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.