Heating Element Control

[cadman]

An interesting technique I once saw was to use burst control with a
relay but switch on a cheap low power triac for the first couple of
cycles to stop therelay arcing. The relay would then last for millions
of cycles instead of 100's of thousands as it is not burnt on switch
on or off.

 

[Ross Herbert]

On Thu, 25 Oct 2007 20:48:52 GMT, "Farticus" <localhost@127.0.0.1>
wrote:

"Baron" <baron.nospam@linuxmainiac.nospam.net> wrote in message
news:ffqh7q$tnt$1@registered.motzarella.org...
Ross Herbert wrote:

On Thu, 18 Oct 2007 23:02:34 GMT, "Farticus" <localhost@127.0.0.1
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.

 

[Tim Perry]

"Farticus" <localhost@127.0.0.1> wrote in message

... 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?


why not just wire in a VARIAC?. no design engineering required.


.. too expensive for an isolated (not auto-transformer type).
Thanks for the suggestion.

How about just put a large diode in series?

The heaters probably dont care if they see AC or pulsating DC.

Might want to add an RF choke to cut down on radio noise.