An electric heater assembly for tools having a removable heated
tip includes a tubular metal base having an open end adapted to
removably receive and hold the end of the tool tip. A coating of
ceramic electrical insulation material is provided on a portion
of the exterior of the base and a length of Nickel-Iron alloy resistance
wire is wound on the ceramic coated portion. Electrical lead wires
are connected in lapped relationship to the respective ends of the
resistance wire. Each lead wire includes a first portion lying between
several turns of resistance wire and the ceramic coating on the
base and a second portion bent back and overlaying the several turns
of resistance wire. The second portion is wrapped with at least
one additional turn of the resistance wire. A coating of ceramic
electrical insulation material is provided over the resistance wire
and the terminals to seal the same from the deliterious effects
of the atmosphere.
What is claimed is:
1. A heater assembly for tools having a heated tip comprising:
(a) a tubular metal base having an open end adapted to receive
and hold the end of a tool tip in heat conductive relationship therewith,
(b) a coating of insulating material on a portion of the exterior
of said base,
(c) a length of resistance-type wire wound on said coated portion
and around said base,
(d) lead wires connected to respective ends of said resistance
wire, each of said lead wires being in lapped relationship to the
respective end of said resistance wire and including a first portion
lying between several turns of resistance wire and said insulating
coating, a second portion overlaying said several turns of said
resistance wire, said second portion in turn being wrapped with
at least one additional turn of said resistance wire to form a terminal
(e) a coating of insulating material on said resistance wire and
said terminal formed at each end of said wire to seal same from
the deleterious effects of the atmosphere.
2. A heater assembly as set forth in claim 1 wherein said insulating
material is ceramic.
3. A heater assembly as set forth in claim 1 wherein said resistance
wire is Nickel-Iron.
BACKGROUND OF THE INVENTION
This invention relates to a heater assembly for heated tipped tools
and its method of manufacture and more specifically to a heater
assembly whose resistance value in ohms remains substantially constant
throughout its useful life.
It is well known that the electrical properties of resistance alloy
conductors vary with the type of alloy used and that the resistance
value changes with temperature. It is also known that the resistance
of the assembly will vary with a change in the tension of the wire
due to thermal shock or with oxidation that forms in the junction
between the resistance wire and the lead conductors supplying current
thereto or corrosion of the resistance wire itself. Thus, experiments
have shown that the total resistance of such prior art heater assemblies
utilizing resistance alloy conductors will vary as a result of one
or more of the aforementioned factors.
In order to improve electrical contact and reduce the possibility
of oxidation forming in the junction between the resistance wire
and the lead conductors, the prior art heater assemblies employed
the technique of spot welding the two together. This was generally
satisfactory in reducing the effects of oxidation, however, the
technical problems of spot welding a fine wire 0.009 inch in diameter,
for example, to another wire are significant and require expensive,
complex machines requiring operation by skilled workers. No satisfactory
solution has been found to the problem of changing resistance due
to thermal shock or corrosion of the resistance wire itself. The
wrapping of the resistance wire with mica or similar insulating-type
material has not been shown to solve these problems.
Such variations in resistance would normally not be of serious
consequence in normal soldering iron applications, however, when
the heater assembly is used to heat the tips of tools used for delicate,
precise soldering or desoldering-type applications where the temperature
of the tip must be precisely controlled by electronic circuitry,
such variations of the resistance of the heater assembly are intolerable.
In addition, because the temperature controlling circuitry associated
with each tool is preset to maintain a specific temperature, consistency
of resistance between heater assemblies is essential to enable interchangeability
of heater assemblies associated with various types of designs of
Applicant has discovered that by connecting the resistance wire
to the lead conductors by a pressure contact or junction resulting
from lapping one upon the other several times and surrounding the
resistance wire on the heater assembly bobbin as well as the junction
between the resistance wire and lead conductors with a coating of
insulating material, the junctions are sealed against oxidation,
the whole assembly is substantially shock resistant, considerably
cheaper, and the overall resistance value remains substantially
constant during the life of the assembly in contrast to similar
assemblies known in the art as aforementioned which are much more
expensive due to the use of complex, expensive machines with skilled
operators required when spot welding techniques are used.
It is, therefore, the primary object of the present invention to
provide a superior heater assembly for heated tipped tools.
It is another object of the present invention to provide a heater
assembly whose resistance value remains substantially constant throughout
its useful life.
It is yet another object of the present invention to provide a
heater assembly which utilizes a ceramic-type coating to seal the
assembly from the deleterious effects of the atmosphere such as
oxidation and corrosion to thereby ensure constant resistance.
It is a still further object of the present invention to provide
a heater assembly wherein the resistance wire is connected by pressure
to the conductor wires to provide a junction which when sealed results
in an assembly which is superior yet less expensive than those assemblies
utilizing spot-welding techniques at such junctions.
These and other objects and advantages will become apparent to
those skilled in the art from the following description considered
in connection with the accompanying drawings in which a presently
preferred embodiment of the invention is illustrated by way of example.
It is to be expressly understood, however, that the drawing is for
the purpose of illustration and description only, and is not intended
as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view in partial cross-section of the
heater assembly of the present invention.
FIG. 2 is an enlarged view of a portion of the heater assembly
encircled in FIG. 1.
FIG. 3 is a cross-sectional view of the heater assembly taken along
the lines 3--3 of FIG. 1.
FIG. 4 is a cross-sectional view of the heater assembly taken along
lines 4--4 of FIG. 4.
FIG. 5 is a plan view of a portion of the heater assembly secured
in a chuck of a winding machine.
FIG. 6 is a side elevational view of the heater assembly of FIG.
FIGS. 7(a)-7(f) is an illustration of the steps involved in the
method of making the heater assembly of the present invention.
FIG. 8 is a schematic of a control circuit for the heater assembly.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Referring now to the drawings wherein like reference numerals designate
identical or corresponding parts throughout the several views, FIG.
1 shows the heater assembly 10 of the instant invention. The heater
assembly 10 comprises a tubular metal base 12 of stainless steel
or the like having a center section 14 to which are adjacent end
sections 16,18 separated from the center section 14 by circumferentially
extending ribs 20,22 respectively. Ribs 20,22 are integrally formed
with the metal base 12.
The end section 18 has a bore 24 through the side thereof and a
metal collar 26 positioned thereon and secured thereto. The collar
26 also has a threaded bore 28 which is in registry with bore 24
into which is screwed a set screw 30 for maintaining a hollow desoldering
tip 32 or the like within the tubular portion of the metal base
12 and in heat conductive relationship thereto. The other end section
16 is insertable in a handle (not shown) which can be grasped by
the hand of the user.
The center section 14 has a coating 34 of insulating and sealing
type material placed over the outside surface thereof. The material
is preferably ceramic in nature, a typical example of which would
be CERAMACOAT 512* manufactured by Aremco Products, Inc. A winding
of heater wire 36 is spirally wound on the ceramic coating 34 in
spaced-apart relationship to adjacent turns and is made preferably
of a Nickel-Iron alloy having a composition of 70 percent nickel
and 30 percent iron. This composition of heater wire 36 was chosen
because its resistance varies in a linear manner relative to heat
whereas Nickel-Chromium wire, although less susceptible to oxidation,
has resistance characteristics which vary greatly with temperature
changes. The ends of the heater wire 36 are connected to lead wires
38,40 at terminals 42,44 in a manner to be more fully described
later. The lead wire 40 has a portion 41 which extends from one
end of the heater assembly over the top of the heater windings 36
to the other end and is insulated from the heater windings 36 by
means of a sleeve 46 made of, for example, silicone rubber. The
silicone sleeve 46 also extends over the entire length of both lead
wires 38,40 to the electrical connectors 48,50 connected to the
respective ends thereof. The terminals 42,44 at opposite ends of
the center section 14 as well as the entire winding of heater wire
36 and the portion 41 are covered with a coating 52 of the same
ceramic material as coating 34 to seal same from the deleterious
effects of the atmosphere such as oxidation and corrosion. Such
oxidation, etc. forming in the area of the terminals 42,44 would
cause variations in the resistance and overall useful life of the
heater assembly 10 as aforementioned. The coating 52 eliminates
any possibility of such oxidation.
Referring now to FIGS. 5 and 6, the method of making the heater
assembly 10 will be described. The tubular base 12 of the heater
assembly is shown secured in a chuck 54 of a winding machine (not
shown). Prior to winding the heater wire 36 a bare section 56 of
lead wire 38 is bent substantially U-shaped and positioned as shown
in FIG. 6 so that the remainder of the lead wire 38 extends within
a channel 58 formed in the chuck 54 and the curved portion 60 of
the bare section 56 rests against the coated surface 34 as will
subsequently be described. In addition, after the desired length
of winding 36 has been wound on the curved portion 60 to form terminal
42 and on the coated surface 34, the curved portion 62 of the bare
section 64 of lead wire 40 is positioned adjacent the coated surface
34 and the terminal 44 is formed as will now be described in detail.
With regard to the actual steps of making the heater assembly 10,
after the bare section 56 of lead wire 38 is positioned adjacent
the coated surface 34 in the manner shown in FIG. 6, an end of the
heater wire 36 is placed in the curved portion 60 and the chuck
54 is rotated so as to wind three turns of the heater wire 36 as
shown in FIG. 7(a). The bare section 56 is then bent back over the
top of the three turns of heater wire 36 and pressed thereagainst
and two more turns of heater wire 36 are wound thereon as shown
in the second step shown in FIG. 7(b). The bare section 56 is then
bent back over the aforementioned two turns as shown in FIG. 7(c)
and the entire lap connection thus formed is pressed together to
form the complete terminal 42. The desired number of turns of heater
wire 36 (approximately 14) are then spirally wound along substantially
the length of the center section 14 in spaced-apart relationship
until the proper resistance value is achieved. This can be accomplished
by use of an ohmmeter (not shown) connected across the heater wire
The bare section 64 of lead wire 40 is then positioned adjacent
the coated surface 34 as shown in FIG. 6 and three turns of the
heater wire 36 is wound in the curved portion 62 as shown in FIG.
7(d). The bare section 64 is then bent back over the three turns
and an additional two turns are wound on the bent portion as shown
in FIG. 7(e). The bare section 64 is then bent over the two turns,
the entire is pressed together to form the complete terminal 44.
The lead wire 38 is then bent back adjacent the heater wire 36 to
a position substantially adjacent lead wire 40 and the entire assembly
consisting of heater wire 36, terminals 42,44 and lead wire 38 are
covered with coating 52.
Referring to FIG. 8, there is shown illustrative temperature control
circuitry 75. The circuitry for controlling the heating current
to and thus the temperature of heating element 36 is such that the
heating current is supplied at a high rate when the heating element
is below a selected temperature and at a very low rate when it is
above the selected temperature. The material employed for heating
element 36 has a positive temperature coefficient of resistance
and typically varies from about 1 ohm when cold to about 4 ohms
when the maximum temperature has been reached, although, of course,
other resistance ranges may be utilized. Typically, the heater element
material may be a nickel-iron alloy. The circuitry of FIG. 8 senses
the change in resistance by sensing the change in current through
the heating element to thereby control its temperature. A digital
system as opposed to an analog system is employed to thereby lessen
power consumption. Accordingly, pulsed power is continuously applied
to element 36 to obtain samples of the heater temperature. The sampling
is effected by an oscillator comprising a comparator 76, a 10K resistor
78, a diode 80, a 0.027 microfarad capacitor 82 and a voltage divider
comprising a 1 megohm resistor 84 and a 100K resistor 86, it being
understood the foregoing values are for purposes of illustration.
When heater 36 is raised to its selected temperature, comparator
76 turns off so that its output goes positive. This allows the capacitor
82 to charge through resistors 84 and 86. As capacitor 82 charges
positively, the inverting input of comparator 76 becomes more positive.
When it becomes more positive than the non-inverting input, the
output of comparator 76 goes to ground thereby discharging capacitor
82 through resistor 78 and diode 80, the discharge time being much
faster than the charging time.
During the time capacitor 82 is discharging, the output of comparator
76 remains at ground. This causes the non-inverting input of a comparator
88 to be at ground while the inverting input thereof is fixed at
typically about 4 volts. Accordingly, the output of comparator 88
is grounded. This, in turn, causes a transistor 90 to be cut off.
Accordingly, current flowing through a 200 ohm resistor 92 no longer
flows into the base of a transistor 94 where transistors 94 and
96 comprise a Darlington pair which supply current to heating element
If the temperature is high enough so that the inverting input to
a comparator 98 remains below the input setting at the non-inverting
input thereof, the sampling of the heater temperature will last
only until capacitor 82 is discharged. The input setting at the
non-inverting input of comparator 98 may be controlled by a potentiometer
100 or it may be fixed by 27K resistor 102 and 1K resistor 104.
If, as stated above, the temperature is high enough so that the
inverting input to comparator 98 remains below the non-inverting
input setting, the inverting input of comparator 76 will become
negative and the output will go positive and capacitor 82 will again
start charging positive through resistors 84 and 86. The diode 80
prevents positive charging through resistor 78 on the output of
If the temperature is low, the sampling current will be high due
to the low resistance of heater element 36. Thus, the inverting
input of comparator 98 will be more positive than the non-inverting
input thereof and the output will be grounded. This will cause the
non-inverting input of comparator 76 to be more negative and the
output of comparator 76 will stay negative. Hence, capacitor 82
will not charge positive as long as the inverting input of comparator
98 is more positive than the non-inverting input.
As the temperature of heating element 36 increases, its resistance
increases and the current decreases. Eventually, the inverting input
of comparator 98 will become negative with respect to the non-inverting
input and the output will move positive. This will allow comparator
76 to switch so that its output will be positive and the capacitor
will again charge positive. Also comparator 88 will switch so that
its output will become positive and turn on transistor 90 so that
the power pair 94 and 96 are turned off and no current is supplied
to heating element 36. After capacitor 82 is charged to thereby
switch comparator 76, another sample will occur and the cycle is
As can be appreciated, the current through heating element 36 is
not switched on or off until the current reaches the input setting
established at the non-inverting input of comparator 98. Hence,
LED 106 is not switched on and off until the heating element has
been raised to its operating temperature. Accordingly, the operator
is notified when the desolderer is ready for use by the blinking
on and off of LED 106. Although shown conveniently mounted on the
handle of the desolderer in FIG. 3, LED 106 can also be mounted
on the front panel of the portable power pack (not shown).
Other elements employed in the temperature controller circuitry
of FIG. 8 are a 0.1 resistor 108, 2K resistor 110, 200K resistor
112, 100K resistor 114, comparator 116, 10K resistor 118, 10K resistor
120, 100K resistor 122, 10K resistor 124, diode 126, 10K resistor
128, 100K resistor 130, 100K resistor 132 and 200K resistor 134,
it again being understood the foregoing values are given for purposes
Obviously many modifications and variations of the present invention
are possible in light of the above disclosure. It is therefore to
be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described.