From: rherber1@bigpond.net.au   
      
   On Thu, 18 Oct 2007 23:02:34 GMT, "Farticus"    
   wrote:   
      
   >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.   
   >   
      
   It seems to me that your customer is trying to be a cheap-skate and   
   doesn't understand that trying to do what he wants means making   
   compromises which could drastically affect the safety of anyone using   
   his cooker.  In the end he will end up paying more for his bodged up   
   job becaue it will either fail or electrocute someone. He would be   
   well advised to simply spend the money and do it properly in   
   accordance with electrical standards.   
      
   Since each of the 2 elements are required to be operated separately as   
   needed it does not make sense to try to do it with a common temerature   
   controller. This would mean that either element would require some   
   means of disconnection when not required. It makes more sense, and I   
   believe that local electrical appliance standards wiring would require   
   it, that each element be controlled from a separate temperature   
   controller. Now that doesn't solve the problem of trying to power the   
   two 120Vac/5A elements from a 240Vac supply and to my mind the best   
   way to do this is to have a 240V to 120V step-down transformer.   
   Unfortunately, this requires a 1500Va continuous transformer and that   
   won't come cheap.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)