WO2001031805A1

WO2001031805A1 - Universal switch interface

Info

Publication number: WO2001031805A1
Application number: PCT/US2000/029076
Authority: WO
Inventors: Tracy L. Lentz; Edward L. Schwarz
Original assignee: Honeywell Inc.
Priority date: 1999-10-26
Filing date: 2000-10-20
Publication date: 2001-05-03
Also published as: AU1098401A

Abstract

A switch interface for a powered humidifier or other device whose operation is controlled by a remotely located switch, is compatible with both a remote electromechanical switch and with a remote electronic switch. Each type of switch may be connected to the interface by a single pair of wires. The remote electronic switch is designed to receive operating power from the interface on the single pair of wires.

Description

UNIVERSAL SWITCH INTERFACE

BACKGROUND OF THE INVENTION

Certain types of systems operate under the control of a remotely located switch. Typically these switches sense some condition local to the switch and influenced by the condition, open and close so as to operate the system in a way which maintains the local condition within a predetermined set point range. The classic example is a space-heating furnace or an air conditioner operating to maintain the temperature local to a thermostat controlling the space temperature within the set point setting of the thermostat . The temperature is the local condition and the thermostat is the remotely located switch with sensing capabilities . With humidity control more common now than formerly, humidifiers and dehumidifiers operating upon demand from a humidistat are another type of such system. The controlled condition is local humidity which is sensed by the humidistat, whose switch closes to activate the humidifier or dehumidifier when necessary to bring the local humidity value within the set point range . In HVAC systems such as furnaces, air conditioners, humidifiers, etc, the norm in the US is 24 v. AC for the control voltage switched by the remote switch. These systems' controllers require relatively low amperage when operating at 24 v. AC power, so low amperage remote switches are more than adequate .

Remote switches can be classified into two different groups that we will call passive and powered. An example of a passive switch is the typical electromechanical thermostat having a coiled bimetal strip which operates to tilt a mercury bulb switch and in this way open and close a switch responsive to temperature changes. Passive humidistats use a humidity-sensitive thread or other element to operate a switch which closes to activate the humidifier or dehumidifier.

Powered switches receive power from some source to operate circuitry which opens and closes a switch electronically. The switch could be relay contacts, or could be a transistor or other solid state device. Many powered switches have on-board batteries which provide power for operating the circuitry. As a further example, many different models of electronic thermostats have been developed. By the use of low current circuit components, battery life is typically at least one heating or cooling season. But the need to occasionally replace batteries is inconvenient, so there have been developed alternative remote powered switches which use permanently available power sources . In new construction, it is easy to install the extra pair of wires for supplying power to a powered remote switch. But where the remote switch is installed in a completed building, installing power supply wires is more difficult. Because of this problem, remote switch designs often employ some system for supplying power on the same pair of conductors that indicate the state of the remote switch. Some of these designs employ a power stealing circuit, which uses a small portion of the low voltage AC waveform to keep a capacitor charged, which then provides the small amount of DC power needed for the electronic circuitry. In another design, the controller for the system supplies DC power on the pair of conductors which a powered remote switch can use to operate its electronics. So as to constantly have power available, such a "switch" is designed to signal a demand for environmental or other control by changing its impedance from a higher finite value to a lower finite value rather than from infinite to zero impedance as switches typically do. The voltage drop remaining across the variable impedance at its lower value is sufficiently large to operate the switch's electronics. Of course, the controller for interfacing with such a remote switch must be specifically designed to detect such impedance changes. Presently this type of remote switch is becoming available for use in humidifiers.

U.S. Patent No. 5,635,896 having a common assignee with this application, shows a communication system having a two wire connection between a remote switching module operating on DC power and a local decoding module which also supplies operating power to the remote module.

BRIEF DESCRIPTION OF THE INVENTION We have realized that it is helpful if the controller in a humidifier or other system is compatible with both powered and passive remote switches. Our invention comprises a switch interface for the controller which is compatible with any type of passive switch, and with properly designed powered switches as well. Even though such a system may be slightly more complex, the fact that only one type of controller need be manufactured and stocked allows substantial savings to scale, so the actual effective cost may be less than having two different types of controllers. Although our design is specifically intended for a powered humidifier, it is also directly relevant to thermostatically controlled furnaces, and perhaps other controlled systems as well .

We implement our invention in a switch interface for switching power to a local system responsive to the conduction status of a remotely located switch in electrical connection to first and second sensing terminals on the controller. The interface is compatible with both passive and powered versions of the remotely located switch. The interface provides through the sensing terminals, DC voltage to operate powered versions of the switch. This interface comprises a power supply providing a DC voltage of at least a predetermined level across first and second DC power terminals. The second DC power terminal serves as the second sensing terminal. A first impedance has a first terminal connected to the first power terminal, and a second terminal which serves as the first sensing terminal .

A switch status detector is connected across the first impedance ' s terminals and provides a voltage level signal having a first value when the voltage across the first impedance ' s terminals is above a preselected fraction of the power supply DC voltage, and a second value otherwise.

A switching circuit receives the detector's voltage level signal at a control terminal . The switching circuit has first and second switching terminals . The switching circuit sets the impedance between the switching terminals to high and low levels responsive respectively to the first and second levels of the voltage level signal.

Thus the impedance across the sensing terminals controls the amount of current which flows through the impedance. When this impedance is low or zero, current flow is relatively high through the controller impedance and the voltage across the impedance is also high. This high voltage is detected by the status detector, which provides the first value of the voltage level signal to the switching circuit. This value signals the switching circuit to electrically connect the switching terminals, which allows electrical power to flow to a selected device. When the impedance rises across the sensing terminals, such as when a humidistat or thermostat opens, then the detector senses a reduced voltage across the impedance and changes its voltage level signal to the second value. The switching circuit opens the connection between its switching terminals disconnecting the selected device from electrical power.

At the same time, the voltage provided at the sensing terminals allows a powered switch to extract sufficient power from the power supply to operate the circuitry within the switch. Such circuitry may in the case of a humidistat provide an electronic display, electronic humidity sensing, and relative humidity or dew point calculation, as well as perform the switching which signals a call for humidification. When there is no call for humidification the powered humidistat version operates by drawing a relatively small amount of current and which is below the level which will cause the voltage across the first impedance to exceed the preselected voltage level . When humidification is required, a switch internal to the humidistat is closed which reduces the impedance across the sensing terminals. This decreased impedance across the sensing terminals increases current flow through the humidistat and the first impedance sufficiently to increase the voltage across the first impedance to above the preselected level. The detector senses this condition and provides the first value of the voltage level signal, causing the switching circuit to connect the switching terminals .

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a humidifier including blocks comprising the interface forming the subject of the invention.

Fig. 2 is a circuit diagram of the interface comprising the invention, along with a circuit and block diagram of a representative powered remote switch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The block diagram of Fig. 1 shows the relevant portions of a simple powered humidifier 1 including its fan motor 22 for drawing air through an evaporation chamber and its valve solenoid 14 for controlling flow of water to the evaporation chamber. Humidifier 1 includes a a special interface 10 allowing either a powered or a passive remote condition- sensing humidistat switch 11 to activate humidifier 1. Interface 10 performs a quite simple control function for humidifier 1 which is to simultaneously activate the line power fan motor 22 and a 24 v. valve controlled by solenoid 14 when humidity is to be added to the air. While the system shown is quite rudimentary, the principles are equally applicable to more complex systems such as air circulating systems, lighting controls using timers and occupancy sensors, and HVAC systems using thermostats. The switch 11 is connected to interface 10 through two wires 60a and 60b, providing compatibility with two existing wires when a new humidifier replaces an existing humidifier controlled by a mechanical (passive) humidistat switch. Conductors 14 provide line AC power to fan motor 22 through Kl relay contacts 20b and to a transformer 17 providing nominal 24 v. AC to the remainder of the system. 24 v. AC operates valve solenoid 14 under the control of Kl relay contacts 20a. A DC power supply 32 receives 24 v. AC power from transformer 17, and provides around 30 - 40 v. DC on conductors 35b and 35c, conductor 35c being chosen as positive in this configuration. Conductor 35b may be considered to be the neutral or ground conductor.

As those with skill in the art know well, the 24 v. designation is RMS voltage, with peak voltage being _2 x RMS voltage or _2 x 24 = 34 v. For this reason it is possible to provide with even a half wave rectifier, a properly filtered DC voltage that is higher than the RMS input AC voltage, provided the load impedance is high relative to the effective power supply internal impedance. Because various types of step-down transformers 17 provide anywhere from 24 to 28 v. AC, the DC voltage provided by power supply 32 can be as high as 40 v. DC.

Conductor 35c is connected to a first terminal of a current-sensing impedance 30. A second terminal of impedance 30 is connected to conductor 35a. We expect that in most cases impedance 30 will comprise a simple resistor with a value of a few hundred ohms . The value of impedance 30 must be low enough to provide sufficient current for operating a powered switch 11. At the same time, the value of impedance 30 must be high enough to avoid overloading the mechanical contacts of a passive switch 11, and for that matter, causing impedance 30 to generate excessive heat. We currently believe that a typical powered switch 11 will operate reliably on about 60 ma. Current through impedance 30 in the range of 50 to 100 ma. when used with a passive version of switch 11 is probably acceptable both for operating a powered switch 11 and for limiting the amount of heat generated by impedance 30. For a power supply 32 voltage of 34 v. , this suggests that a resistor serving as impedance 30 might have a value in the range of 300 - 700 _. Once the operating parameters of a powered switch 11 have been specified, a person of skill in the art can easily specify a suitable value for impedance 30.

Impedance 30 converts the current flow from power supply 32 into a voltage across impedance 30. Conductors 35a and 35b are to be connected by wires 60a and 60b to terminals 61a and 61b of a remote condition-sensing switch 11. A pull-down resistor 25 whose value will usually be a couple of orders of magnitude greater than that of impedance 30 is connected between conductors 35a and 35b. Resistor 25 prevents the voltage at conductor 35a from floating when conductor 35a is not attached to a definite voltage potential, which can happen when a passive switch 11 is connected to terminals 35a and 35b and is open.

A switch status detector 27 is connected to compare the voltage between the first and second terminals of impedance 30 with a first preselected voltage value, which might be perhaps half the voltage provided by power supply 32 across conductors 35c and 35b. Detector 27 provides a switch status signal on conductor 28 having a first value when the voltage across impedance 30 is greater than the first preselected voltage value, and a second value otherwise. The switch status signal is used to activate a Kl switch operator 20. We expect that switch operator 20 will typically be a relay, for which dotted lines 20c symbolize control of the normally open switch contacts 20a and 20b. In line with customary practice, this relay is designated the Kl relay; the contact pairs of the Kl relay are designated as Kl contacts. When the first value of this switch status signal is present, operator 20 causes its contacts 20a and 20b to close. When the second value of the switch status signal is present, the switch operator 20 causes the contacts 20a and 20b to open.

When impedance is placed across conductors 35a and 35b, or impedance already present is reduced, additional current flows through impedance 30. If this impedance across conductors 35a and 35b is sufficiently small, this additional current increases the voltage drop across impedance 30 to above the first preselected voltage level. Detector 27 senses this increased voltage and responds by setting the switch status signal on path 28 to its first level. The first level of the switch status signal activates switch operator 20 to close contacts 20a and 20b. Closed contacts 20a and 20b electrically connects fan motor 22 to line power AC and valve solenoid 14 to stepped down AC voltage from transformer 17 allowing humidifier 1 to operate .

If the impedance across conductors 35a and 35b is provided by a simple passive humidistat or other mechanical switch functioning as the condition-sensing switch 11, voltage across impedance 30 rises to essentially the voltage at conductor 35b, which is far above the first preselected level when switch 11 closes. In response, detector 27 provides the first level of the switch status signal to operator 20.

An electronic humidistat may also function as the powered switch 11 forming the varying impedance across conductors 35a and 35b. Such a switch must be designed to present a relatively high impedance to conductors 35a and 35b corresponding to no demand for operation of the controlled device, and a relatively low impedance, but substantially greater than no impedance, corresponding to a demand for operation of the humidifier or other controlled device. The higher switch 11 impedance must limit current flow to a level which produces across impedance 30, a voltage lower than the first preselected voltage level. The low impedance level presented by a powered switch 11 must allow enough current flow to generate a voltage across impedance 30 exceeding the first preselected voltage level. However, the low impedance of the powered switch must also be chosen to provide adequate voltage across terminals 35a and 35b for operating the powered switch's electronics. As an example only, if power supply 32 provides 40 v. DC at terminals 35b and 35c, then the higher impedance of switch 11 might be perhaps three times the value of impedance 30. The lower impedance of switch 11 might be a third the value of impedance 30. This will cause the voltage across impedance 30 to vary from 3/4 of the power supply 32 voltage to 1/4 the power supply 32 voltage. Even with only 1/4 of the voltage across power supply 32 available for operating the electronics of a powered switch 11, this will still be over 8 v. for a power supply 32 voltage of 40 v. AC. This will be more than adequate for operating a variety of electronic circuit components including many suitable microcontrollers . Fig. 2 is a circuit diagram showing a preferred embodiment for the interface 10 of Fig. 1, as well as a circuit configuration for a powered condition-sensing switch 11 compatible with interface 10. Similar elements in Fig. 2 have the reference numbers used in Fig. 1. As in the block diagram of Fig. 1, the positive voltage on conductor 35c is provided to the first terminal of impedance 30. Conductor 35c is also connected to a first terminal of a resistor 40. Resistors 40, 41, and 42 are connected in a series string to form a voltage divider 43 between conductors 35c and 35b. A conductor 44 is connected to the connection between resistors 40 and 41, and provides a voltage whose level relative to conductor 35b, equals the voltage drop across resistors 41 and 42. The voltage between conductors 44 and 35b provides operating power for a comparator 50 at the V+ and V- power terminals of comparator 50. A typical comparator suitable for use here may operate with power voltage in the range of 30 - 40 v. DC and draw no more than a few ma .

The voltage across resistor 42 provides a constant threshold or reference voltage (referred to as the first preselected voltage in the discussion of Fig. 1) relative to conductor 35b on conductor 45. The value of resistor 40 is selected to limit the voltage to within the specified operating range for comparator 50. The reference voltage at conductor 45 has only a small amount of current flow because the load connected to it is very small so resistors 41 and 42 may be large relative to resistor 40. The relative values of these resistors and of impedance 30 are well within the ability of an experienced electronic designer to specify.

Comparator 50 is conventional, and which in this embodiment is of the type providing a high output voltage at terminal 50c when its + input terminal at 50a is more positive than its - input terminal at 50b, and a relatively low output voltage, typically ground or 0 v., otherwise. The signal at terminal 50c forms the switch status signal, and is carried by conductor 28. The high output voltage at terminal 50c corresponds to the first level of the switch status signal. The low output voltage of terminal 50c corresponds to the second level of the switch status signal. Comparator 50 receives the reference (first preselected) voltage from conductor 45 at + terminal 50a. - input terminal 50b receives the voltage across impedance 30, which is the voltage indicating the conduction status of switch 11, high when switch 11 impedance is high, and low when switch 11 impedance is low.

Conductor 28 is connected to the base of a transistor 62. Transistor 62 controls flow of current from conductor 35c through the winding 59 of the Kl relay mentioned during the discussion of Fig. 1. Winding 59 is connected between conductor 35c and the collector of transistor 62. The emitter of transistor 62 is connected to conductor 35b. Transistor 62 functions as a switch controlling current flow through winding 59. When the high output voltage from comparator 50 is applied to conductor 28, transistor 28 conducts and causes current to flow through winding 59 and relay contacts 20a and 20b of Fig. 1 to close. Diode 55 is connected across winding 59 simply to prevent the reverse inductive voltage surge which arises when shutting off transistor 62, from damaging transistor 62 or other components of detector 27. When the impedance of switch 11 falls, current flow through impedance 30 increases, causing an increased voltage drop across impedance 30. This causes the voltage at - terminal 50b to fall below the reference (first preselected) voltage at terminal 50a and the voltage at terminal 50c to rise to its high level. Transistor 62 conducts and switches 20a and 20b (Fig. 1) close.

The circuit for switch 11 shown is one possible configuration for a powered switch. Conductors 35a and 35b connected to wires 60a and 60b which are connected to input terminals 61a and 61b of switch 11 respectively. Input terminals 61a and 61b form the input terminals for a diode bridge 79. Bridge 79 is connected to conductors 70a and 70b which serve as its output terminals. Bridge 79 inverts the DC voltage on conductors 60a and 60b so that conductor 70a of switch 11 is positive relative to conductor 70b regardless of the polarity of conductors 35a and 35b relative to terminals 61a and 61b. Bridge 79 is not absolutely necessary, but considering that at least one of the electronic components (microprocessor 83) of powered switch 11 must receive power of the predetermined polarity for operation, and that it may often be difficult for an installer to determine the polarity of the conductors 35a and 35b during installation, bridge 79 will substantially simplify installation.

A voltage dropping resistor 80 provides a reduced voltage from conductor 70a to an input conductor 81a of a voltage regulator 81. Regulator 81 provides a regulated voltage between its output conductor 81b and conductor 70b with conductor 81b positive with respect to conductor 70b. We prefer that regulator 81 provide a voltage difference between conductors 81b and 70b of approximately 5 v. DC, because preferred choices for circuit components 88 and microprocessor 83 are designed to operate on 5 v.

Circuit components 88 for a humidistat implementation may include a keyboard or set point selector potentiometer, display unit, humidity sensor, and A-D converter for the humidity value. For HVAC applications, components 88 may be temperature sensors, and for lighting controls, light level sensors and timers. Microprocessor 83 receives signals from the various circuit components and may also supply signals to such circuit components 88 say if such a component is a display unit .

A first impedance control resistor 75 is connected to conductor 70a and to one terminal of a second impedance control resistor 77. The other terminal of resistor 77 is connected to conductor 70b. Microprocessor 83 controls the conduction of a transistor 73 whose emitter and collector are connected across resistor 77. When microprocessor 83 detects that the humidifier should operate, it drives transistor 73 into conduction. This shorts resistor 77 and pulls the voltage on conductor 70a closer to the voltage on conductor 70b. The actual voltage on conductor 70a when transistor 73 is conducting depends on the value of resistor 75. Resistor 75 should be chosen so that the voltage on conductor 70a will be at least 6 - 7 v. higher than that on conductor 70b, which allows regulator 81 to provide the 5 v. preferred for components 88 and microprocessor 83. In fact, it is necessary to select the value for resistor 75 relative to the value of resistor 30 to allow transistor 73 to pull the voltage on conductor 35a to below the reference voltage on conductor 45. At the same time it is necessary to select the values of resistors 30, 75, and 80 to assure that the voltage at conductor 81a will be at 6 - 7 v. even when transistor 73 is fully conducting (assuming 5 v. logic) .

If switch 11 is of the passive type, when that switch closes, then the voltage on conductor 35a is pulled to the voltage on conductor 35b. Comparator 50 can easily detect this voltage and again drives transistor 62 into conduction, closing relay contacts 20a and 20b. In this way, the interface circuit 10 provides compatibility both for the powered switch 11 shown, as well as for any version of a passive switch such as an electro-mechanical humidistat. It is possible to provide the circuit of Fig. 2 as a stand-alone interface for allowing a powered electronic thermostat for example to operate a conventional furnace or air conditioning control which was originally designed to operate on the 24 v. AC switched by a passive electromechanical thermostat. In this format, power supply 32 will be connected to the existing transformer 17 secondary terminals, and the Kl relay switch terminals serve to switch power to the furnace control .

Claims

We claim :

1. A switch interface for switching power to a local system responsive to the conduction status of a remotely located switch in electrical connection to first and second sensing terminals of the interface, said interface compatible with both passive and powered versions of the remotely located switch, said interface providing through the sensing terminals, DC voltage to operate said powered version of the switch, said interface comprising: a) a power supply providing a DC voltage of at least a predetermined level across first and second DC power terminals thereof, said second DC power terminal serving as the second sensing terminal; b) an impedance having a first terminal connected to the first power terminal, and a second terminal serving as the first sensing terminal; c) a detector connected across the impedance's terminals and providing a switch status signal having a first value when the voltage across the impedance's terminals is above a preselected level, and a second value otherwise; and d) a switching circuit receiving the switch status signal at a control terminal, and having a pair of switching terminals, said switching circuit setting the impedance between the switching terminals to high and low levels responsive respectively to the first and second levels of the switch signal.

2. The interface of claim 1, including a power transformer having primary terminals for receiving AC line voltage and providing reduced voltage AC power across first and second secondary terminals, and wherein a first of the pair of switching terminals is connected to the first secondary terminal .

3. The interface of claim 2, wherein the switching circuit comprises a relay having coil terminals receiving the voltage level signal, said relay having a pair of power terminals forming the pair of switching terminals.

4. The interface of claim 2, wherein the power supply has first and second input terminals, and wherein the power transformer secondary terminals are connected to the power supply input terminals.

5. The switch interface of claim 3 adapted for connection to a humidifier having a fan motor, wherein the switching circuit has a first pair of switching terminals for electrically connecting the fan motor to the AC line voltage.

6. The switch interface of claim 5, wherein the humidifier has a valve solenoid, and wherein the switching circuit has a second pair of switching terminals for electrically connecting the valve solenoid to the transformer secondary terminals.

7. The switch interface of claim 2, wherein the value of the impedance is in a range of 300 to 700 _.

8. A powered switch for connection to the interface of claim 2, said switch having an electronic component requiring DC power of predetermined polarity for operation, and a diode bridge having a pair of input terminals forming the connection to the interface and a pair of output terminals for connection to the electronic component .