From: localhost@127.0.0.1   
      
   .. too expensive for an isolated (not auto-transformer type).   
   Thanks for the suggestion.   
      
      
   "Tim Perry"  wrote in message   
   news:47188d60$0$9537$4c368faf@roadrunner.com...   
   >   
   > "Farticus"  wrote in message   
   > news:JDURi.1622$CN4.620@news-server.bigpond.net.au...   
   >>   
   >> "Farticus"  wrote in message   
   >> news:eCRRi.1529$CN4.1347@news-server.bigpond.net.au...   
   >> >I need to control two 120VAC/5A heating elements off a 240VAC supply.   
   >> > Two elements will be plugged into one control box and ideally one   
   >> > "temperature control" is to be fitted to the box. The elements can be   
   >> > unplugged so that only one element might be operated at any one time.   
   > Cost   
   >> > as always is a factor (as is with most customers!).   
   >> >   
   >> > The customer suggested a cooker type infinitely variable simerstat, as   
   >> > this would be good enough for the type of temp control they require.   
   >> > The only simerstats available locally are 240V and the manufacturer   
   >> > advises that they will not function properly on120V. Then there would   
   >> > be   
   >> > the need for interface relay/contactor to switch the element etc.   
   > etc....   
   >> >   
   >> > My first thought were to use a 240/120 transformer with a thyristor   
   >> > circuit on the 120V output.   
   >> >   
   >> > My second thought was to use a thyristor without a transformer,   
   >> > restricting the the thyristor output to a max of 120V via the thyristor   
   >> > control circuit. The problem with this is that a short-circuited   
   > thyristor   
   >> > would apply 240V to the elements causing problems. A fuse in circuit   
   > yes,   
   >> > however this equipment is to be used in a rough industrial environment,   
   >> > meaning that the operators WILL replace expired fuses with fencing   
   >> > wire!   
   >> >   
   >> > Any ideas, past experiences, circuit suggestions using triacs,   
   >> > back-to-back SCR's or whatever would be appreciated.   
   >> >   
   >> ... having given it a bit more thought, I'm thinking of using a solid   
   > state   
   >> relay triggered by a 555.   
   >> The slower the pulse rate the lower the temp - PWM?   
   >> Any thoughts on this idea?   
   >>   
   >   
   > why not just wire in a VARIAC?.   no design engineering required.   
   >   
   >   
      
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