Energy efficient thermostat

A thermostat method and apparatus has one or more demand circuits, an energy storage device; a DC regulator connected to the energy storage device, and a thermostat control connected to the DC regulator and to the energy storage device. Current is drawn from the one or more demand circuits when demand associated with the demand circuits is not active and the energy storage device is charged with the current drawn from the one or more demand circuits. If energy stored in the energy storage device is below the first predetermined threshold, activity in the thermostat is reduced and if energy stored in the energy storage device is above the second predetermined threshold, activity in the thermostat is allowed to increase.

BACKGROUND

Digital thermostats need power. Operating power is typically provided from battery or from the thermostat wiring. A typical HVAC system runs on low voltage 24 VAC system and has a 110/220 VAC to 24 VAC transformer. The two sides of the transformer are typically marked as R (Return) and C (Common). Newer house wirings routes both taps of the transformer to the thermostat and thus the thermostat has direct access to this 24 VAC system and can derive its required internal supply voltages from the 24 VAC directly.

However, older houses do not typically have the C wire routed to the thermostat. Instead the C side of the terminal is routed through various demand controls, such as Fan, Heat, Cool, etc. The thermostat activates a relay and shorts these connections to the R, thus signaling a demand. When the contacts of the relays are open, the full 24 VAC is available between the various demand lines and the R. When the contacts are closed, the voltage drops to 0 VAC and the current flows from the C terminal of the 24 VAC transformer via the demand wires back to the R terminal of the transformer.

There have been on the market various power stealing methods that allow stealing power from these demand wires when the relay is open (voltage driven) and even when the relay is closed. The problem with these solutions is that they only allow a ‘small’ amount of power to be harvested, because if the current increases above approximately 10 mA or so in the demand line, the HVAC controller might detect a false demand on the control line. Most digital thermostats are very low power and may survive on this small amount of power harvested from one or more control lines. They may also be supported with battery backup and power stealing may be used just extend the battery life. There is also a solution that steals power from systems with a single demand line when the demand is not active, storing some of the energy in a rechargeable battery or super capacitor, and then powers the thermostat from this battery when the demand is active.

Newer thermostats are now getting network attached. Some network attached thermostats use a wireless interface and nowadays Wi-Fi is popular. The problem with a Wi-Fi attached thermostat is that it needs more power than can be stolen from an HVAC system without the C terminal. Thus this thermostat either requires the presence of the C wire or requires an external wall mount power supply.

What is needed is a system and method for powering a digital thermostat in the absence of an external power source such as a C wire or an external power supply.

DETAILED DESCRIPTION

In the following detailed description of example embodiments of the invention, reference is made to specific examples by way of drawings and illustrations. These examples are described in sufficient detail to enable those skilled in the art to practice the invention, and serve to illustrate how the invention may be applied to various purposes or embodiments. Other embodiments of the invention exist and are within the scope of the invention, and logical, mechanical, electrical, and other changes may be made without departing from the subject or scope of the present invention. Features or limitations of various embodiments of the invention described herein, however essential to the example embodiments in which they are incorporated, do not limit the invention as a whole, and any reference to the invention, its elements, operation, and application do not limit the invention as a whole but serve only to define these example embodiments. The following detailed description does not, therefore, limit the scope of the invention, which is defined only by the appended claims.

An example heating, ventilation and cooling (HVAC) system is shown inFIG. 1. In the example shown inFIG. 1, system10includes a heating unit12, a cooling unit14and a ventilation unit16connected to the ventilation system18used to control a building's climate. In the example shown inFIG. 1, system10includes a thermostat system100that controls each of heating unit12, cooling unit14and ventilation unit16.

An example thermostat system100is shown inFIG. 2. In the example shown inFIG. 2, thermostat system100includes a first demand circuit102and a second demand circuit112connected to a first limited current source104and a second limited current source114, respectively. In the example shown, limited current sources104and114include a bridge rectifier106connected to a current limiter108. In the example shown, first demand circuit102includes a switch connected between wires RH and W; in the example shown, first demand circuit102serves to power a HVAC unit such as a heating unit off and on. Similarly, demand circuit112is connected between wires RC and Y; in the example shown inFIG. 2, second demand circuit112serves to power a HVAC unit such as a condenser or other cooling unit off and on. In one example embodiment, wires RH and RC provide 24 VAC to their respective HVAC units via their corresponding demand circuits102and112.

In one embodiment, demand circuits102and112include relays. In another embodiment, semiconductor devices such as triacs are used in demand circuits102and112to provide power to the HVAC units.

In the example thermostat system100ofFIG. 2, current sources104and114store energy into energy storage116. In one such embodiment, current flows from limited current sources104and114only when the corresponding demand of the HVAC unit is turned off.

In the example embodiment shown inFIG. 2, when energy stored in energy storage116passes a particular threshold, thermostat control120wakes from a low power sleeping state. Typically, the threshold is selected to be a sufficient number of volts over the output of DC regulator118to ensure that DC regulator118is capable of driving sufficient current for a predetermined minimum time at the desired voltage to drive thermostat control120. For a 5V power supply, the number might be 2 volts above the desired voltage, or 7 volts DC.

In one embodiment, thermostat control120is placed into a reduced power mode (sleep mode) if the voltage across energy storage116falls below a predetermined threshold.

In one embodiment, energy storage116is a rechargeable battery. In another embodiment, energy storage116is a capacitor.

As noted above, previous attempts to power thermostats from power stolen from the demand lines required very low powered thermostat controls. It is difficult to extend such a mechanism so that it can include higher powered features such as Wi-Fi, Zigbee or other wireless devices. Thermostat system100solves this problem by providing at least two sources of the power needed to store energy into energy storage116. It is unlikely that an HVAC system that supports both heating and cooling would be doing both simultaneously. The assumption is that both of these demands will rarely be activated simultaneously, thus at least one of the relays are always open providing 24 VAC to power current source104or114.

In one embodiment, additional demand lines (such as second stage cooling or heating) can be used in similar configurations to provide additional power sources.

In addition, as shown inFIG. 2, in one embodiment thermostat control120includes sleep/wakeup logic124used to power down thermostat100when the energy stored in energy storage116drops below a particular threshold and wake up when it rises above a particular threshold. Such an approach allows a network attached wireless digital thermostat to work without battery, C wire or external power supply. This approach also is capable of employing a low power requirement RF network, such as a Zigbee network that can sleep most of the time and wake-up periodically, resume the network connection quickly, transfer the required data and then go back to deep sleep. The power profile of such system is for low power consumption for an extended period followed by a burst power demand for a short period in time, followed by another low power period, etc.

This burst demand for power can be harvested via power stealing over a longer period of time. By carefully selecting the ratio of the deep sleep and the active burst power, an improved power stealing system can harvest enough energy from the HVAC system without a C wire or external power supply to maintain a wireless RF Digital Thermostat operation.

In one embodiment, the system employs a constant current limiting network via current limiter108(adjustable, but typically less than 10 mA) to make sure that no false demand would be presented. This constant current source than would charge a rechargeable battery or a storage cap. The output of energy source116is then fed into a high-efficiency, wide input range, DC/DC controller118providing required operating voltages.

Another example embodiment of a thermostat system is shown inFIG. 3. In the example shown inFIG. 3, thermostat system200includes a first relay202as a first demand circuit and a second relay212as a second demand circuit. First relay202and second relay212are connected to a first current source104and a second current source114, respectively. In the example shown, current sources104and114include a bridge rectifier106connected to a current limiter108. In the example shown, first relay202is connected between wires RH and W, and serves to power a HVAC unit such as a heating unit off and on. Similarly, relay212is connected between wires RC and Y, and serves to power a HVAC unit such as a condenser or other cooling unit off and on. In one example embodiment, wires RH and RC provide 24 VAC to their respective HVAC units via their corresponding relays202and212. Additional current sources may be implemented by duplicating circuit104for additional demand lines, such as second stage cooling or heating, if available.

In the example thermostat system200ofFIG. 3, current sources104and114store energy into charge capacitor216. In one such embodiment, current flows from current sources104and114only when the corresponding demand of the HVAC unit is turned off.

In the example embodiment shown inFIG. 3, when energy stored in energy storage216rises above a particular threshold, thermostat control120wakes from a sleeping state. Typically, the threshold is selected to be a sufficient number of volts over the output of DC regulator118to ensure that DC regulator118is capable of driving sufficient current at the desired voltage for a predetermined minimum cycle time to drive thermostat control120. For a 5V power supply, the number might be 2 volts above the desired voltage, or 7 volts DC.

In one embodiment, system200provides an active monitoring of the energy stored in the charge capacitor216and forces the system to go to sleep when the energy stored in the charge capacitor216drops below a predetermined critical level. In one such embodiment, system200includes a feature that wakes the system up when the energy stored in the cap reaches a preset level. This feature may not be required in all applications, because selecting the proper duty cycle might be sufficient. Such an approach can, however, be helpful during periods when more power is needed, such as during, for instance, a code download or a Flash update.

An example of such an active monitoring approach is shown inFIG. 4. InFIG. 4, at300, a controller detects the voltage across energy storage116(or charge capacitor216inFIG. 2) and, at302, determines if the voltage is above a first threshold T1. If so, the controller moves to304, the thermostat processing engine126is awakened and control moves to306.

If the voltage at302is not above a first threshold T1, the controller waits at302until the voltage is above the first threshold T1.

At306, a check is made to determine if the voltage across energy storage116is below a second threshold T2. If the voltage is below that threshold, control moves to308and the thermostat processing engine126is placed in a low power state, or is put to sleep. Control them moves to300.

If the voltage at302is not below the second threshold T2, the controller waits at306until the voltage is below the second threshold T2.

In one embodiment, as is shown inFIG. 5, thermostat400includes a wireless interface402. In one such example, the wireless interface is a Wi-Fi interface. In one such embodiment, thermostat400establishes the thermostat as a wireless node. In one embodiment, the wireless interface is a Zigbee interface.

In the example shown inFIG. 5, thermostat400includes a first relay202and a second relay212connected to a first current source104and a second current source114, respectively. In the example shown, current sources104and114include a bridge rectifier106connected to a current limiter108. Additional current sources may be implemented if additional demand lines are available. In the example shown, first relay202is connected between wires RH and W, and serves to power a HVAC unit such as a heating unit off and on. Similarly, relay212is connected between wires RC and Y, and serves to power a HVAC unit such as a condenser or other cooling unit off and on. In one example embodiment, wires RH and RC provide 24 VAC to their respective HVAC units via their corresponding relays202and212.

In the example thermostat system400ofFIG. 5, current sources104and114store energy into energy storage116. In one such embodiment, current flows from current sources104and114only when the corresponding demand of the HVAC unit is turned off.

In the example embodiment shown inFIG. 5, when energy stored in energy storage216rises above a particular threshold, thermostat control120wakes from a sleeping state. Typically, the threshold is selected to be a sufficient number of volts over the output of DC regulator118to ensure that DC regulator118is capable of driving sufficient current at the desired voltage to drive thermostat control120. For a 5V power supply, the number might be 2 volts above the desired voltage, or 7 volts DC. Since wireless interface402interface typically requires a significant amount of power, in one embodiment wireless interface402is only enabled when the voltage across energy storage116is above a second, higher, threshold.

In one embodiment, thermostat400provides an active monitoring of the energy stored in the energy storage116and forces the system to go to sleep when the energy stored in the energy storage116drops below a first predetermined critical level. In one such embodiment, thermostat400includes a feature that wakes the system up when the energy stored in energy storage116reaches a first preset level and that enables wireless interface402to operate when the energy stored in energy storage116reaches a second higher preset level. In one such embodiment, shut down is stepped as well. If the energy stored in energy storage116drops below a preset level, the wireless interface is powered down. In one such embodiment, if the energy stored in energy storage116drops further, the thermostat is put into a sleep mode.

An example of such an active monitoring approach is shown inFIG. 6. InFIG. 6, at500, a controller detects the voltage across energy storage116and, at502, determines if the voltage is above a first threshold T1. If so, the controller moves to504, the thermostat processing engine126is awakened and control moves to506.

If the voltage at502is not above a first threshold T1, the controller waits at502until the voltage is above the first threshold T1.

At506, a check is made to determine if the voltage across energy storage116is above a second threshold T2or below a threshold T4. If the voltage is above the threshold T2, control moves to508and wireless interface402is enabled. Control them moves to510.

If the voltage at506is below threshold T4, the controller moves to516and the thermostat is put to sleep. Control then moves to500.

If the voltage at506is not above the second threshold T2and not below threshold T4, the controller waits at506until the voltage is above threshold T2or below threshold T4.

At510, a check is made to determine if the voltage across energy storage116is below a threshold T3. If the voltage is below that threshold, wireless interface402is turned off at512to conserve power. Control then moves to control moves to514.

If the voltage at510is not below the threshold T3, the controller waits at510until the voltage is below the threshold T3.

At514, a check is made to determine if the voltage across energy storage116is above threshold T2or below threshold T4. If the voltage is above the threshold T2, control moves to508and wireless interface402is enabled. Control them moves to510.

If the voltage at514is below threshold T4, control moves to516and the thermostat processing engine126is placed in a low power state, or is put to sleep. Control them moves to500.

If the voltage at514is not above threshold T2and not below the threshold T4, the controller waits at514until the voltage is above threshold T2or below threshold T4.

As noted above, other thermostat systems typically have fairly constant power requirements. For low power they can survive on a traditional power stealing. For higher power they require the C wire or an external power supply. The solutions described above rely on the bursty power profile of an RF system and the harvesting of the required energy over time for the burst operation, thus eliminating the need for the C wire or external power supply. The system also monitors the energy stored in the storage cap and can wake the system up or forces it to go to sleep based on the level.

In addition, the above described thermostat system and method makes installation easier, faster, more bulletproof, thus lower cost. It also eliminates the need for external power supply when the C wire is not available.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. The invention may be implemented in various modules and in hardware, software, and various combinations thereof, and any combination of the features described in the examples presented herein is explicitly contemplated as an additional example embodiment. This application is intended to cover any adaptations or variations of the example embodiments of the invention described herein. It is intended that this invention be limited only by the claims, and the full scope of equivalents thereof.