"Farticus" wrote in message
...
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.

.... having given it a bit more thought, I'm thinking of using a solid state
relay triggered by a 555.
The slower the pulse rate the lower the temp - PWM?
Any thoughts on this idea?