Patent Publication Number: US-6908164-B2

Title: Power control circuit for printers and other devices

Description:
TECHNICAL FIELD 
   The present invention relates generally to power control circuits and, more particularly, to a circuit for controlling the application of power to the primary electronics in devices such as printers. 
   BACKGROUND OF THE INVENTION 
   Energy standards as applicable to printers currently provide that standby power consumption should be below 1 watt, and as applicable to printer/fax combinations currently require that standby power consumption be below 2 watts. 
   Accordingly, it would be advantageous to provide a relatively simple power control circuit for use in printers and other electronic devices to provide a low power off state for the devices. 
   SUMMARY OF THE INVENTION 
   In one aspect, a printer includes a DC power source, an electronic controller for controlling operations of the printer including a printing mechanism of the printer, and a power control circuit connected between the DC power source and the electronic controller for controlling delivery of DC power to the electronic controller. The power control circuit includes a power control transistor, a user closeable switch and latching circuitry. The power control transistor is connected between the DC power source and the electronic controller. An ON state of the power control transistor effects DC power delivery to the electronic controller and an OFF state of the power control transistor prevents DC power delivery to the electronic controller. The user closeable switch is connected in line with at least one other circuit component that receives power from the DC power source when the user closeable switch is closed to set up a voltage condition across terminals of the power control transistor that places the power control transistor in its ON state. The user closeable switch is biased into an open condition. The latching circuitry is connected in the power control circuit and latches into a conducting state upon closure of the user closeable switch to maintain the voltage condition even after the user closeable switch returns to its open condition. 
   In another aspect, a printer includes a DC power source, an electronic controller for controlling operations of the printer including a printing mechanism of the printer, and a power control circuit including a power control transistor connected between the DC power source and the electronic controller. An ON state of the power control transistor effects DC power delivery to the electronic controller and an OFF state of the power control transistor prevents DC power delivery to the electronic controller. Latching circuitry is connected to the power control transistor and has a latched state that maintains a voltage condition across terminals of the power control transistor to hold the power control transistor in its ON state. A kick-start circuit is connected to the power control circuit and is configured to temporarily set up the voltage condition when DC power is initially output by the power source. The kick-start circuit sets up the voltage condition for a time period sufficient to cause the latching circuitry to latch into its latched state. 
   In yet another aspect, a printer includes a DC power source and an electronic controller for controlling operations of the printer including a printing mechanism of the printer. A power control circuit is connected between the DC power source and the electronic controller for controlling delivery of DC power to the electronic controller. The power control circuit includes latching circuitry for latching the power control circuit in a power delivery state to deliver DC power through the power control circuit to the electronic controller. A control path is connected between the electronic controller and the latching circuitry enabling the electronic controller to unlatch the power control circuit into a power non-delivery state that causes cessation of DC power delivery through the power control circuit to the electronic controller. 
   In a further aspect, a printer includes a DC power source, an electronic controller and a power control circuit connected between the DC power source and the electronic controller. The power control circuit has both a latchable power delivery state in which power is delivered from the DC power source to the electronic controller and a power non-delivery state in which DC power is not delivered from the DC power source to the electronic controller. The power control circuit includes a user closeable switch connected therein and a kick-start circuit portion. When the power control circuit is in the power non-delivery state, temporary closure of the user closeable switch causes the power control circuit to latch into its power delivery state. When the power control circuit is in the power non-delivery state, initial output of power from the DC power source causes the kick-start circuit to latch the power control circuit into its power delivery state. 
   In another aspect, a power control circuit for use in an electronic device includes an input for connecting to a power source to receive power therefrom, an output for connecting to an electronic controller of an electronic device, and a power control transistor connected between the input and the output. An ON state of the power control transistor makes a power path from the input to the output and an OFF state of the power control transistor breaks the power path. A switch is connected in line with at least one other circuit component and the input for setting up a voltage condition across terminals of the power control transistor when the switch is closed to place the power control transistor in its ON state. The switch is biased into an open condition. Latching circuitry is connected in the circuit for latching into a conducting state upon closure of the switch to maintain the voltage condition even after the switch returns to its open condition. 
   In yet another aspect, a power control circuit for use in an electronic device includes an input for connecting to a power source to receive power therefrom, an output for connecting to an electronic controller of an electronic device, and a power control transistor connected between the input and the output. An ON state of the power control transistor making a power path from the input to the output and an OFF state of the power control transistor breaking the power path. Latching circuitry is connected to the power control transistor and has a latchable conducting state for maintaining the power control transistor in its ON state. A kick-start circuit is connected to temporarily set up a voltage condition across terminals of the power control transistor when power is initially received at the input, the voltage condition placing the power control transistor in its ON state. The kick-start circuit configured to set up the voltage condition for a time period sufficient to cause the latching circuitry to latch into its conducting state. 
   In still another aspect, a printer includes a DC power source, an electronic controller for controlling operations of the printer including a printing mechanism of the printer, and a power control circuit connected between the DC power source and the electronic controller for controlling delivery of DC power to the electronic controller. The power control circuit includes a switching device connected between the DC power source and the electronic controller, where a closed state of the switching device effects DC power delivery to the electronic controller and an open state of the switching device prevents DC power delivery to the electronic controller. A user controlled switch is connected in line with at least one other circuit component that receives power from the DC power source when the user controlled switch is closed to set up a condition in the power control circuit that places the switching device in its closed state, the user controlled switch being normally open. Latching circuitry is connected in the power control circuit, the latching circuitry latching into a conducting state upon closure of the user controlled switch to maintain the condition even after the user controlled switch opens. 
   In a further aspect, a printer includes a DC power source, an electronic controller for controlling operations of the printer including a printing mechanism of the printer and a power control circuit including a switching device connected between the DC power source and the electronic controller. A closed state of the switching device effects DC power delivery to the electronic controller and an open state of the switching device prevents DC power delivery to the electronic controller. Latching circuitry is connected to the switching device and having a latched state that maintains a circuit condition that holds the switching device in its closed state. A kick-start circuit is connected to the power control circuit and configured to temporarily set up the circuit condition when DC power is initially output by the power source. The kick-start circuit sets up the circuit condition for a time period sufficient to cause the latching circuitry to latch into its latched state 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit schematic of a printer system including one embodiment of a power control circuit. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a printer system  10  is shown and includes a power source  12 , an electronic controller  14  and a power control circuit  16  connected between the power source  12  and the electronic controller  14 . The electronic controller  14  is configured to control the operations of the printer and is connected to various components  18   a - 18   n  of the printer for that purpose. In one embodiment the electronic controller  14  includes both an analog ASIC portion and a digital ASIC portion. By way of example, the components  18   a - 18   n  could be various motors, solenoids, lights, display(s) and sensors located throughout a typical printer, as well as the print mechanism(s) of the printer. In one embodiment the printer is an inkjet printer and the print mechanisms are inkjet printheads mounted on a carriage for movement across a paper path as is well known in the art. However, it is recognized that in other embodiments printer system  10  could represent another type of printer such as a dot-matrix printer, a thermal printer, a laser jet printer or any other type of printer. As used herein the term “printer” is intended to encompass any device which is capable of placing indicia on a media, regardless of the type of print technology and printer mechanism used. Further, the term “printer” specifically encompasses both stand alone units and multi-function devices incorporating the capability of placing indicia on a media (e.g., devices commonly referred to as combination fax/printers). 
   In the illustrated embodiment the power source  12  is provided at the output of an AC/DC power supply/adapter having an input to receive AC power, typically from a standard 120 V outlet. However, other power sources could be provided, and in some cases the power source could simply be an input line that is intended to be connected to a source of power when the printer is installed for use (e.g., a printer that does not have is own power supply but instead receives power from a separate device when installed). 
   The power supply circuit  16  includes a primary power supply circuit portion  20  and a kick-start circuit portion  22  connected thereto. The output of the AC/DC power supply is connected to an input  24  of the power supply circuit  16  and the electronic controller  14  is connected to an output  26  of the circuit. A power control transistor Q 1  is connected between the input  24  and output  26 . An ON state of the power control transistor Q 1  makes a power path from the input  24  to the output  26 , thereby delivering DC power from the power source  12  to the electronic controller  14 . An OFF state of the power control transistor Q 1  breaks the power path, preventing the delivery of DC power from the power source  12  to the electronic controller  14 . The remainder of the circuit  16  is primarily configured to establish the manner in which the ON/OFF state of the power control transistor Q 1  is set as will be described in detail below. 
   In the illustrated embodiment power control transistor Q 1  is a P-Channel Enhancement Mode Field Effect Transistor such as the NDT2955 available from Fairchild Semiconductor. Other type of transistors or other switching devices could be used. A user closeable switch S 1  or other user controlled switching device is connected to the power source  12  in line with resistors R 1 , R 2  and R 3 . The switch S 1  may be biased into the normally open condition shown such that when a user depresses a button on the printer the switch S 1  closes, and when the user releases the button the switch S 1  opens. The resistor R 2  is one component of latching circuitry  28  provided in the circuit portion  20 . The latching circuitry  28  has a latchable conducting state that can be used to maintain or latch the power control transistor in its ON state as will become apparent below. 
   Assuming the power control transistor Q 1  is in its OFF state and the latching circuitry  28  is likewise in its non-conducting state, the output  26  is low (i.e., DC power is not being delivered to the output  26 ). Temporary closure of the switch S 1  by a user will effect power delivery to the electronic controller  14  as follows. When the switch S 1  initially closes, current flow along the path defined by resistors R 1 , R 2 , R 3 , switch S 1  and resistor R 4  causes a voltage condition across the source S to gate G terminals of the power control transistor Q 1  that places the power control transistor Q 1  in its ON state, the voltage condition being a voltage drop from source S to gate G in the illustrated embodiment. DC power is then delivered through the transistor to the electronic controller  14 , causing a latching circuitry control input  30 , which connects through resistor R 5  to the base (i.e., the control input) of transistor Q 2 , to be set in a high impedance state. In the illustrated embodiment control input  30  is connected to the open collector of a normally OFF transistor Q 4  in an analog ASIC of the electronic controller  14 , and the electronic controller  14  includes a digital ASIC that can turn transistor Q 4  ON. 
   At the same time, current flow through resistor R 2  causes a voltage drop from the emitter E to base B of transistor Q 3 , which in the illustrated embodiment is a PNP transistor, thereby biasing transistor Q 3  into its ON state, conducting state. When transistor Q 3  conducts, current flows through resistors R 7  and R 8  and the voltage across R 8  is applied at the base of transistor Q 2 . In the illustrated embodiment, Q 2  is an NPN transistor, and the voltage biases transistor Q 2  into its ON, conducting state. Once transistor Q 2  is turned ON, current flows from the power source  12  through resistors R 1 , R 2  and R 9 , even if the user releases the switch S 1  to its open condition. Current flow through transistor Q 3  keeps transistor Q 2  in its ON state and current flow through transistor Q 2  keeps transistor Q 3  in its ON state, thereby latching the circuitry  28  in its conducting state to maintain or latch the power control transistor Q 1  in its ON state. 
   Assuming power control transistor Q 1  is latched in its ON state by latching circuitry  28 , temporary closure of switch S 1  by a user can effect cessation of power delivery to the electronic controller  14 . As shown, a switch condition feedback path  32  is provided from the circuit portion  20  to the electronic controller  14 . In the illustrated embodiment the feedback path  32  is input to the digital ASIC of the electronic controller  14 . 
   In one embodiment, the electronic controller  14  may take different actions depending on whether the switch S 1  is closed momentarily or closed for a longer, threshold time period such as a few seconds. When the switch S 1  is momentarily closed a high signal is momentarily applied to path  32  due to the voltage drop across resistor R 4  and the electronic controller  14  responds by placing the printer in a “soft off state” by powering down various of the printer components for energy conservation. During such a soft off state the power control transistor Q 1  maintains its ON state and a power on LED color may be changed to visually indicate to users the soft off state of the printer  10 . The electronic controller  14  wakes up, returning the printer  10  to its operating power state when it receives USB data (e.g., data to effect a print operation). When the switch S 1  is closed for the longer, threshold time period, the electronic controller  14  responds by completely turning off power in the manner to be described below. 
   In another embodiment the electronic controller  14  may respond by turning off power completely upon momentary closure of the switch S 1 . 
   Regardless of the particular embodiment, when electronic controller  14  recognizes a high signal on path  32  as an indication that a user wants to power off the printer  10  completely, the electronic controller  14  first runs through a power shut down sequence (e.g., parking the print head carriage, expelling media, communicating with the print driver, etc.). Then the electronic controller  14  causes the control input  30  of the latching circuitry  28  to be pulled low through transistor Q 4  by turning ON transistor Q 4 . This operation grounds the base of transistor Q 2  causing it to turn OFF. When transistor Q 2  turns OFF, current flow through resistor R 2  stops, causing transistor Q 3  to also turn OFF. At that point, current stops flowing through resistor R 1  and the source to gate voltage drop across power control transistor Q 1  is eliminated causing the power control transistor Q 1  to turn OFF. When transistor Q 1  turns OFF, power delivery to the electronic controller  14  stops. It is recognized that the electronic controller  14  could be configured to also initiate the shutdown via control input  30  based upon the detection of some other condition if desired. 
   In circuit portion  20 , capacitor C 1  acts as a filter for the incoming DC voltage from the power source  12 . Capacitor C 2  limits the turn on speed of transistor Q 1  in order to maintain inrush current to the transistor at an acceptable level. Capacitor C 3  serves a dual function of stabilizing the voltage at the base of transistor Q 3  and assisting capacitor C 2  in inrush current control for transistor Q 1 . Capacitor C 4  acts a filter to stabilize the base-emitter voltage of transistor Q 2 . 
   Referring now to the kick-start circuit portion  22 , the purpose of this portion of the circuit  16  is to automatically place the power control transistor Q 1  in its ON state whenever power is initially applied by the power source (e.g., when the user plugs a power adapter into the machine, when machine&#39;s power adapter is plugged into a power strip and the power strip is cycled off and then back on again, or when the AC power to the power adapter has an interruption or outage). Before power is applied to input  24 , transistor Q 5  is in its OFF state and the voltage across capacitor C 5  is zero. When power is first applied to input  24  the applied DC voltage appears on both sides of capacitor C 5  with respect to ground. Because the potential across the capacitor C 5  cannot change instantaneously, the side of capacitor C 5  connected to resistor R 10  discharges to ground through resistors R 10  and R 11 , which resistors act to slow down the discharge rate. The resistors R 10  and R 11  set up a voltage divider that is connected to the base of transistor Q 5  to reduce the voltage at its base. The transistor Q 5  is turned ON temporarily while the capacitor C 5  discharges. The duration the transistor Q 5  is turned on can be calculated as follows:
 
 t =−( R   10 )( C   5 )1 n[Vc/Vi]   (1)
 
where Vc is the voltage across the divider at which the transistor Q 5  will shut OFF, and Vi is the voltage applied at the input  24 . Resistor R 11  does not factor into the equation because a potential of approximately 0.7 volts, due to the base-emitter voltage of transistor Q 5 , sets up across resistor R 11  and does not change until Q 5  shuts off. If resistor R 10  is selected as 100 kΩ, resistor R 11  is selected as 10 kΩ, and the Vbe to turn OFF transistor Q 5  is 0.65 volts, then when the voltage across the divider is 7.15 volts the transistor Q 5  will shut OFF. Using 7.15 volts as Vc, assuming an input voltage of +30 volts, and selecting 2.2 μF for capacitor C 5 , the on time of transistor Q 5  calculated per equation (1) will be about 315.5 msec. The temporary ON state of transistor Q 5  is designed to be long enough to draw current through resistors R 1  and R 2  so that the power control transistor Q 1  is latched into its ON, conducting state by the latching circuitry  28  in the same manner described above relative to the closure of switch S 1 .
 
   R 12  is provided in the kick-start circuit  22  to bleed off the high potential side of capacitor C 5  if the power source is disconnected or AC power is lost. The diode D 1  quickly discharges the low potential side of capacitor C 5  and clamps at most to −0.7 volts when power is lost. 
   The described circuit provides an improved technique for controlling delivery of power from a DC power source to an electronic controller in a device capable of printing on media. The power control circuit  16  is connected between the DC power source and the electronic controller and has a latchable power delivery state (e.g., transistor Q 1  ON) in which power is delivered from the DC power source to the electronic controller. The power control circuit also has a power non-delivery state (e.g., transistor Q 1  OFF) in which DC power is not delivered from the DC power source to the electronic controller. The power control circuit includes a user closeable switch S 1  and a kick-start circuit portion  22 . When the power control circuit is in the power non-delivery state, the power control circuit latches into its power delivery state in response to temporary closure of the user closeable switch. When the power control circuit is in the power non-delivery state, the power control circuit latches into its power delivery state in response to initial output of power from the DC power source. 
   Although the invention has been described above in detail referencing the illustrated embodiment thereof, it is recognized that various changes and modifications could be made.