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   "Ross Herbert"  wrote in message   
   news:ipt0i35hctnqttdh4fa3j53krsokrjvj26@4ax.com...   
   > 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.   
      
   I couldn't agree more!   
      
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