Heating Element Control

[Farticus]

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.

 

[Farticus]

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?

 

[PeterD]

I need to control two 120VAC/5A heating elements off a 240VAC supply.

That won't last long, will it?

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!).

With a MTBF on the elements of a few minutes I don't see this as cost
efective in any way.

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

Well, since you are using a 240 volt supply, what's the problem? Oh,
the elements are going to pop like cheap lightbulbs, but hey, that's
life.

that they will not function properly on120V. Then there would be the need
for interface relay/contactor to switch the element etc. etc....

Why do they say that?

 

[Farticus]

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

 

[Farticus]

From the point of view of cost, I like the idea of a single
phase-controlled TRIAC driving both heaters in parallel since the
pair of heaters, in parallel, will only need 10A from the mains and,
with proper timing, the loads will never see more than 120V.

When one is unplugged, of course, the TRIAC need only pass 5A.
Even better.

But what's a simerstat? Is it a device with feedback which will
maintain a temperature once set, or is it like an open-loop lamp
dimmer?

A simerstat is the term given by electrical wholesalers for a control
typically used to control a cooker plate. It is open loop.
The simmerstat has a bi-metal strip built into it that does the "control".
The simerstat usually incorporates a 1amp(or thereabouts) rated contact that
is used to control the cooker plate (element).
F.

 

[Baron]

---
I'm confused.

If the simerstat is used to control temperature and the bimetallic
switch is used to switch the element on and off in order to maintain
that temperature, then that's closed loop.

An example of 'open loop' would be a lamp dimmer where there is no
feedback used to control the brightness of a lamp. Or, for another
example, a VARIAC.

Hi John,

The simmerstat is only open loop in the sense you describe. There is no
sensing of the actual hotplate temperature. The current through the
hotplate passes through a secondary heater wound around a bimetallic
strip. This simply makes/breaks the circuit. A cam controls the
distance the bimetallic moves.

 

[Baron]

Your'e welcome.

 

[Farticus]

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.

Thank you for your suggestion.
Unfortunately its not that simple.
The gear has to be constructed to meet certain standards and customer
expectations.
Here in Australia, the neutral and earth are bonded at the power supply
station and at all earth points enroute from the supply to the users outlet
meaning that the PD between line and either neutral or earth is 240VAC.
The heaters belong to a customer, not mine. He purchased them ex-USA and the
supplier only manufactures 120V units.
Yes, indeed, I do know what a contactor is. The first contactor I ever came
across was in 1963 when I was an apprentice. It was a really robust piece of
gear with built in overload that used oil in a damping mechanism (called a
dash pot) and was if I remember correctly made by GE.
I do not use contactors or relays in applications as described here as they
are a bit out-of-date as opposed to solid state devices which last much
longer.
One must also bear in mind that when making up equipment for a customer (who
is always correct!) a number of factors have to be taken into account.
Firstly final price to the customer, and of course enough profit.
Then there are the many compliance requirements to ensure the equipment
meets all local electrical laws etc.
The equipment must be professionally presented, built to meet the
expectations of the customer and to mechanically fit the industry into which
it will be used.
Taking these and a few other aspects into consideration one then sets about
designing the equipment, endeavouring to meet all requirements and deliver a
quality piece of apparatus.
I have finalized my design and run a test which works adequately for the
application and meets the customers expectations.
The design uses a simple LM555 circuit configured to provide a PWM signal of
approximately 100Hz which drive a solid state 30A relay with built in
features such as zero crossing, overload protection etc. The mechanical
loading (heatsink) provided for the heating elements by the work being
performed is more than adequate to ensure the circuit is kept busy
constantly thereby providing reasonably accurate temperature maintenance
without any closed loop configuration. Further protection is provided by a
circuit incorporated into the control that allows the device to operate for
30 minutes at a time. This means that the operator resets the control every
30 minutes - this is to prevent uneccessary use of the heating elements when
no in active use (customers request).
The operators of the equipment learn through experience the optimum setting
for the various materials they run through the machine/heating elements.

Thanks to all for their valued comments.
F.

 

[Ross Herbert]

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.

 

[Baron]

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.

 

[Farticus]

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 couldn't agree more!

 

[Farticus]

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.

Try to convince one of those customers that is "always right" - I agree with
your sentiments exactly.

 

[Baron]

Try to convince one of those customers that is "always right" - I
agree with your sentiments exactly.

Yes I get them too! Wouldn't it be nice to tell them to "Sod Off" !!!

I must admit that I have pointed a couple at my competitors. One of
those is now a very good client. But he had to get burned to realise
where he was better off.

Good Luck.

 

[cadman]

I would control the elements from a pair of triacs.
Phase control is very good but burst control would work just as well.
In fact burst control would be simpler as phase control needs to
adjust for the phase angle as simply adding linear steps to the phase
angle doesnt increase the heating pwoer linearly.

www.ckp-railways.talktalk.net/pcbcad28.htm

 

[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]

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.