From: rherber1@bigpond.net.au
On Thu, 25 Oct 2007 20:48:52 GMT, "Farticus"
wrote:
>
>"Baron" wrote in message
>news:ffqh7q$tnt$1@registered.motzarella.org...
>> Ross Herbert wrote:
>>
>>> 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.
>>
>> I'm in general agreement. Why not simply use 230v elements. There
are
>> a few companies that will make up whatever size/shape you want.
Then
>> you can use the "Simmerstat" to do the controlling of temperature.
>>
>> --
>> Best Regards:
>> Baron.
>Try to convince one of those customers that is "always right" - I
agree with
>your sentiments exactly.
>
If I were in your position I would explain the safety factors, and
that due to possible legal action against you, you are unable to
accede to his request to carry out the job in the manner he requires.
The customer who thinks he is always right sometimes has to be shown
that such is not always the case - even at the risk of losing his
business.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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