From: localhost@127.0.0.1   
      
   "John Fields"  wrote in message   
   news:519ih3dgdj0lg36f01a48e6887ehk8a988@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.   
   >   
   >   
   > From the point of view of cost, I like the idea of a single   
   > phase-controlled TRIAC driving both heaters in parallel since the   
   > pair of heaters, in parallel, will only need 10A from the mains and,   
   > with proper timing, the loads will never see more than 120V.   
   >   
   > When one is unplugged, of course, the TRIAC need only pass 5A.   
   > Even better.   
   >   
   > But what's a simerstat?  Is it a device with feedback which will   
   > maintain a temperature once set, or is it like an open-loop lamp   
   > dimmer?   
   >   
   >   
   > --   
   > JF   
   A simerstat is the term given by electrical wholesalers for a control   
   typically used to control a cooker plate. It is open loop.   
   The simmerstat has a bi-metal strip built into it that does the "control".   
   The simerstat usually incorporates a 1amp(or thereabouts) rated contact that   
   is used to control the cooker plate (element).   
   F.   
      
   --- SoupGate-Win32 v1.05   
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