US3758844A - Control circuit for load having measureable coefficient of resistance - Google Patents
Control circuit for load having measureable coefficient of resistance Download PDFInfo
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- US3758844A US3758844A US00242694A US3758844DA US3758844A US 3758844 A US3758844 A US 3758844A US 00242694 A US00242694 A US 00242694A US 3758844D A US3758844D A US 3758844DA US 3758844 A US3758844 A US 3758844A
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- circuit
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- power control
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
Definitions
- FIG. 2 is a DC wave form taken from a full wave rectified AC supply.
- FIG. 6 is a combination of the wave forms shown in FIGS. 4 and 5 wherein the load is under simulated actual on and off" controllingconditions, and
- Power is applied to the power supply 12 in the form of an AC voltage coupled to a bridge rectifier l9.
- the unfiltered DC output of rectifier 19 via lines 20 and 21 is shown in FIG. 2. This, at the appropriate time, is switched to the load 11 by the power control circuit 14.
- a filtered and zener regulated voltage source for the various control circuits is supplied from a capacitor 23, resistor 24 and zener diode 25.
- the regulated voltage in one instance, is applied via leads 26 and 27 to the amplifier 28 in the pulsing circuit 13.
- Amplifier 28 maybe of the DC high gain integrated circuit type having a plus input 29 and a minus'input 30.
- a plus input passes the signal to the amplifier output without changing polarity.
- the minus input passes the signal to the amplifier output but reverses its polarity.
- Negative input 30 of the amplifier is coupled to the bridge rectifier output 20 and receives the unfiltered full wave DC voltage 32 (FIG. 2).
- amplifier 28 will achieve its most positive voltage state. This results in the pulsed amplifier output as shown at 31 (FIG. 3).
- the amplifier sampling signal or pulse 31 is applied through conductor 36 to the base 37 of a transistor 38.
- Output current of transistor 38 is applied to a transistor 39 which in turn causes a transistor 40 to switch or conduct at pulsed intervals.
- the transistor 40 has its output circuitcoupled between the full wave DC power supply line 20 and the monitoring circuit 15 (the latter containing the load 11).
- the current pulses 41 occur near orat the zero voltage crossings of the supply voltage.
- amplifier 46 When the voltage at point Y reaches a level just slightly less than point X, amplifier 46 will switch to its low state. This turns off transistors 38, 39 and 40 in the power control circuit 14. The power control circuit will then stay off until the next sampling pulse 41 at half wave intervals causes-it to turn on momentarily.
- the amplifier 46 will turn on the power control circuit 14 via conductor 49. Full wave unfiltered DC voltage is then applied to the heater load 11 as shown at 50 in FIG. 5.
- means are provided through the safety circuit 17 for the automatic shut-down of the load 11 should it become shortexceedsX by a predetermined level such as when the load in its coldest condition. For example, if the heater is shorted or carries excess current, point'Y will be more positive than it would be during its coldest operating design temperature level.
- Amplifier 53 then switches to its maximum positive output which is applied to drive transistor 59 into heavy conduction. This diverts current normally supplied to switching circuit transistor 38 by either or both of the amplifiers 28 and 46. The current diversion will turn off transistor 38 thereby turning off the power control circuit l4 and consequently removing power from load 11.
- a unijunction pulser 60 (FIG. 7) or similar relaxation generator device may be substituted for the power supply 12 and amplifier 28.
- the unijunction pulser would'connect at points 61-, 62, 63 and 64 in the circuit of FIG. 1.
- a DC power supply 64 is used to charge a capacitor 65 through a resistor 66.
- the RC circuit is coupled to the emitter 67 of a unijunction 68.
- the sampling pulse rate is determined by the characteristics of the resistor and capacitor.
- comparator means coupled to the output of said monitoring means and coupled to the input of said power control means to energize said power control means independently of said pulsing means when said DC output reaches said predetermined low level.
- a circuit comprising,
- pulsing means for generating a sampling pulse which generally coincides with said predetermined DC low value as said AC supply approaches said zero crossings
- said pulsing means being operably coupled to the input of said power control means so as to energize said power control means when said AC supply is close to the zero crossings thereof to thereby apply power pulses to said load,
- monitoring means coupled to said load for determining the relative resistance of said load and adapted to provide a signal as to the condition thereof as said load is energized by said pulsing means
- comparator means coupled to said monitoring means and to said power control means operable to maintain said power control means energized independently of said pulsing means when said load has a predetermined change in resistance.
- a circuit comprising,
- power control means having an input and an output with the input being energized by pulses from said pulsing means
- monitoring means for determining the relative resis tance of said load and adapted to provide a signal indication thereof
- comparator means coupled to the output of said monitoring means and operatively coupled to said power control means to keep said power control means energized independently of said pulsing means so as to apply power to said load when the load resistance varies a predetermined amount as supply furnishes fluctuating DC voltage and said pulsing means sends a pulse when the voltage drops to a lower level.
- said monitoring means includes a bridge and wherein said load is in one leg of said bridge.
- said comparator means includes a feed-back type amplifier having its input coupled to said bridgeand its output coupled to said power control means.
- protection means includes means coupled to said pulsing means and to said comparator means to divert current from both of said means to de-energize said power control means.
- a circuit as claimed in claim 12 wherein said protection means includes a transistor having its input coupled to the output of an amplifier and its output coupled to the output of said pulsing means and said comparator means.
Abstract
A control circuit wherein the load acts as its own transducer and wherein the circuit may be provided with means for zero switching and overload protection.
Description
[11] 3,758,844 1 1 Sept. 11, 1973 United States Patent Harkenrider et a1.
- CONTROL'CIRCUIT FOR LOAD HAVING 3,125,715 3/1964 'Br06ks............
MEASUREABLE'COEFFICIENT 0F RESISTANCE 3,309,602 3/1967 Euvino et a1. 3,381,226 3,390,275
4/1968 Jones et a1. 6/1968 11/1968 5/1971 12/1971 Baker........... Crane.......... 323/22 ZS 323/22 T 323/22 T 307/252 UA Grady, Jr.
Moe, both of Winona, Minn.
Assignee: Waynco, Inc., Winona,
3,626,278 Matsumura Appl. No.: 242, 94
22 'Filed:
Primary ExaminerWi11iam M. Shoop, Jr.
AttorneyRichard .1. Renk 323/22 T, 307/133, 307/252 UA O Z //U M 3 TL "C U .a u e U s L h C 10 .M t C U h m 111 2 00 [56] References ducer and wherein the circuit may be prc'wided with 1 means for zero switching and overload protection. UNITED'STATES PATENTS 6/1972 Hendrickson................ 307 252 UA 14 Claims 7 Drawing Figures PATENTEU SEN I 1975 saw 11H 1 CONTROL CIRCUIT FOR LOAD HAVING MEASUREABLE COEFFICIENT OF RESISTANCE BACKGROUND OF INVENTION SUMMARY OF INVENTION The present invention eliminates noise andresolution problems inherent with AC circuit applications in controls wherein the load also acts as'its own sensor by providing a novel means of switching at or near the zero crossings of the AC supply. This is accomplished by sending a sampling pulse' through the load as the AC voltage is close to a zero voltage crossing. The inven tion also provides for the automatic shut-down of power to the load by a safety circuit which is referenced to the maximum load current desired and shuts the circuit down in the event it is overloaded or shortcircuited.
DESCRIPTION OF DRAWINGS. I
FIG. 1 is a schematic circuit illustrating the principles of the invention.
FIG. 2 is a DC wave form taken from a full wave rectified AC supply.
FIG. 3 is a wave form which isused as a pulsing signal initiated as the DC wave of FIG. 2 approaches the zero crossing of its AC supply.
FIG. 4 shows the wave form of the pulse power or current applied to the load when the load'is above its control point.
FIG. 5 is a wave form of the power or current applied to the load when the load is below its control point and requires full power.
FIG. 6 is a combination of the wave forms shown in FIGS. 4 and 5 wherein the load is under simulated actual on and off" controllingconditions, and
FIG. 7 is a schematic view of an alternate embodiment of the invention-wherein a DC supply is used instead of an AC supply.
PREFERRED EMBODIMENTS Reference is now made to FIG. 1 of the drawings which shows a representative circuit embodying the concepts of the invention. While not necessarily limited thereto, the circuit indicated at 10 is described in conjunction with the temperature control of a heater load 11. The load has a measureable temperature coefficient of resistance; in the present case it has a positive coefficient. In other words, the higher the temperature the greater the load resistance. As shown in FIG. 1, the embodiment includes a power supply 12, a pulsing or sampling signal source 13, a power control or switching circuit 14, a monitoring circuit 15, a comparator circuit 16, and a safety shut-down circuit 17.
Power is applied to the power supply 12 in the form of an AC voltage coupled to a bridge rectifier l9. The unfiltered DC output of rectifier 19 via lines 20 and 21 is shown in FIG. 2. This, at the appropriate time, is switched to the load 11 by the power control circuit 14.
A filtered and zener regulated voltage source for the various control circuits is supplied from a capacitor 23, resistor 24 and zener diode 25. The regulated voltage, in one instance, is applied via leads 26 and 27 to the amplifier 28 in the pulsing circuit 13.
A sampling or pulse signal indicated at 31 (FIG. 3) is generated by the amplifier 28. The pulse signal output 31 is made to trigger as the output 32 (FIG. 2) from the bridge rectifier 19 approaches zero (Point 33, FIG. 2); in other words as the AC line input from terminals 18 nears o'r approaches the zero voltage crossings thereof.
To achieve the pulse '31, the positive input at amplifier terminal 29 is referenced to the voltage level of about 5 volts across resistor 34. It is derived from the regulated supply including the zener 25 and may be typically set at'about 5 volts by the voltage divider resistors 34 and 35.
In function, each time the amplifier 28 sends out a pulse 31 (FIG. 3), the power control circuit 14 switches on for the same duration. Current pulses as shown by the solid lines 41 in FIG. 4 are then applied to the moni-.
toring circuit 15 and load 11. As shown in FIGS. 2 4, the current pulses 41 occur near orat the zero voltage crossings of the supply voltage.
As shown in FIG. 1, the monitoring circuit 15 includes a bridge 42' having the load 11 in one leg, followed by a low value resistor 42 in an adjacent leg, a resistor 43 in the next leg, and a resistor 44 and resistance potentiometer 45 in a fourth leg. Resistor 42 must pass all of the load or heater current; for high efficiency and minimum power loss it is typically less than 1/ 10 the resistance of the load 11. It thus acts as a current shunt.
To detect the condition of the monitoring circuit 15, the bridge output at points X and Y is coupled to the comparator circuit 16 and its amplifier 46. The amplifier 46 is used as a switching differential comparator. It acts as a switch by using a positive feedback resistor 47 to return a portion of the amplifier output to the plus input 48.
When the bridge output voltage at point Y is, for example, millivolts more positive than the voltage at point X, the amplifier 46 output will be at its most positive value. It will stay in such condition until the voltage at point X is slightly more positive than the voltage at point Y. By the amplifier switching from a low to a high output in response to the output of the bridge 42, the comparator circuit provides an indication of thebridge condition. I
In operation, assume the load resistor 11 is a heater being controlled and it is below the temperature set by the potentiometer 45. In such case, the resistance of the load. 11 is low and the pulse 41 (FIG. 4) applied thereto will appear more positive at Y than at X. Amplifier 46 will then switch to its more positive output value which is then applied via conductor 49 to the base 37 of the transistor 38. Transistor 38 is now held in a conductive mode thereby turning on transistors 39 and 40. Full-wave rectified DC voltage as at 50 (FIG. 5) is then applied to the monitoring circuit including the bridge and heater 11. Initial power switching via the pulse 41 is always done near or at the zero crossings of the AC supply voltage to reduce the generation of RF I effects.
As the load 11 increases in temperature its resistance will increase. Current through the load will then decrease as will the voltage at point Y.
When the voltage at point Y reaches a level just slightly less than point X, amplifier 46 will switch to its low state. This turns off transistors 38, 39 and 40 in the power control circuit 14. The power control circuit will then stay off until the next sampling pulse 41 at half wave intervals causes-it to turn on momentarily.
If the load is too hot, the amplifier 46 will stay in its low state and no further power (except for the pulse 41) will be applied to the load for the remaining portion of the half wave supply cycle. The wave form of the pulse 41 applied to the heater is shown in FIG. 4 for the condition of the heater or load being above the setpoint temperature. Dotted lines 51 indicate the time the power is off.
Once the heater 11 has cooled sufficiently to provide the predetermined positive output from the bridge at point Y, the amplifier 46 will turn on the power control circuit 14 via conductor 49. Full wave unfiltered DC voltage is then applied to the heater load 11 as shown at 50 in FIG. 5.
. After the load has reached its set-point temperature, the power applied to the load may take the form shown in FIG. 6. The ratio ofon to off time to keep the heater at the correct average temperature will depend on the condition being controlled. Excellent control resolution is achieved since the power control means 14 may be turned off at any point in the voltage cycle. An example is shown in FIG. 6 wherein comparator circuit 16 detects point X being more positive than Y midway in the cycle as indicated at 52 and immediately shuts off the power control circuit 14. This can occur at any point in the half cycle (except for the pulse period 41).
As another feature of the invention, means are provided through the safety circuit 17 for the automatic shut-down of the load 11 should it become shortexceedsX by a predetermined level such as when the load in its coldest condition. For example, if the heater is shorted or carries excess current, point'Y will be more positive than it would be during its coldest operating design temperature level.
Amplifier 53 then switches to its maximum positive output which is applied to drive transistor 59 into heavy conduction. This diverts current normally supplied to switching circuit transistor 38 by either or both of the amplifiers 28 and 46. The current diversion will turn off transistor 38 thereby turning off the power control circuit l4 and consequently removing power from load 11.
Once amplifier 53 switches to its most positive state, positive feedback via resistor 54 causes it to latch-up and neither the positive nor the negative inputs can affect its high positive output. To place the circuit in operation again, it is necessary to reset by turning the circuit off and then turning it on again. This causes circuit 17 to unlatch.
For use with a DC power supply such as found in automobiles, trucks, aircraft, etc., a unijunction pulser 60 (FIG. 7) or similar relaxation generator device may be substituted for the power supply 12 and amplifier 28. The unijunction pulser would'connect at points 61-, 62, 63 and 64 in the circuit of FIG. 1. In this embodiment a DC power supply 64 is used to charge a capacitor 65 through a resistor 66. The RC circuit is coupled to the emitter 67 of a unijunction 68. The sampling pulse rate is determined by the characteristics of the resistor and capacitor.
While the invention has been described in conjunction with the controlling of a heater type load, the concepts can also be applied to controlling other loads where the resistance changes with a change in condition. The load is thus used as the sensor to initiate control action.
It is of course obvious that different type amplifiers and voltages may be used than those described hereinabove.
We claim:
1. A circuit comprising,
a power supply,.
means coupled to said power supply for supplying a fluctuating DC output,
pulsing means coupled to said means for producing said fluctuating DC output, said pulsing means being adapted to send an energizing pulse when said DC output reaches a predetermined low level,
power control means having input and output circuits with the input circuit being energized by pulses from said pulsing means,
a load having a pre-selected coefficient of resistance coupled to the output circuit of said power control means,
means for monitoring the relative resistance of said load and adapted to provide a signal indication thereof when said power control means is energized by said pulsing means, and.
comparator means coupled to the output of said monitoring means and coupled to the input of said power control means to energize said power control means independently of said pulsing means when said DC output reaches said predetermined low level.
2. A circuit comprising,
an AC power supply,
means for converting said AC supply to a DC voltage with the DC voltage dropping periodically to a predetermined low value as the AC supply approaches its zerocrossings,
pulsing means for generating a sampling pulse which generally coincides with said predetermined DC low value as said AC supply approaches said zero crossings,
a load having a pre-selected coefficient of resistance which changes as the condition .of said load changes,
power controlmeans operably connected between said load and said power supply, said power control means having an input and an output,
said pulsing means being operably coupled to the input of said power control means so as to energize said power control means when said AC supply is close to the zero crossings thereof to thereby apply power pulses to said load,
monitoring means coupled to said load for determining the relative resistance of said load and adapted to provide a signal as to the condition thereof as said load is energized by said pulsing means, and
comparator means coupled to said monitoring means and to said power control means operable to maintain said power control means energized independently of said pulsing means when said load has a predetermined change in resistance.
3. A circuit comprising,
a power supply pulsing means being adapted to send an energizing pulse at spaced intervals,
power control means having an input and an output with the input being energized by pulses from said pulsing means,
a load having a pre-selected coefficient of resistance coupled to the output of said-power control means,
monitoring means for determining the relative resis tance of said load and adapted to provide a signal indication thereof,
comparator means coupled to the output of said monitoring means and operatively coupled to said power control means to keep said power control means energized independently of said pulsing means so as to apply power to said load when the load resistance varies a predetermined amount as supply furnishes fluctuating DC voltage and said pulsing means sends a pulse when the voltage drops to a lower level.
5. A circuit as claimed in claim 3 wherein said pulsing means includes a relaxation type pulse generator.
6.A circuit as claimed in claim 5 wherein said pulsing means includes a unijunction transistor.
7. A claim as claimed in claim 3 wherein said power supply includes an AC source and wherein said power control means may be turned off at any point in the AC wave.
8. A circuit as claimed in claim 3 wherein said monitoring means includes a bridge and wherein said load is in one leg of said bridge.
9. A circuit as claimed in claim 3 wherein said comparator means includes a feed-back type amplifier having its input coupled to said bridgeand its output coupled to said power control means.
10. A circuit as claimed in claim 3 wherein said protection means includes means coupled to said pulsing means and to said comparator means to divert current from both of said means to de-energize said power control means.
12. A circuit as claimed in claim 3 wherein said protection circuit once energized prevents said power control means from applying power to said load until the circuit is re-set.
13. A circuit as claimed in claim 12 wherein said protection means includes a transistor having its input coupled to the output of an amplifier and its output coupled to the output of said pulsing means and said comparator means.
14. A circuit as claimed in claim 13 wherein said protection means has the amplifier input coupled to said monitoring means.
Claims (14)
1. A circuit comprising, a power supply, means coupled to said power supply for supplying a fluctuating DC output, pulsing means coupled to said means for producing said fluctuating DC output, said pulsing means being adapted to send an energizing pulse when said DC output reaches a predetermined low level, power control means having input and output circuits with the input circuit being energized by pulses from said pulsing means, a load having a pre-selected coefficient of resistance coupled to the output circuit of said power control means, means for monitoring the relative resistance of said load and adapted to provide a signal indication thereof when said power control means is energized by said pulsing means, and comparator means coupled to the output of said monitoring means and coupled to the input of said power control means to energize said power control means independently of said pulsing means when said DC output reaches said predetermined low level.
2. A circuit comprising, an AC power supply, means for converting said AC supply to a DC voltage with the DC voltage dropping periodically to a predetermined low value as the AC supply approaches its zero crossings, pulsing means for generating a sampling pulse which generally coincides with said predetermined DC low value as said AC supply approaches said zero crossings, a load having a pre-selected coefficient of resistance which changes as the condition of said load changes, power control means operably connected between said load and said power supply, said power control means having an input and an output, said pulsing means being operably coupled to the input of said power control means so as to energize said power control means when said AC supply is close to the zero crossings thereof to thereby apply power pulses to said load, monitoring means coupled to said load for determining the relative resistance of said load and adapted to provide a signal as to the condition thereof as said load is energized by said pulsing means, and comparator means coupled to said monitoring means and to said power control means operable to maintain said power control means energized independently of said pulsing means when said load has a predetermined change in resistance.
3. A circuit comprising, a power supply pulsing means being adapted to send an energizing pulse at spaced intervals, power contrOl means having an input and an output with the input being energized by pulses from said pulsing means, a load having a pre-selected coefficient of resistance coupled to the output of said power control means, monitoring means for determining the relative resistance of said load and adapted to provide a signal indication thereof, comparator means coupled to the output of said monitoring means and operatively coupled to said power control means to keep said power control means energized independently of said pulsing means so as to apply power to said load when the load resistance varies a predetermined amount as determined by said monitoring means, and protection means coupled to said power control means for de-energizing said power control means to keep excess current from being applied to said circuit.
4. A circuit as claimed in claim 3 wherein said power supply furnishes fluctuating DC voltage and said pulsing means sends a pulse when the voltage drops to a lower level.
5. A circuit as claimed in claim 3 wherein said pulsing means includes a relaxation type pulse generator.
6. A circuit as claimed in claim 5 wherein said pulsing means includes a unijunction transistor.
7. A claim as claimed in claim 3 wherein said power supply includes an AC source and wherein said power control means may be turned off at any point in the AC wave.
8. A circuit as claimed in claim 3 wherein said monitoring means includes a bridge and wherein said load is in one leg of said bridge.
9. A circuit as claimed in claim 3 wherein said comparator means includes a feed-back type amplifier having its input coupled to said bridge and its output coupled to said power control means.
10. A circuit as claimed in claim 3 wherein said protection means includes means coupled to said pulsing means and to said comparator means to divert current from both of said means to de-energize said power control means.
11. A circuit as claimed in claim 10 wherein said protection means includes a feed-back type amplifier which has its input referenced to a voltage taken from a bridge in said monitoring circuit.
12. A circuit as claimed in claim 3 wherein said protection circuit once energized prevents said power control means from applying power to said load until the circuit is re-set.
13. A circuit as claimed in claim 12 wherein said protection means includes a transistor having its input coupled to the output of an amplifier and its output coupled to the output of said pulsing means and said comparator means.
14. A circuit as claimed in claim 13 wherein said protection means has the amplifier input coupled to said monitoring means.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US24269472A | 1972-04-10 | 1972-04-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| US3758844A true US3758844A (en) | 1973-09-11 |
Family
ID=22915829
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| US00242694A Expired - Lifetime US3758844A (en) | 1972-04-10 | 1972-04-10 | Control circuit for load having measureable coefficient of resistance |
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| US (1) | US3758844A (en) |
Cited By (6)
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| US3914625A (en) * | 1973-08-13 | 1975-10-21 | Westinghouse Electric Corp | Zero crossover circuit |
| US4065805A (en) * | 1975-04-16 | 1977-12-27 | Sprecher & Schuh Ag | Circuit arrangement in an electrical device operated with direct-current, especially in a timing relay |
| US4736091A (en) * | 1986-09-10 | 1988-04-05 | Watlow/Winona, Inc. | Integral sensor controller for an electrical heater |
| US6580255B1 (en) * | 2002-02-12 | 2003-06-17 | Technical Witts, Inc. | Adaptive gate drivers for zero voltage switching power conversion circuits |
| WO2003085813A3 (en) * | 2002-04-04 | 2004-02-19 | Thomson Licensing Sa | Line frequency switching regulator |
| EP1692920A2 (en) * | 2003-11-21 | 2006-08-23 | Watlow Electric Manufacturing Company | Two-wire layered heater system |
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| US4065805A (en) * | 1975-04-16 | 1977-12-27 | Sprecher & Schuh Ag | Circuit arrangement in an electrical device operated with direct-current, especially in a timing relay |
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| US6580255B1 (en) * | 2002-02-12 | 2003-06-17 | Technical Witts, Inc. | Adaptive gate drivers for zero voltage switching power conversion circuits |
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| EP1692920A4 (en) * | 2003-11-21 | 2008-01-16 | Watlow Electric Mfg | Two-wire layered heater system |
| US20070138166A1 (en) * | 2003-11-21 | 2007-06-21 | Watlow Electric Manufacturing Company | Two-wire layered heater system |
| US20120292308A1 (en) * | 2003-11-21 | 2012-11-22 | Watlow Electric Manufacturing Company | Two-wire layered heater system |
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