United States Patent (19) (11) Patent Number: 4,795,886

Carter, Jr. 45) Date of Patent: Jan. 3, 1989

(54. TEMPERATURE CONTROLIN WHICH THE 50-84373 7/1975 Japan .
CONTROL PARAMETER IS THE DEGREE
OF IMPERFECTION IN THE IMPEDANCE OTHER PUBLICATIONS
MATCHING Low Cost Wideband Dual Directional Coupler, by
75 Inventor: Philip S. Carter, Jr., Palo Alto, Calif. Robert S. McDonald, pp. 34-36 of the May/Jun. 1982
issue of r.f. design.
73) Assignee: Metcal, Inc., Menlo Park, Calif. Specification Sheet for National Semiconductor
(21) Appl. No.: 943,755 MM54C14/MM74C14 Hex Schmitt Trigger, pp. 1-10,
i-11, and 1-12.
22 Filed: Dec. 19, 1986 Specification Sheet for National Semiconductor
51 Int. Cl." ............................................... H05B 1/02 MM54C221/MM74C221, Dual Monostable Multivibra
52 U.S. C. .................................... 219/505; 219/504; to.
219/501; 219/494; 219/10.75; 324/58 B Application of William D. Hall, Ser. No. 749,637, filed
58) Field of Search ............... 219/501, 497, 499, 494, Jun. 28, 1985, entitled Ferromagnetic Element with
219/504,505, 506, 508, 509, 491,490, 10.75; Temperature Regulation.
324/58.5 B, 58 B Application of Clappier Ser. No. 684,730, Filed Dec.
(56) References Cited 21, 1984, entitled Constant Current RF Generator.
U.S. PATENT DOCUMENTS Primary Examiner-M. H. Paschall
Attorney, Agent, or Firm-Hall, Myers & Rose
3,584,190 5/1971 Marcoux ............................. 219/233
3,686,460 8/1972 Lamparter et al. .............. 219/10.77 57 ABSTRACT
3,800,218 3/1974 Shekel ............................... 324/58 B A temperature control system is disclosed for control
3,924,102 12/1975 Hanekom ............................ 219/501
4,002,882 1/1977 McCutchen ........................ 219/499 ling heating current to a heater having an impedance
4,211,911 7/1980 Dehn ...... 219/10.55 F which varies substantially with temperature. The
4,499,358 2/1985 Scott ................................ 219/10.77 change in impedance of the heater changes the degree
4,507,546 3/1985 Fortune et al. ..................... 219/497 of mismatch between the power transmission system
4,546,238 10/1985 Ahs ..................................... 219/497 and the heater resulting in a change in the voltage
4,616,120 10/1986 Maruyama et al..... ... 324/58 B
4,626,767 12/1986 Clappier et al. .................... 323/280 which is reflected back to the power transmission sys
tem. The reflected voltage is monitored and its magni
FOREIGN PATENT DOCUMENTS tude compared to a reference source and the power to
O087099 2/1983 European Pat. Off. . the heater is controlled in response to this comparison.
2806159 1/1980 Fed. Rep. of Germany .
2527916 6/1977 France . 21 Claims, 4 Drawing Sheets

Threshold
Detector (und
Pulse Generator Detector
Diff. Amp a
Regulator
20
U.S. Patent Jan. 3, 1989 Sheet 1 of 4 4,795,886

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|-pd0157|-p7olz
07

ZºfTHI

02
 U.S. Patent Jan. 3, 1989 Sheet 2 of 4 4,795,886

Fig.5
hectercoil 23

non-magnetic
COre 25
t capacitor
ferromagnetic layer z
To detector To directional Coupler
Fig.6
3.
To
detector ferrite Toroid 30

6
heater Coil lead 3 Fig 7
Outershielding layer 33
inner shielding layer 32
heater Coil ?)-35
leads 3
2
NS
O

Current SensOrleads 6
-Z, 7

21Nsgfegyétéfet as a
22 S3323222222

24
U.S. Patent Jan. 3, 1989 Sheet 3 of 4 4,795,886

Cut slots in PCB

for toroids

-- OUTPUT

D.C. AMPLFER

W in
U.S. Patent Jan. 3, 1989 Sheet 4 of 4 4,795,886

|-—lº
…?U8J/m0
izZIT6?EI
 1.
4,795,886
2
driver circuit 52 for a fixed period of time to allowing
TEMPERATURE CONTROL IN WHCH THE the heater to cool. After to, the driver turns on again,
CONTROL PARAMETER IS THE DEGREE OF thus producing a pulsating RF current in the load.
MPERFECTION IN THE IMPEDANCE In some applications, Hall's approach using a pulsed
MATCHING RF generator has a significantly higher electrical effi
BACKGROUND OF THE INVENTION
ciency than the non-switching constant current mode of
operation of resistance ferromagnetic heater described
This invention relates particularly to temperature in said U.S. Pat. No. 4,256,945 patent. In the constant
control systems. One use for the invention is the provi 10
current mode, the heater of U.S. Pat. No. 4,256,945
sion of a soldering iron which operates at a relatively when "idling' with a small thermal load near the Curie
constant temperature. temperature, has a relatively low impedance and there
It is well known that a piece of high permeability fore presents a large impedance mismatch to the RF
ferromagnetic material may be held at a relatively con generator. This causes the generator to operate at a
stant temperature near its effective Curie temperature if 15
relatively low efficiency. In an application such as the
the radio frequency current of proper frequency and hand soldering tool, the circuit is operating in this mode
magnitude are passed through it. Such a device, how for extended periods. Hall's approach, when properly
ever, maintains its temperature constant only over a implemented, avoids operation into a badly mismatched
very limited range of heat power (in watts) extracted load and consequently achieves higher efficiency.
from the load. Generally, a typical such device operates My present invention includes an improvement upon
attemperatures above the effective Curie when the heat 20 the aforesaid Hall application.
extracted by the load is very low, operates at or near the
effective Curie over a very limited range in which heat SUMMARY OF THE INVENTION
in watts is extracted from the load, and its temperature There are heating devices in which the load impe
falls off sharply as the heat in watts extracted by the dance varies with the temperature of the load. The
load increases above said very limited range. 25
aforesaid U.S. Pat. No. 4,256,945 and the aforesaid ap
The very limited range referred to above may be plications are such devices. Another such prior art de
extended by using a composite structure of a ferromag vice is a heater of a doped barium titanate in which the
netic layer clad upon a ccpper conductor as taught in resistance rises precipitously within a given temperature
U.S. Pat. No. 4,256,945, issued Mar. 17, 1981 to Philip S. range, as the temperature rises.
Carter and John F. Krumne entitled: Alternating Cur 30 My invention involves supplying a radio frequency
rent Electrically Resistive Heating Element Having heating current to heat the load of a heater whose impe
Intrinsic Temperature Control. dance varies substantially with temperature. The
In two later applications I have described such a change in impedance of the load changes the degree of
composite device in which I observed that there was a
voltage standing wave ratio (VSWR) in a composite 35 mismatch between the power transmission system and
heater of the type such as was described in the aforesaid the load resulting in a change in the voltage which is
U.S. Pat. No. 4,256,945. Two such applications are my reflected back to the power transmission system by the
U.S. patent applications Ser. No. 586,715 (filed Mar. 6, load.
1984) and Ser. No. 666,346 (filed Oct. 30, 1984) both The reflected voltage is sensed and its magnitude is
entitled High Efficiency Autoregulating Heater. compared to the voltage of a reference source. The
A prior application of Clappier, Ser. No. 684,730 difference between the reference source and the re
filed Dec. 21, 1984), now U.S. Pat. No. 4,626,767, enti flected voltage is used to turn off the power to the load
tled Constant Current RF Generator, assigned to the when the reflected voltage exceeds the reference volt
same assignee as the present application, mentions the age. The load temperature then drops and the power to
destructive effect on a power supply due to changes in 45 the load is restored after a selected predetermined time.
the VSWR due to changes in the load impedance. The power continues this on-off cycling operation to
The application of William D. Hall, Ser. No. 749,637 produce a pulsating current that holds the load tempera
(filed June 28, 1985), entitled Ferromagnetic Element ture relatively constant.
With Temperature Regulation, is assigned to the same A directional coupler may be employed to isolate the
assignee as the present application. A continuation-in 50 reflected voltage. The isolated reflected voltage is com
part application, Ser. No. 003,288 was filed on Jan. 14, pared with a reference voltage, to produce a difference
1987, and the parent application abandoned. FIG. 6 of signal. The power supply is turned on and off depend
the parent application, teaches (1) means for maintain ing on the magnitude of the difference signal, thereby
ing a constant RF load current, and (2) a means for producing a pulsating current that holds the load tem
switching the RF power off when the temperature of 55 perature constant as explained above. The reflected
the ferromagnetic heating elements is at, or close to, its voltage is isolated from the power supply.
Curie temperature. This is accomplished by means of a A modified form has an excellent impedance match at
current sensor which develops an output voltage pro the desired operating temperature. The regulator cir
portional to the current through the load. The current is cuit, holds the temperature constant by holding the
higher than necessary to raise the temperatures of the 60 reflected voltage at its minimum.
load up to the effective Curie point of the material. The The modifications described in the last paragraph are
magnitude of voltage across the load decreases as the more expensive than the preferred form described here
- temperature rises in the neighborhood of the effective inafter in conjunction with FIG. 1. By reason of their
Curie Temperature due to the decrease of the load added cost they are not regarded as the preferred form.
impedance magnitude. A signal is produced which is 65 BRIEF DESCRIPTION OF THE DRAWINGS
proportional to the difference between the load voltage
and a reference voltage. This difference voltage acti FIG. 1 is a schematic drawing of the preferred form
vates a pulse generator circuit which shuts down the of the invention.
 4,795,886
3 4.
FIG. 2 is a schematic drawing of the impedance impedance matching circuit 14 and a voltage KVrat the
matching circuit of FIG. 1. reflected voltage output port 18 of the directional cou
FIG. 3 is the impedance vector diagram for the cir pler 13. k is the voltage coupling coefficient of the cou
cuit of FIG. 2 where the temperature is in a narrow pler 13. The coupler reflected voltage KVactivates the
range around Tc. 5 detector 19, D.C. amplifier 20, threshold detector and
FIG. 4 is a graph showing the relation of the reflected pulse generator 21 circuits, producing a rectangular
voltage (along the Y axis), and the temperature (X-axis). voltage V of duration to and of the appropriate polarity
FIG. 5 is a perspective view of a soldering iron tip, to deactivate, i.e., turn off, the driver 11.
plus a schematic view of the electrical circuit involved. The threshold detector 21 has a selectable threshold
FIG. 6 is a view of a modified system for the current O voltage 22. When AV, exceeds the threshold voltage 22
sensor of FIG. 1. the pulse generator delivers a single pulse V which
FIG. 7 is a cross-sectional view of a shielded solder turns the driver 11 off for a fixed, but selectable, time to.
ing iron tip which may be employed in place of the By selecting the threshold voltage 22 one can control
heater of FIG. 1. the temperature at which the pulse is triggered and thus
FIG. 8 is a schematic diagram of the directional cou 15 achieve another useful purpose, i.e. to vary the nomi
pler 13 of FIG. 1. nally constant control temperature over a range in the
FIG. 9 is a layout diagram of the directional coupler vicinity of the effective Curie temperature. By properly
13 of FG 1. selecting to the variation of the nominally constant
FIG. 10 is a schematic diagram of the D.C. Amplifier temperature can be controlled.
20 of FG, 1. 20 The reflected voltage V, at the input to the impe
FIG. 11 is a schematic diagram of part of the circuit dance matching circuit is given by the equation:
of the Detector, Differential Amplifier and Regulator
of FIG. 1. W=load (Zload -Zo)/2 (Equation 1)
FIG. 12 is a detailed diagram of block 21 of FIG. 1.
25 where:
DETALED DESCRIPTION OF THE Zload=simpedance at the input to the impedance
INVENTION matching circuit.
In FIG. 1, a conventional power supply comprises an Zo=characteristic impedance of coupler, usually 50
oscillator and buffer 10, a driver 11 and a Class C ampli ohms.
fier 12. The output of the Class C amplifier, which has 30 load=heater current.
a frequency preferably in the range of 13 to 14 MHz, is It is instructive to consider a specific example, for
fed through directional coupler 13 and impedance instance the equivalent circuit of a soldering iron, illus
matching-circuit 14 to the load 15 which comprises a trated in FIGS. 2 and 5. In this example the heater 15 is
heater composed of high permeability ferromagnetic a simple cylinder of magnetic material 23. The heater 23
material with an inductance L and resistance Rh. As 35 is hollow and the tip 24 with the non-magnetic core 25
will appear, the current from the Class Camplifier 12 to are held by heater 23. Parts 24 and 25 may be in one
the load 15 is pulsed on and off. When the current is piece and made of copper. The impedance matching
"on' its magnitude is held constant by the combination circuit includes a multi-turn coil 26A inductively cou
of current sensing coil 16 and "Detector, Differential pled to the heater 15 (also 23), plus a series capacitor 27.
Amplifier and Regulator' 17. When the current to the The function of the capacitor 27 is to tune out the induc
load exceeds the desired constant magnitude, the volt tive reactance of the transformer 26A, 23.
age induced across coil 16 causes the regulator 17 to It is understood that the circuit is designed so that at
decrease the voltage fed to the collector of the final temperature T2 (described later) the circuit elements 14,
stage of the power supply, Lhnamely the Class Campli 15 have their impedances matched at 50 ohms.
fier 12. To achieve this function, the detector, differen 45 In operation, in the circuit of FIG. 1, the coil 26 and
tial amplifier and regulator 17 has an adjustable refer the capacitor 27 will be chosen so that at temperature
ence voltage against which the voltage across coil 16 is T2, well below the effective Curie temperature, Ziadis
compared. If the voltage across coil 16 exceeds the very nearly equal to 50 ohms and therefore V is very
reference voltage, the regulator 17 lowers the collector small-not large enough to activate the threshold detec
voltage to the Class C amplifier 12, until the two volt 50 tor 21. As the temperature approaches the effective
ages are equal. Similarly, if the voltage across coil 16 is Curie temperature Zload departs from 50 ohms and the
lower than the reference voltage, the regulator senses magnitude of Vincreases to a level at which the thresh
the difference and raises the voltage to the Class C old detector 21 is triggered. In the example selected, the
amplifier until the voltage across coil 16 equals the inductance L and the resistance Rh of the heater 15
reference voltage. If the Class C amplifier is of the 55 both decrease causing Zload to be capacitive. A vector
vacuum tube type, the regulator 17 controls the plate diagram of the impedance is shown in FIG. 3. The
voltage of that amplifier. magnitude Zload has increased above its value Zo
A directional coupler 13 is employed to sense reflec below the Curie temperature and its phase is now nega
tions at the input to the impedance matching circuit 14. tive. This results in an impedance difference vector
The reflected voltage V,is very small when the temper Zload-Z which generates a reflected voltage vector
ature of the heater of the heater 15 is well below the according to Equation 1.
effective Curie temperature. This condition i.e., small FIG. 4 illustrates several factors affecting the opera
Vr is obtained by means of the impedance matching tion of the system. FIG. 4 shows a plot of the magnitude
circuit 14, the details of which are discussed below. As of the reflected voltage as a function of temperature
the temperature approaches the effective Curie temper 65 over a range from ambient T1, through the effective
ature of the ferromagnetic heater 15, the resistnce Rh Curie temperature T. In this illustration we have
and the inductance Lh of the heater 15 both decrease shown a residual or stray component or reflected volt
producing a reflected voltage V, at the input of the age AV at low temperatures which is due either to
 5
4,795,886
6
(1) imperfect matching of the load 15, (2) imperfect Designs for the directional coupler shown in FIG. 1
directivity of the coupler 13, or a combination of (1) and are well known and documented. At frequencies in the
(2). It is obvious that the threshold detector circuit 21, 1-100 MHz range designs employing ferrite cores are
22 must be adjusted so as not to be activated by this well known. See R. McDonald "Low Cost Wideband
residual reflected voltage. As the temperature ap Directional Coupler' RF Design pp. 34-36, May/June
proaches T, V, will increase above this residual volt 1982. Designs employing capacitors and lengths of
age. The threshold detector 21, 22 can be set to trigger transmission line are useful at these frequencies and at
at any temperature in the range perhaps T3 to T. Thus lower and higher frequencies, since lumped element
we have available a range of possible operating temper equivalents of the transmission lines can be used. See C.
atures near T. 10 Y. Ho "Design of Lumped Quadrature Couplers' Mi
Though it is theoretically possible to vary the operat crowave Journal pp. 28-31, September 1979. At lower
ing temperature over the entire range from approxi frequencies, below 1 MHz, bridges are very useful, See
mately T3 to Te it is probably desirable to maintain it Dunwoodie and Baxter "Measure Small SWRS With
substantially below the effective Curie tempearature Tc Great Accuracy' Microwaves, June 1978.
at all times in order to maintain the good amplifier effi 15 FIG. 1 employs an inductive loop or winding 16 as a
ciency and stability referred to previously as one of the current sensor. One possible implementation of this
advantages of this approach. A large value of V, corre sensor coil is shown in FIG. 5 for the case of the solder
sponds to a high degree of mismatch between the Class ing tool (shown in FIG. 5, without the shielding enclo
C amplifier 12 and the load 14, 15. This in turn lowers sure). In this illustration the heater coupling or impe
the efficiency of the amplifier output. Thus operation at 20 dance matching coil 26A and the current sensor coil 16
temperatures ranging from T3 up to a temperature T4, at are both wound around the ferromagnetic heater layer
which amplifier efficiency and stability are still high, is 23. Another possibility is shown in FIG. 6 where a
desirable. ferrite torrid 30 is wound with the sensor pickup coil 16,
The choice of the RF generator design 10, 11, 12 is a heater coil lead 31 being passed through the torrid.
subject to many considerations including cost, both 25 Toroidal current sensors of this type are well known
inital and operating, weight, size, etc. High efficiency is and commercially available, for instance Pearson Elec
almost always desirable in an RF generator since size, tronics, Inc., Model 410.
weight, and cost reductions often result. In general a I use the term "effective Curie' to refer to the tem
high efficiency power amplifier, such as Class C ampli perature below the published Curie temperature, at
fier, is usually desirable in the output stage. Recent 30 which the temperature of the ferromagnetic element
developments have made it possible to achieve even will remain substantially constant.
higher efficiencies than are achievable with the Class C. Instead of the load 15 being a ferromagnetic element
See, Kraus, Bostian and Raab “Solid State Radio Engi such as 23, the load 15 may be any electrical conductor
neering' Chap. 14, pp. 432-471, John Wiley and Sons, whose impedance changes when the temperature
New York 1980. I have not yet determined whether 35 thereof changes, for example a doped barium titanate
these designs are applicable to this proposal or to our conductor.
previous heaters. Instead of turning current on and off for controlling
FIG. 5 shows an impedance matching circuit which the temperature of the load 15, the apparatus may be
consists of a coil 26 surrounding the cylindrical heater designed to simply reduce the current.
23 plus a series capacitor 27 for tuning the circuit to On some applications RF shielding is required to
series resonance at ambient temperature. When a heater prevent the leakage and radiation of the electromag
according to FIG. 5 is used in FIG. 1, the capacitor 27 netic field into the space outside the heater. This re
is added to FIG. 1 as shown in FIG. 5. This circuit is quirement and means for achieving it have been pres
described more fully in my patent application Ser. No. ented and described in the patent application of Carter
666,346, and has been found to operate well in this 45 and Krumme Ser. No. 243,777 filed Mar. 16, 1981, a
soldering instrument application. continuation-in-part of the U.S. Pat. No. 4,256,945 of
The regulating circuit described here is useful in ap Carter and Krumme. This problem was also dealt with
plications where the entire heater is in close thermal in the application of John F. Krumme Ser. No. 430,317
contact with the object being heated, i.e., the work entitled "Autoregulating Electrically Shielded Heater”
piece. In this case the good thermal connection between 50 filed on Sept. 30, 1982. Finally a description of a shield
the work piece and the heater produces the same ing arrangement which is appropriate for use in connec
change in temperature over the entire extent of the tion with the heater shown in FIG. 5 is described in the
heater and thus causes the maximum change in V. The patent application of Carter, Ser. No. 666,346 C-I-P to
soldering tool described in my U.S. patent application application Ser. No. 586,715. FIG. 7 shows in a cross
Ser. No. 666,346, is a good example of this type of appli 55 sectional view the essential features of a shield which is
cation since the entire heater is in good thermal contact applicable to the heater shown in FIG. 5. This shield
with the working surface at the tip of the tool. An exam comprises a two layer tube having an inner shielding
ple of a non-optimum application is a physically and layer 32 and an outer shielding layer 33. The inner
thermally long heater in a situation where more heat is shielding layer is composed of a high permeability fer
required in some sections of the heater than others. In romagnetic conducting material which remains mag
this case those sections of the heater which require the netic at all time. This inner layer, the thickness of which
least heat may produce a large enough value of V to should be at least one skin depth serves two functions
turn off the generator. Other parts of the heater which simultaneously. The first is to substantially decrease the
require more heat will be at a lower temperature. This current which is induced in the outer shielding layer by
situation can be partially remedied by operating the 65 the current flowing in the heater coil. This prevents the
heater at a current high enough to maintain the hotter outer shielding layer from causing a short circuiting
parts of the heater close to the effective Curie tempera action on the heater. The second function is of course to
ture. contribute to the shielding action. The outer shielding
 4,795,886
7 8
layer 33 may be either magnetic or non-magnetic. It is creases. This in turn causes the output of the Class C
preferably a relatively low thermal conductivity non amplifier to increase, and the output of the current 16
magnetic material such as a Series 300 stainless steel. and the voltage at 17b to increase, reducing the differ
Very good shielding can be obtained at 10 MHz with an ence between the voltages at 1f and 17g to a value at
inner shield layer which is two thousandths of an inch which this difference voltage amplified by the gain of
thickness of 1010 steel and an outer layer which is five the amplifier exactly equals the output votage at 17b.
thousandths of an inch thickness of a Series 300 stainless Since the voltage gain of the amplifier can be quite high,
steel. The outer layer tube 33 can be extended past 34 to e.g. 500, this difference or "error" voltage can be quite
enclose the heater coil and detector leads as far as de small. Thus the reference voltage at 17g and the equilib
sired, probably to a connector interface. 10rium voltage at 17f which is proportional to the load
At 35 a washer shaped extension to the heater layer current, are very nearly equal and constant. A similar
23 may be added. This washer contributes to the heat progression of events occurs when the reference volt
ing and prevents large short circuiting currents from age at 17g is reduced, producing a corresponding reduc
being induced in the low resistivity material 24 with tion in the Class Camplifier output and a corresponding
which it is in electrical contact. 15 reduction of the current in the heater to a level at which
The directional coupler 13 is shown in more detail in the voltage at 17fis nearly 17g.
FIGS. 8 and 9. It is essentially the same device that is The output 17h feeds a conventional voltage regula
described on pages 34-36 of the May-June issue of the tor, for example National Semiconductor LM350K
magazine "rf, design,” which is incorporated by refer which in turn raises or lowers the collector voltage of
ence as showing directional coupler 13. A copy of this the Class Camplifier 12 as necessary to keep the voltage
magazine article is being filed with this application. across sensor 16 constant. In other words the current
A directional coupler is a device which samples RF fed to heater 15 is held constant,
current and voltage flowing in one direction but is in A typical example of the DC Amplifier 20 is shown in
sensitive to current and voltage flow in the reverse FIG. 10. The gain of the amplifier is selectable using the
direction. Directional couplers are very old and well 25 design equations as follows:
known and any of many designs are equally good as a
component of my invention.
The schematic of a simple dual directional coupler is
given in FIGS. 8 and 9. Two inexpensive toriod trans
formers T1 and T2 in the lines L1-L2, and L3-L4, respec 30 The apparatus of block 21 is essentially made of three
tively, provide inductive coupling. The unit is built on a components as shown in FIG. 12. The first component
printed circuit board which provides the necessary
microstrips L1, L2, L3 and L4. The toroids T1 and T2 are circuit insuresathat
is made up of 2.7K resistor and a 1N 4148 diode. This
only positive voltages appear at the
not coupled to each other. When voltage is reflected by
the load 15 it appears on wire 13a, and current flows 35 input of the Hex Schmitt trigger. The second circuit is
through lines L4-L3 inducing voltage in toroid T2 a threshold detector 21a. It receives the signal from
which is connected via line L2 to output 18. Also re directional coupler output 18, which is then detected by
flected voltage on wire 13a passes current through the detector 19, amplified by d.c. amplifier 20, compares it
coil of toroid T inducing current in the circuit 118, Li, to a reference voltage, and produces an output when the
L2, 18. The two signals thus fed to output 18 combine to signal exceed the reference voltage. The reference volt
produce a voltage proportional "to' the reflected volt age source 22 is shown in FIGS. 1 and 12. There are
age, assuming correct design of the components. commercially available devices that will perform the
I will next describe added details of the Detector, stated comparison, for example the National Semicon
Differential Amplifier and Regulator 17 of FIG.1. This ductor MM 54C14/MM74C14 Hex Schmitt Trigger
block 17 performs three functions. The first of the three manufactured by National Semiconductor. A copy of
functions is that of an r.f., detector which will detect the the specification sheet for this device is being filed with
signals from sensor 16. Assuming that the output 12a of this application.
Class Camplifier 12 operates at the preferred frequency When the reflected voltage signal from output 18
of 13.56MHz, the rif detector 17a would also operate produces a voltage that exceeds the reference voltage
at that frequency. A schematic of the rif, detector is 50 from circuit 22, the Schmitt trigger produces an output
shown in block 17a of FIG. 11. The d.c. output 17b of signal which actuates a monostable multivibrator 21b
the rif detector 17a is fed to differential amplifier 17c. which produces an output signal Vof a predetermined
The differential amplifier 17c has a 2000 ohm potienti length, to Vecan turn off the driver transistor by using
ometer 17d for providing a d.c. reference voltage it to bias the base of a common emitter amplifier stage.
against which the output 17b of the rif detector 17 may 55 The monostable multivibrator 21b may be National
be compared. A differential amplifier 17e has input 17f Semiconductor Dual Monostable Multivibrator
for the voltage at output 17b and input 17g for the refer MM54C221/MM74C221. This device enables the dura
ence voltage. The differential amplifier 17e may be tion of the output signal Veto be adjusted to the desired
Fairchild Model MA 741 or National Semiconductor time to. The output signal V will shut off driver 15 to
Model LM 741. The output of the differential amplifier 60 cool. If the cooling load on the heater 15 is a heavy
17e has a voltage that is proportional to the difference cooling load (for example the soldering iron tip is in
between the input voltage 17 fand the reference voltage contact with a cool copper conductor) the cooling will
17g. If these two voltages are exactly equal, the output be much greater than if there is very little cooling load
voltage at 17h is zero. However, in our case a finite on the heater 15. If the cooling load on heater 15 is very
voltage is required to operate the voltage regulator 65 small the reflected voltage at output 18 will cause the
which in turn controls the output of the Class C ampli multivibrator 21b to emit a signal to driver 11 for a time
fier. If the reference voltage at 17g is suddenly increased period to when the effective Curie of heater 15 is
the output of the differential amplifier suddenly in reached. Since the cooling load is small the heater 15
 9
4,795,886
10
may still be at the effective Curie when time period to 6. In a temperature regulator as defined in claim 1,
expires in which event the driver and Class C amplifier said responsive means repeatedly turning the current to
will be turned back on, but since there would then still said device on and fully off to provide a pulsating cur
be a reflected voltage at output 18 the mulitvibrator rent to said device, the energy of which is varied to the
would be turned back on almost instantly starting a new extent necessary to hold the temperature of said device
off period for a time period to. Thus, the radio fre substantially constant.
quency power to heater 15 would be cycled on and off 7. In a temperature regulator as defined in claim 6 in
with the "off" periods being relatively long as com which said control means includes a reference voltage
pared to the "on' periods. and means for comparing the reflected voltage with
On the other hand if the cooling load was large the 10
said reference voltage and also includes means for con
same events would occur except that there would be trolling said temperature in accordance with said com
greater cooling of heater 15 during the time periods to, parison.
and the “off” periods would usually be shorter than the 8. In a temperature regulator as defined in claim 7,
"on' periods. means for holding the current to said device constant
The exact relation of on and off intervals depends on 15 when it is on.
the cooling load, the selected time period to, and the 9. In a temperature regulator as defined in claim 1 in
magnitude of the current in heater 15. which said responsive means repeatedly turns the cur
With this arrangement of this invention, the magni rent on and off, said responsive means including means
tude of the current fed to the soldering iron (heater 15) for holding the current off for a determinable and vari
may be much larger than it would be in the case of 20 able time during each off period.
Carter-Krumme U.S. Pat. No. 4,256,945. This is a great 10. The method of controlling the temperature of a
advantage as it enables the soldering iron to come up to load comprising,
operating temperature fast when it is first turned on. providing the load with an electrical component hav
Moreover, there are serious power losses with the de ing an impedance that changes with the tempera
vice of said Carter-Krumme patent. Due to the changes 25 ture of the load,
of load impedance when the soldering iron passes from
the condition where most of the current is in the mag feeding the load with alternating current and produc
netic layer to the condition where most of the current is ing a signal created by any impedance mismatch at
in the copper, there is an impedance matching problem. the load, and
This arises since the load impedance changes and the 30 detecting said signal and varying the amplitude of
impedance cannot easily be matched to both conditions. said current in response to changes in the magni
With the present application, the mismatch in the impe tude of said signal to thereby control the tempera
dance is much smaller that with the Carter-Krumme ture of said load.
U.S. Pat. No. 4,256,945 resulting in greater efficiency. 11. The method of claim 10 in which said signal is a
I claim: 35 reflected voltage and in which the amplitude of the
1. In a temperature regulator: current to the load is varied to hold said reflected volt
a device having an electrical impedance which varies age substantially constant to thereby hold the tempera
as the temperature of the device varies over a spec ture of the load substantially constant.
ified region of its temperature range, 12. The method of claim 11 in which said reflected
means for applying pulsating current to said device to 40 voltage is held constant by comparing it with a refer
heat the same, said means having an impedance ence voltage and varying said current to keep the re
which is mismatched with the impedance of said flected and reference voltages equal.
device over at least part of the range over which 13. The method of claim 12 in which said reference
the impedance of said device varies as a result of voltage is varied to vary the operating temperature of
temperature change, whereby as the temperature 45 the load.
of said device varies over said region the extent of 14. The method of claim 12 in which the following
mismatch of the impedances also changes, series of events repeatedly occurs: said current to the
said means further comprising means for producing a load is turned off when said reflected voltage reaches
signal indicative of reflected power that varies as a equality with said reference voltage and when the load
function of the impedance mismatch, and 50 cools to a predeterminable extent the current to the load
control means responsive to said signal for varying is restored.
said pulsating current in order to regulate the tem 15. The method of claim 13 in which the current is a
perature of said device. pulsating current which intermittently goes on and off
2. A temperature regulator as defined in claim 1 in to hold the load temperature substantially constant.
which said device is a ferromagnetic element whose 55 16. The method of claim 14 in which said load is
permeability changes with temperature to thereby provided with magnetic properties so that its impe
change the impedance of the device to said pulsating dance to said current varies with changes in the temper
current when its temperature changes. ature of the load.
3. A temperature regulator as defined in claim 1, in 17. The method of claim 15 in which the periods of
which said device comprises a doped barium titanate time during which the current is off are substantially
resistance element. equal.
4. A temperature regulator as defined in claim 1 in 18. The method of claim 15 in which each off period
which said device is a non-magnetic device which has a is for a predetermined time.
substantial temperature coefficient of resistance. 19. A device having an electrical impedance which
5. In a temperature regulator as defined in claim 1, in 65 varies with variations in temperature of the device,
which said responsive means controls said current to means for supplying pulsating current to said device
said device to hold the temperature of said device sub to heat the same and comprising a source of radio
stantially constant. frequency current,
 4,795,886
11 12
an impedance matching device between said source proaching the Curie temperature of said ferromag
and said device for at least approximately matching netic material.
the impedance of said device to said source, 21. In a temperature control
whereby there is a reflected voltage that has a a ferromagnetic element having a permeability above
magnitude dependent on the degree of mismatch, 5 100 when said element is well below its effective
said means having an impedance which is mis Curie temperature and a permeability of about
matched with the impedance of said device over at unity near its effective Curie, and
least part of the range over which the impedance of means for feeding said element with pulsating cur
said device varies, whereby as the temperature of 10 rent,
said device changes the extent of mismatch of the said first-named means producing a voltage that in
impedances also changes, and creases when the temperature of said element in
control means being responsive to the magnitude of ind approaches said effective Curie ten
said reference voltage to at least reduce the current said first-named means including means for detecting
supplied by said source to the load in response to 15 when said voltage increases to a selected threshold
one amount of mismatch, said current being re- and for at least reducing the magnitude of said
stored in response to another amount of mismatch, current when said threshold is reached, with said
whereby the current to said device changes to current being restored to its former value when
control the temperature of said device. said element cools, whereby said current occurs in
20. In a temperature regulator as defined in claim 1920 a series of pulses of a length and spacing that will
or claim 6 or claim 9 wherein hold the temperature of said element substantially
said device includes a ferromagnetic material heated Constant.
by said alternating current to a temperature ap
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