[0001] The present invention proposes a method to produce class 2 resistors with PTC, in
which:
- a double layer of insulating material, particularly kapton or teflon, is arranged
inside a tube;
- a heating element constituted of one or more PTC elements clamped between a pair of
diffusers is placed inside the tube, centring the assembly using centring rings;
- the space around the heating element is filled with pulverized insulating material,
in particular magnesium oxide;
- said insulating material is compacted;
and in which the dimensions of the magnesium oxide grains are below 200 microns,
and the average dimension of the grains is preferably around 40 microns.
[0002] In this way resistors are obtained in which it is no longer necessary to provide
control and/or safety devices such as thermostats and/or thermal fuses as the necessary
insulation is guaranteed in any case.
[0003] In particular the use of magnesium oxide with the indicated grain size makes it possible
to compact the insulating material inside the tube containing the PTC elements without
damaging the external coating in kapton.
[0004] The invention also relates to the resistors obtained with said method.
[0005] Generally, to produce class 2 resistors (resistors which must have double insulation)
insulating materials, such as magnesium oxide in grains, are used combined with a
second material, generally constituted by a double turn of kapton or teflon.
[0006] A first type of resistor in this category comprises a winding of resistive wire fitted
inside a tubular container generally made of metal filled with pulverized magnesium
oxide.
[0007] This tube is then fitted inside a second tube with a larger diameter and the space
between them is also filled with magnesium oxide which is subsequently pressed and
compacted.
[0008] This type of resistor has a diameter which for some applications is considered excessive.
[0009] In a second type of resistor the winding of resistive wire is again placed inside
a tube which is filled with pulverized magnesium oxide and this tube is then coated
with a double layer of kapton or teflon before being fitted inside a second tube,
which constitutes the outer casing.
[0010] This second type of construction makes it possible to obtain resistors with smaller
diameters.
[0011] Class 2 resistors in which the heating element is constituted of one or more PTC
elements have recently been proposed.
[0012] These elements have the considerable advantage of not requiring control and/or safety
devices such as thermostats and/or thermal fuses, even if they still present all the
problems mentioned above, in relation to the need to provide double insulation.
[0013] A first solution to this problem is described in the German patent N. 19737241, which
relates to a resistor comprising one or more PTC resistors clamped between a pair
of heat diffusing elements, with a first layer of insulating material composed of
a plurality of rings in material based on magnesium oxide and silicone rubber, which
are fitted on the pack composed of the PTCs and relative diffusers and a second insulating
layer composed of a double sheet of kapton wound around these rings.
[0014] This is all fitted inside a common container composed of a metal tube and then compacted.
[0015] This solution has various advantages compared with prior art, as temperature control
devices are no longer necessary and moreover, its dimensions are also smaller than
prior art resistors.
[0016] However there is still the problem of a somewhat laborious production process, as
the insulating rings in a material based on silicone rubber and magnesium oxide must
be produced in advance, making it possible to further improve this solution.
[0017] In fact, it would be useful to produce a class 2 resistor with PTC heating elements
using, as insulating material, common magnesium oxide and kapton or teflon.
[0018] Experiments have been conducted in this sense, although these provided somewhat disappointing
results as during the phase to compact the magnesium oxide, which is generally performed
by hammering, the grains perforated the layer of kapton, thus invalidating the insulating
properties of this material.
[0019] To find a solution to this problem the applicant conducted numerous experiments,
varying all the parameters which may influence the final result and, in particular,
varying the magnesium oxide grain size.
[0020] In fact, all the resistors produced to date use magnesium oxide in which the average
dimension of the grains is around 180 microns and the fraction with dimensions below
45 microns is equivalent to around 6% of the total.
[0021] The quantity of fine component is intentionally limited, as this makes filling the
element more difficult, so that the general and common tendency of all producers is
to use magnesium oxide with grains of a certain dimension.
[0022] As an example, tables A and B below provide two examples of typical grain sizes used
for this purpose.
TABLE A |
TABLE B |
> 500 im ........ 0.0% |
>500 im ........ 0,0% |
425 - 500 ....... 0.0% |
>425 im ......... <0.1% |
250-425 im ...... 31.4% |
425-355 im ...... 4% |
180-250 im ...... 18.3% |
355-250 im ...... 24% |
100 - 180 im .... 26.2% |
250-180 im ...... 20% |
75-100 im ........ 8.9% |
180-106 im ...... 24% |
45-75 im ......... 9.0% |
106-75 im ....... 13% |
<-45 im .......... 6.3% |
75-450 im ....... 10% |
|
<45 im .......... 5% |
[0023] As previously mentioned, the applicant hypothesized that by varying the grain size
appropriately it might be possible to obtain satisfactory results without running
into the problems that previously caused this course of action to be abandoned.
[0024] Experiments conducted have confirmed this hypothesis to be correct, and have led
to the development of the following method, which shall now be illustrated with reference
to the attached figures in which:
- figure 1 shows the section along the axis of a resistor according to the invention;
- figure 2 is the section of the resistor in figure 1, according to a direction orthogonal
to the previous one.
[0025] In accordance with the invention, a class 2 resistor with heating elements constituted
by PTCs is produced as follows.
[0026] A layer of insulating material 2 composed of a few turns of kapton or teflon, are
wound in a winding and fitted into a tube 1, which forms the external casing of the
resistor, in order to obtain, on the internal surface of the tube, at least two complete
turns of insulating material.
[0027] One or more centring elements 3 composed for example of rings made of teflon, silicon
rubber or other material capable of withstanding the temperatures developed by the
resistor are then fitted into the tube and subsequently the heating element, composed
of one or more PTC elements 4 clamped between a pair of diffusers 5, for example in
aluminium, is fitted into the tube.
[0028] These diffusers will preferably have the form, in section, of a circular sector in
which the bending radius corresponds to the radius of the internal surface of the
centring elements.
[0029] Preferably, the ends of the diffusers will be shaped to define a pair of projecting
longitudinal edges, indicated with no. 7 in figure 2, which allow more secure assembly
of the PTC plates, preventing them from escaping either entirely or in part from the
diffusers and thus guaranteeing optimum electric contact.
[0030] These centring elements will have an annular internal surface while the external
surface will preferably be polygonal, for example hexagonal, to facilitate the passage
of the magnesium oxide dust which is subsequently introduced to provide the primary
insulating layer indicated with 6 in the figures.
[0031] Once the heating element has been fitted into the tube, the finely triturated magnesium
oxide is then introduced to fill all the available spaces around the heating element
to provide the primary insulation.
[0032] In conformity with one characteristic of the invention, this magnesium oxide has
an average grain dimension of around 40 microns, and has no grains with dimensions
above 200 microns and preferably no grains with dimensions above 192 microns.
[0034] Once the inside of the resistor has been filled with magnesium oxide this is compacted,
for example by hammering after which the tube can be closed and the resistor is ready.
[0035] Using magnesium oxide with the grain size indicated it was observed that the subsequent
operations to compact and press the grains do not cause any damage to the external
coating in kapton or teflon, which thus maintains its integrity.
[0036] In this manner, it is possible to obtain a class 2 resistor with PTCs, with compact
dimensions and which can be produced at a relatively low cost, as it may be produced
using inexpensive materials, already known to and used in the specific sector for
some time.
[0037] Those skilled in the art may then devise different modifications and variants, which
however must all be considered as coming within the scope of the present invention.
1. Method for producing class 2 resistors with PTCs,
characterized in that it includes the following phases:
- fitting, inside a tube which constitutes the container of the resistor , at least
two layers of insulating material in correspondence with the internal surface of said
tube;
- fitting a heating element composed of one or more PTC elements clamped between a
pair of diffusers inside said tube, after fitting one or more centring elements;
- filling the spaces between said heating element and said insulating layer with pulverized
magnesium oxide;
- compacting said magnesium oxide;
in which the dimensions of the magnesium oxide grains are below 200 microns.
2. Method as claimed in claim 1, characterized in that at least 50% of the magnesium oxide has dimensions below 200 ìm.
3. Method as claimed in claim 1, characterized in that the grains of magnesium have dimensions below 192 im.
4. Method as claimed in each of the previous claims, characterized in that the magnesium grains have mean dimensions between 30 and 50 microns, in particular
40 ìm.
6. Class 2 resistors with PTCs, produced with the methods as claimed in one or more of
the previous claims.
7. Class 2 resistors
characterized in that they have:
- a heating element composed of one or more PTC resistors clamped between a pair of
diffusers;
- a tubular container for said heating element;
- an insulating layer in kapton applied to the internal wall of said tube;
- an insulating layer in magnesium oxide with grains with dimensions below 200 microns,
between said heating element and said insulating layer in kapton.
8. Resistor as claimed in the previous claim,
characterized in that to maintain the heating element positioned, it has one or more centring elements
composed of annular elements and that said centring elements must allow the magnesium
oxide grains to pass and flow along the entire length of the tubular container.
9. Resistor as claimed in the previous claim, characterized in that the ends of said diffusers are shaped so as to define pairs of projecting longitudinal
edges, suitable to hold the PTC plates in place, preventing them from escaping either
entirely or in part from the diffusers.