ELECTRICAL CONNECTION CIRCUIT
Field of the invention
The present invention relates to an electrical circuit for connection beLween a source of electrical power and an appliance, the circuit being capable of being incorporated in a connector such as a plug or a wall socket or within an extension lead or a power supply lead. i0
Background of the invention
Several appliances are currently designed to operate in a standby mode in which the appliance is dormant but can be is returned to a normal operating mode without the inconvenience of a warm-up period. In the case of computers, for example, having a standby mode avoids the need for cold rebooting, and therefore saves the user time and avoids inconvenience. Other appliances, such as televisions, can be returned to a normal operating mode by using an infrared remote control when they are in standby mode, which is not possible when their main ON-OFF switch is turned to the OFF position.
While having a standby mode offers advantages, such as those described above, it results in unnecessary consumption of electrical power. It is a popular misconception that appliances consume negligible electrical power when they are operating in standby mode. In reality, their power consumption is typically 85% of that during normal operation. Allowing appliances to operate in standby mode for prolonged periods is therefore hugely wasteful of energy and, having regard to current concerns about global warming, legislation is currently being considered in some countries to prohibit the provision of a standby mode in appliances manufactured in the future.
While the problem of energy wastage may be reduced in the future by modifying the design of appliances, there are currently numerous appliances in use which are left on in standby mode for prolonged periods of time, often inadvertently, resulting in unnecessarily high fuel bills to the consumer as well as harm to the environment.
Summary of the invention
With a view to mitigating the foregoing problem, the present invention in its broadest aspect provides a circuit for connection between an appliance and a power supply, which circuit includes means for detecting when the appliance is operating in a standby mode and means for automatically isolating the appliance from the power source either immediately or after a predetermined length of time of operation in the standby mode.
While it would possible to inhibit standby operation totally, by setting the predetermined length of time to be negligible or no more than the time taken to ascertain that the appliance is operating in a standby mode, this is not preferred as there are benefits to the user in having an appliance on standby for a short time. It is only prolonged operation in standby mode that is unjustified and likely to be unintentional.
It is therefore preferred for the circuit to incorporate a timer and for it to disconnect the appliance from the power supply only after several minutes of operation in the standby mode have lapsed.
In a preferred embodiment of the invention, the means for detecting when the appliance is operating in a standby mode includes means for sensing the current drawn by the appliance and means for determining when the current drawn from the power supply drops below a threshold.
The means for isolating the appliance from the power supply may be selfresetting. This would he suitable for any appliance which, having been disconnected from the power supply, requires its own ON-OFF switch to be turned ON before it can again be operated.
For appliances that would automatically return to a standby mode on reconnection to the power supply, it is possible for the means for isolating the appliance from the power supp]y to include a manual reset button or for it to be reset automatically if disconnected from the power supply. As a further alternative, the isolating means may be reset in response to toggling of the ON-OFF switch of the appJ iance.
The connection circuit of the invention may either be incorporated into the plug or the power lead of an appliance or it may be incorporated into a wall socket or an extension lead. If built into a plug or power lead dedicated to one appliance, the threshold level of the connection circuit can be calibrated for that appliance and will enable the appliance to be plugged into any power socket. If built into a socket, on the other hand, the connection circuit can prevent standby mode operation of any appliance plugged into it but the current threshold indicative of standby operation cannot be preset as it may vary with the appliance.
To overcome this problem, it is preferred for the connection circuit to include means for determining the maximum current drawn by an appliance after initial activation and means for setting a threshold indicative of operation in a standby mode as a predetermined fraction of the maximum sensed current.
When appliances are first switched on, their current consumption may remain high for some time until a steady state is reached. For this reason, it is preferred for the connection circuit not to rely on instantaneous measurements of appliance current but on measurements averaged over a period of time sufficiently long to ensure a reliable reading, unaffected by brief transients.
Brief description of the drawings
The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a block diagram of a connector in accordance with the invention.
Detailed description of the preferred embodiment(s) The single figure shows a connection circuit having two inputs and two outputs. The two inputs are connected to the Live and Neutral of a mains supply and the two outputs lead to the Live and Neutral inputs of the appliance, which requires no internal modification. The illustrated circuit can be quite small physically and can therefore be built into the mains supply lead of the appliance or into the plug fitted to the mains supply lead. As a further alternative, the circuit can be built into a mains wall socket or into an extension lead.
Within the circuit of the invention, there is an uninterrupted connection between the Neutral input and output terminals whereas a resistor 10 and an electronic switch 12 are arranged in series with one another in the connection between the input and output Live terminals. The switch 12 is shown as being a triac but other forms of switch or relay may alternatively be used.
A DC power supply circuit 14 connected across the mains supply serves to provide reference Ov and 5v DC rails which are required for other components to be described below. To avoid the need for a transformer, the DC supply circuit may simply comprise a Zener diode and a smoothing circuit.
As the resistor 10 is connected in series with the appliance, the voltage developed across it is a measure of the current drawn by the appliance. This voltage is amplified by an amplifier 16 and supplied to a voltage sensing input of a PIC12F675 micro-controller 18 which sends a control siqnal to the triac 12 by way of a resistor 20.
The micro-controller 18 is a small computer which can be pre-programsned to perform certain tasks, as described below, and includes, in addition to a processor, E-PROM memory to hold the instruction program and random access memory.
One of the inputs of the micro-controller 18 is connected to the Neutral line of the mains supply and serves to detect zero crossings. This information is used for initial powering up of the circuit to send a signal to the triac 20 over the resistor 20, to render the triac 12 conductive, the signal being timed relative to a zero crossing of the mains voltage. This output of the micro- controller remains in a high state for as long as power is to be supplied to the appliance and its return to a low state will result in the triac 12 being switched off to isolate the appliance from the mains supply.
While current is drawn from the mains supply, a voltage proportional to the current drawn by the appliance is developed across the resistor 10. The resistor 10 needs to have a low value of resistance and the voltage drop across it is also small. For this reason, it is amplified by the amplifier 16 before being applied to the micro-controller 18. The micro-controller 18 after initial power up measures the applied voltage and averages it over a period of typically one minute. This period allows the current drawn by the appliance to reach a steady state and also enables short current spikes to be disregarded. This average is then stored in memory and used as the reference for the current consumed by the appliance during normal operation.
When at some time later the appliance is switched to s operate in a standby mode, current will continue to flow through the resistor 10 but the value of the measured current will drop to a fraction of the stored steady state value for normal operation. Once again, it is not desirable to act on instantaneous measurements of the drawn current and it is averaged for a period of typically a minute to produce a steady state current reading. if this readinq is below a certain threshold, typically 85% of the normal operation current, then the micro-controller generates a flag to indicate that standby operation has been detected.
It is possible to act on this flag to switch off the triac 12 without delay but it is preferred to permit standby mode to be used for a reasonable length of time, say thirty minutes. The flag therefore initiates a count of clock pulses which continues until a maximum count is reached or until the flag is cleared by the operator resuming normal operation of the appliance. If the maximum count is reached, corresponding to the lapsing of a maximum period of operation in standby mode, the output of the micro- controller 18 goes low to switch off the triac 12 and isolate the appliance from the mains supply.
The described connection circuit is intended to be incorporated into a plug or a main supply lead of an appliance. Once the appliance has been disconnected, the circuit can be reset by disconnecting the mains supply from the input side of the circuit. This can be done either by pulling the plug out of the wall socket or operating a switch on the wall socket. If the appliance is one that remains in an OFF state once the power supply to it has been discontinued, the circuit can be modified to reset itself after it has been triggered by a period of operation in standby mode.
If the circuit is built into a wall socket, it is not possible to disconnect the mains supply from its input side and in this case it s possible to provide a manual reset button.
The triac 12 could alternatively be controlled using a o small burst fire following the mains zero crossing but such an approach may prove problematic in view of the non-uniform current drawn by some appliances. To overcome this issue the triac gate in the described preferred embodiment of the invention is driven continuously, which places a substantial i5 current burden on the transformer-less power supply unit 14 and makes redundant the need to monitor the mains for zero- crossing. If desired, an opto-isolated triac driver may be used with built-in zero-crossing detection such as the M0N3043-M.
If desired, a small LED may be used to indicate when the connection circuit has removed the power to the appliance. This could periodically flash to inform the user of the system status and would also make the device easier to locate to activate a soft reset if the circuit is fitted with a reset button.