From: localhost@127.0.0.1   
      
   "Baron"  wrote in message   
   news:ffqh7q$tnt$1@registered.motzarella.org...   
   > Ross Herbert wrote:   
   >   
   >> 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'm in general agreement.  Why not simply use 230v elements.  There are   
   > a few companies that will make up whatever size/shape you want.  Then   
   > you can use the "Simmerstat" to do the controlling of temperature.   
   >   
   > --   
   > Best Regards:   
   >                     Baron.   
   Try to convince one of those customers that is "always right" - I agree with   
   your sentiments exactly.   
      
   --- SoupGate-Win32 v1.05   
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