circuit it used 4 SCRs (two in parallel but opposite direction of each other   
   for each line) and on the other end of the circuit it used 2 triacs. Sorry I   
   don't remember which was used for the ac input side and which was used for   
   the dc to ac backup output side.   
      
      
      
   My question is this...   
      
   Why not use SCRs on both sides or use Triacs on both sides?   
      
      
      
   Keep in mind that my level of electronic knowledge is about at a first year   
   electronics student level. I am able to read a schematic and understand the   
   basic idea of what each component in a circuit does and can usually figure   
   out what each part of a schematic is for (which is the tank circuit,   
   rectifier circuit, current limiter circuit, etc.) but the details of each   
   are still above my level of thought. All my knowledge is learn-as-you-go so   
   please keep that in mind.   
      
      
      
   Oh, just though of this, would the 50 pounds or so of caps and coils in the   
   tank circuits main purpose have been to try to generate a 60 cycle   
   sinusoidal output since this circuit looked like it was only in action when   
   power was comming from the battery pack?   
      
      
      
   Sorry I don't have more information but I no longer have access to the   
   schematics.   
      
      
      
   Thanks in advance.   
      
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