Method and apparatus for protecting a solenoid and its driving circuit in an electronic device

This application claims the benefit of U.S. Provisional Application No. 60/486,522 filed on Jul. 11, 2003, the disclosure of which is incorporated herein by reference.

1. Field of the Invention

The present invention generally relates to a method and apparatus for protecting a solenoid and its associated driving circuit in an electronic device and more particularly to protecting a solenoid and the associated driving circuit in a point-of-sale terminal having a solenoid operated cash drawer driven by a circuit in a receipt printer.

2. Description of the Related Art

The use of software to control the operation of electronic business machines, such as point-of-sale (POS) terminals has become widespread. For example, software in these machines is typically used to generate a signal that energizes a solenoid to open a cash drawer.

A fault or malfunction in the software or intervening hardware may cause the signal to continue indefinitely, thereby causing the solenoid to remain energized. If the solenoid is energized for a period that exceeds its rated duty cycle, the driving circuit and the solenoid will likely be damaged. The user will then be required to spend a considerable amount of time and money to repair the cash drawer and/or receipt printer before the POS terminal is able to function properly again.

A partial block diagram of a conventional POS system 10 is shown in FIG. 1 . Under normal operating conditions, application software residing in a computer 12 controls, among other functions, the opening of a cash drawer 14 . Specifically, to open the cash drawer 14 , a popular command issued by the computer 12 to a receipt printer 16 is the character CTRL-G, which is not a printable character.

The receipt printer 16 interprets CTRL-G as a command to output a pulse that opens the cash drawer 14 . Typical parameters of the pulse include a duration T of 100–500 milliseconds and a peak voltage Vcc of 12–24 VDC, which is often applied to the base of a transistor 18 in the receipt printer 16 used to drive an energizing signal 20 between Nodes D 1 and D 2 .

The cash drawer solenoid 22 typically has a relatively high Q or quality factor, which is the ratio of its reactance to it effective series resistance at a given frequency. Specifically, the inductance of the solenoid 22 is high but the resistance is low. Thus, for relatively short pulse durations, such as the preferred 100–500 millisecond energizing signal 20 , the impedance of the solenoid 22 is high, which limits the current through the transistor 18 and solenoid 22 to an acceptable value. However, for pulses having a longer duration, the impedance of the solenoid becomes very low, which may appear as a short circuit to the driving transistor 18 in the receipt printer 16 .

Burnout of the transistor 18 and/or the solenoid 22 occurs whenever the duration of the pulse exceeds a certain value. Burnout also occurs if the output of the transistor 18 is momentarily grounded, which may happen during system installation.

U.S. Pat. No. 5,111,394 to Hilles et al. (Hilles), which is incorporated herein by reference, describes an electronic business machine having a cash drawer and a timeout circuit. The timeout circuit couples the output of a microprocessor to a driver for a programmable period of time. The driver provides a signal that energizes a solenoid to open the cash drawer. In an alternative embodiment, a multivibrator is substituted for the timeout circuit.

Hilles overcomes the problem of damaged transistors and solenoids common in conventional POS systems by limiting the period of time that the solenoid is energized. However, in the first embodiment of Hilles, the timeout circuit requires over thirty digital devices that add to the cost and require a significant amount of board space, as well as a source of power. Although the second embodiment of Hilles implements the timeout function in a single integrated device, that is, a multivibrator, such an active device still represents an unreliable and significant additional cost while requiring its own source of power. These characteristics substantially limit the location of and applications for the solutions proposed in Hilles.

It is an objective of the present invention to provide a much simpler and more reliable method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, which uses inexpensive passive components that do not require a source of power.

It is another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, that requires only a small amount of space and consumes negligible power.

It is yet another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, which can be used to retrofit existing electronic devices, as well as being designed into new equipment.

It is still another objective of the present invention to provide a method and apparatus for protecting a solenoid and/or receipt printer in an electronic device, such as a point-of-sale terminal, that employs a simple, straightforward design that is reliable, easy to test, and relatively maintenance free.

These and other goals, purposes, and objectives are met by the present invention, which provides an apparatus for protecting a solenoid and/or receipt printer in an electronic device. The apparatus includes a resistance and a capacitance. The resistance is electrically coupled in parallel with the capacitance and the parallel network of resistance and capacitance is coupled in series with the solenoid.

An energizing signal is applied to the parallel network to energize the solenoid. The capacitance preferably includes two polarized electrolytic capacitors electrically coupled in a back-to-back series configuration having common polarities of the capacitors connected to each other.

The present invention also provides a method for protecting a solenoid and/or receipt printer in an electronic device. The method includes the steps of coupling a resistance electrically in parallel with a capacitance, coupling the parallel network of resistance and capacitance in series with the solenoid, and applying an energizing signal to the parallel network to energize the solenoid. The capacitance preferably includes two polarized electrolytic capacitors electrically coupled in a back-to-back series configuration with common polarities connected to each other.

These and other objectives, features, and advantages of this invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

FIG. 1 is a block diagram of a conventional point-of-sale (POS) system.

FIG. 2 is a block diagram of a first embodiment of a point-of-sale (POS) system formed in accordance with the present invention.

FIG. 3 is a block diagram of a second embodiment of a point-of-sale (POS) system formed in accordance with the present invention.

FIG. 4 is a block diagram of a third embodiment of a point-of-sale (POS) system formed in accordance with the present invention.

FIG. 5 is a block diagram of a fourth embodiment of a point-of-sale (POS) system formed in accordance with the present invention.

FIG. 6 is block diagram of a fifth embodiment of a point-of-sale (POS) system formed in accordance with the present invention.

FIG. 2 shows a first embodiment of the point-of-sale (POS) system formed in accordance with the present invention. The POS system preferably includes a computer 12 , receipt printer 16 , and cash drawer 24 . The system is similar to the conventional system shown in FIG. 1 , except that an AC-coupled interface circuit is electrically connected in a series between a driving transistor 18 in the receipt printer 16 and a solenoid 22 in the cash drawer 24 .

The interface circuit preferably includes a resistor R, a first capacitor C 1 , and a second capacitor C 2 . It is to be understood that the resistor R could be replaced with a resistive network of two or more resistors and/or additional passive and/or active components in series and/or parallel while remaining within the scope of the present invention. Similarly, it is to be understood that the capacitors C 1 , C 2 could be replaced with a capacitive network of one or more capacitors and/or additional passive and/or active components in series and/or parallel while remaining within the scope of the present invention.

The first capacitor C 1 and the second capacitor C 2 are preferably electrically coupled in series. The series combination of capacitors C 1 , C 2 are preferably electrically coupled in parallel with the resistor R. The parallel combination of the resistor R and series combination of capacitors C 1 , C 2 are preferably electrically coupled in a series between the driving transistor 18 and the solenoid 22 . The remaining terminal of the solenoid 22 is preferably connected to ground and the emitter of the driving transistor 18 is connected to Vcc.

The computer 12 preferably provides a digital control signal 26 to the receipt printer 16 . This signal is then preferably conditioned and/or translated to a signal that is appropriate for driving the base of the transistor 18 to bias the transistor 18 in an on state. When the transistor 18 is biased on, Vcc, which is preferably connected to the emitter, is substantially applied as an energizing signal 28 to the cash drawer 24 .

To deliver a sufficient amount of current to the solenoid 22 to open the cash drawer 24 , the AC-coupled capacitance provided by the first capacitor C 1 and the second capacitor C 2 should preferably be larger than about 470 uf 25 VDC. Such capacitance values are generally only available in a polarized electrolytic capacitor.

However, a non-polarized equivalent capacitance is preferable, since the interface connections (energizing signal 28 and grounding connection 30 ) between the receipt printer 16 and the cash drawer 24 may be reversed, particularly during installation. Accordingly, by connecting two polarized capacitors C 1 , C 2 in a back-to-back configuration, that is, connecting common polarities of the capacitors to each other, the resulting equivalent capacitance is non-polarized. C 1 and C 2 may be replaced by a single non-polarized electrolytic capacitor C 3 , as shown in FIG. 5 . However, such a capacitor is more expensive than two polarized capacitors.

Alternatively, a diode quad or bridge 33 , as shown in FIG. 6 , may be coupled between the receipt printer and cash drawer to toggle a specified voltage polarity to a polarized capacitor C 4 . That is, if the diode quad 33 is coupled as shown in FIG. 6 , that is, with Node 1 coupled to Node A, then the driving voltage at Node A will be positive. Alternatively, if the diode quad 33 is rotated 180° such that Node 3 of the diode quad 33 is coupled to Node A and Node 1 is coupled to the solenoid 22 , then the voltage at Node A will be negative. In this case, the polarity of the electrolytic capacitor C 4 is preferably reversed such that the negative terminal of the capacitor C 4 is coupled to Node A and Node 1 . Incorporation of the diode quad 33 adds to the cost and attenuates the driving signal to the solenoid. Schottky diodes may be used to reduce attenuation, but will also add to the cost.

Preferably, the diode quad 33 includes a first diode 33 A, second diode 33 B, third diode 33 C, and a fourth diode 33 D. A cathode of the first diode 33 A is preferably connected to a cathode of the second diode 33 B at Node 1 , and an anode of the second diode 33 B is preferably connected to a cathode of the third diode 33 at Node 2 . An anode of the third diode 33 C is preferably connected to an anode of the fourth diode 33 D at Node 3 , and a cathode of the fourth diode 33 D is preferably connected to an anode of the first diode 33 A at Node 4 . The energizing signal is preferably coupled to Node 2 , and the parallel combination of the resistance and the capacitance is preferably coupled to Node 1 . Node 4 is preferably coupled to ground, and Node 3 is preferably coupled to the solenoid 22 .

The operation of the interface circuit, as shown in FIG. 2 , is best explained by considering an example. Assuming the value of the first capacitor C 1 is equal to that of the second capacitor C 2 , which are both equal to a capacitance C, the equivalent capacitance of C 1 and C 2 is equal to C, not C/ 2 . For instance, if the polarity of the capacitors C 1 , C 2 are connected as shown in FIG. 2 (the anodes of capacitors C 1 , C 2 are coupled together) and node A is at a positive voltage, such as Vcc, the first capacitor C 1 is reversed biased and thus shorted. In this case, the capacitance at node A is equal to the forward biased second capacitor C 2 or C.

Likewise, if node A is at a negative voltage, the first capacitor C 1 is forward biased and the second capacitor C 2 is reverse biased and thus shorted. In this case, the capacitance at node A is equal to the forward biased first capacitor C 1 or again C.

To open the cash drawer 24 , the computer 12 preferably provides the digital control signal 26 to the receipt printer 16 , which translates the digital signal to an appropriate pulse or energizing signal 28 for application to the AC-coupled interface circuit in the cash drawer 24 . The current generated in the interface circuit results in an exponentially decaying signal that passes through the solenoid 22 .

Assuming that the solenoid 22 has an inductance L with an equivalent series resistance R, the time constant of the RLC circuit is preferably chosen to be about two to five times the 100–500 millisecond driving pulse to assure that the total energy delivered to the solenoid 22 is about the same as that provided by the DC-coupled interface shown in the conventional POS system of FIG. 1 .

When the pulse in the energizing signal 28 returns to the ground state, the first capacitor C 1 and the second capacitor C 2 preferably discharge through the resistor R and return all voltage levels back to their steady-state values. If the duration of the pulse is excessively long, the first capacitor C 1 and the second capacitor C 2 block the DC current that may flow under such circumstances into the solenoid 22 , thus preventing the solenoid 22 from being damaged.

The value of the resistor R is preferably about 680 ohms at 1 watt. A lower value for the resistor R would enable the first capacitor C 1 and the second capacitor C 2 to discharge more quickly, which may be advisable under certain applications, such as when the cash drawer 24 must be repeatedly and rapidly opened. However, the value of the resistor R should be chosen high enough such that if a steady-state voltage level substantially equal to Vcc is applied to the AC coupled interface circuit, the resistor R will still be able to dissipate safely a sufficient amount of power.

FIG. 3 shows a second embodiment of the POS system formed in accordance with the present invention in which the AC-coupled interface circuit is incorporated in a separate interface enclosure 32 , which is preferably located between the receipt printer 16 and the cash drawer 14 . This embodiment is particularly suitable for retrofitting existing POS systems, in which the receipt printer 16 or cash drawer 14 cannot readily be modified.

FIG. 4 shows a third embodiment of the POS system formed in accordance with the present invention, wherein the AC-coupled interface circuit has been incorporated into a modified receipt printer 34 . Although placement of the AC-coupled interface circuit has been shown and described with respect to three embodiments, these examples are not intended to limit the scope of the invention to the specified embodiments.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.