"John Fields" wrote in message
...
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.