If I had a heater with four heating elements in it (but two were
removable for versatility in size) how would the heater need to be
constructed to automatically adjust for the extra elements when they
are added or removed? For arguements sake lets say that the heater is
a 1000W heater that throws out X amount of heat with the two permanent
heating elements installed. When the two removable elements are added
the overall wattage would need to be raised to sustain the heat output
of X. How can you vary wattage in this way? Can a switch activate a
surplus of wattage? If you run the elements in series the wattage
would either be too high with two elements or too low with 4 elements.
You wouldn't be able to have a constant output of X amount of heat.

So to summarize...my question would be what technology would be
required to be able to transfer easily within a small household
appliance, between several heating elements and the required wattage
to maintain a constant output? Please let me know if any clarification
is needed. I realize this may be a slightly confusing posting.

I would consider a complete answer to contain links to websites that
have relevant technology as well as a description (or a link to a
description) of how the technology applies to my question. Please let
me know if my price seems unreasonable.

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Request for Question Clarification by eiffel-ga on 13 Jun 2006 12:30 PDT

Hi forti4040-ga,

First, let me check this: you want to switch in additional heating
elements, yet have the heat output remain unchanged. Is that correct?

If your heating elements were perfect resistors, the answer would be
straightforward, and what you want could be achieved with the correct
switching arrangement.

But heating elements change their resistance as they heat up, and work
best under quite a narrow range of operation. So a quite sophisticated
control circuit would be required to maintain the same heat output
when you add additional elements.

If you can also let me know why you want this unusual design, that may
help to point towards the best solution.

Regards,
eiffel-ga

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Clarification of Question by forti4040-ga on 14 Jun 2006 06:59 PDT

Hello eiffel-ga,

Thanks for the comments. You are correct in your assumption that I
want to be able to add elements and have the output "unchanged". The
term unchanged is used loosely because I know there is change in the
setup. But I want to find a method that will produce a similar output
under these circumstances. My reasoning for this design stems from my
curiosity as well as my drive to invent. :-) I have several ideas that
could work around users being able to add and remove elements
depending on the situation they find themselves in. Please let me know
if more clarification or input is needed. Thanks again for the
comments.

Hi forti4040-ga,

You can do it by using simple switching, provided you adhere to a
number of constraints. I'm assuming that all of your heating elements
have the same wattage. (For heating elements, watts of power input is
virtually identical to watts of heat output, by the way.)

Consider the basic design to use just one heating element, shown here as VVV:

------VVV------

You can then use a combined series/parallel arrangement of four
heating elements to achieve the same heat output:

      ------VVV------VVV------
      |                      |
------+                      +------
      |                      |
      ------VVV------VVV------

In this arrangement, each heating element has half the voltage across
it, and also half the current flowing through it (because each
series-group of two has twice the resistance). Therefore, each heating
elements consumes and dissipates one-quarter of the power that it
would if if were directly connected to the input voltage.

In a similar way, you could use nine heating elements arranged as
three parallel groups, each group comprising three series heating
elements.

But, as I mentioned in the request for clarification, this is only
going to work if the resistance of the heating elements does not
change much as they heat up, and that's unlikely to be true.
Certainly, it won't be true for tungsten heating elements.

You have not stated your reason for wanting to maintain the same power
output whilst changing the number of heating elements, but I'm
guessing it may be because you want to radiate the same amount of heat
from a larger or smaller area. If that is the case, perhaps an
alternative method of achieving this goal will be more practical. You
could use a fixed number of heating elements and move them closer or
further apart, or you could swap between different-sized rear
reflectors that would distribute the same amount of heat over a larger
or smaller area. You could conceivably even use one flexible rear
reflector that could be stretched out or curved up.

If you want to go beyond these designs, you would need to consider
active control of the electrical input. If you will allow the user to
re-balance the heat output, then you can just put all the elements in
parallel and let the user operate a simple SCR-based controller
(effectively the same as a lamp dimmer, but able to handle higher
power).

If the power control must be automatic, you can put all the elements
in parallel and use a constant-power control device. The principles
are explained here:

"SCR Control of Electric Heaters"
http://www.eurotherm.com/training/tutorial/instrumentation/holland/scr.htm

An example industrial product that performs SCR power control with a
constant-power facility is this one:

"PCI Series SCR Power Controllers"
http://instrumentation-central.com/SCRPowerControls/pci_series.pdf

I trust this provides the information that you are seeking. If not,
please request clarification.

Regards,
eiffel-ga

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Clarification of Answer by eiffel-ga on 15 Jun 2006 04:20 PDT

You may also find the following Google search useful:

scr power controller "constant power"
://www.google.com/search?q=scr+power+controller+%22constant+power%22

Regards,
eiffel-ga

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Clarification of Answer by eiffel-ga on 15 Jun 2006 06:00 PDT

If you are driving your heating elements with alternating current, you
could use diodes in a switching arrangement.

For two elements: Just run them in parallel.

For four elements: Run elements 1 and 2 in parallel, but with a
rectifier diode in series with the pair. These elements will get half
of each AC cycle. Run elements 3 and 4 in parallel, but with a
rectifier diode in series with the pair. Put this rectifier diode the
other way round (with the opposite polarity) to the other rectifier
diode. These elements will now get the other half of each AC cycle.

We are driving each heating element with its normal rated voltage, but
only for half the time so again it will be important to choose heating
elements whose resistance varies as little as possible with
temperature.

Regards,
eiffel-ga

This answer is excellent! It explains several methods of attaining my
goal and it gives very relevant examples. I appreciate the research
and help.

Thank you

You can't supply "wattage", but you can use a transformer to change
the voltage fed to the heating elements to compensate for the change
in resistance.