I don't understand why you don't just pull one leg of the 240v and a neutral
to get your 120v? better yet, why not just return the heaters and get 240v
ones? What type of control box? I would use a 20a contactor, and run the
control voltage through the heat control for the contactor. use 120v control
or a transformer to achieve the customary 24v control voltage. if you use
the 240v simmerstat control, make sure it is rated at least 25A. It will
still work. You can go up involtage rating just not down. Do you know what
a contactor is? I would use a contactor, one leg of the 240v hot and one leg
of neutral. this will give you the 120v. hook the t stat to the magnetic
coil connectors. this will pull in the contactor as needed based on the t
stat. you could run two wire connectors out from the contactor to your
outlets, and they will be hot or not based on the t stat setting. I'd even
install some light switches in there for control if he wanted.
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.
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