What are the elements of differential association?

Wattage of multiple interchangable heating elements

• 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.

• Answer:

  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