Abstract:
A dual-mode system for inexpensively operating electrically powered double-insulated devices ( 12 ), such as hand-held power tools and appliances. The system includes a cordless battery power module ( 14 ) that supplies the power and current demands of the device ( 12 ) in a cordless mode or a chopper module ( 21 ) that supplies the necessary power and current demands in a physical envelope commensurate in size and interchangeable with that of the battery power module ( 14 ). The chopper module ( 21 ) is provided with a non-filtered high efficiency converter circuit that allows the chopper module ( 21 ) to generate the power and current required by the driven device ( 12 ). The inductance of the motor ( 11 ) for the driven device ( 12 ) is used to filter the output of the chopper module ( 21 ). Eliminating the output filter from the chopper module ( 16 ) significantly reduces the cost and size of the module ( 16 ). Additionally, the need for multiple battery packs and fast rechargers is minimized by the availability of a low-cost converter. The line power module ( 16 ) includes an EMI filter ( 15 ) for filtering AC or DC input power. The chopper module ( 21 ) chops the filtered voltage, providing a series of voltage pulses having a DC voltage level that is suitable for driving the motor ( 11 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. non-provisional application Ser. No. 09/458,285 filed Dec. 10, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electrically operated power tools and in particular, to portable hand-held power tools which can alternatively operate in either a cordless mode from a self-contained power source or a corded mode from conventional AC mains or an AC/DC generator. 
     BACKGROUND OF THE INVENTION 
     An electrically operated device that functions in a cordless mode typically is powered by a removable battery pack. The battery pack completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recent improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. Using cordless power devices permits work operations to be performed in areas where a conventional AC power source is not available or is inconvenient to use. However, the use of cordless devices is limited by the effective charge capacity of the battery pack and the availability of replacement battery packs. When the battery pack is discharged, it must be recharged or replaced with a fully charged pack. Therefore, to compensate for the limited operating duration; extra battery packs or an optional corded AC converter module must be used with the cordless power device, or a corded power tool must be provided. 
     Both batteries and battery chargers are expensive in comparison to the power device for which they are intended. Batteries for high power applications cost approximately 30% of the cost of the applicable power device. Additional batteries are required to permit cordless mode operation while a battery is recharged and to replace dead batteries. High power levels drawn from batteries during operation of the power tool, the depth of discharge of the battery, the number of charge/discharge cycles, and the speed with which a battery is recharged all contribute to shortening the usable lifetime of a battery. To recharge batteries either a fast charger or a trickle charger must typically be used. A fast charger can be a significant portion of the cost of the power tool or appliance that is powered by the battery. A trickle charger is significantly less expensive than a fast charger, however a trickle charger requires approximately ½ day to recharge a battery pack. A fast charger on the other hand can recharge a battery pack within approximately one hour or less. Therefore, a trade off must be made between using a trickle charger with a large number of battery packs versus using a costly fast charger with very few replacement battery packs. 
     An optional corded AC converter module has only recently been provided for portable cordless power tools. The AC converter module connects to an AC power source and is designed to be interchangeable with the battery pack. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. Such a device allows a tool operator to use the tool in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of such a device enables the operator of a cordless tool to complete a project when the battery pack has been discharged, or to continue to use the tool while the battery pack is charging and a fully charged backup battery pack is unavailable. Hence, by using a corded converter module the need for extra battery packs is reduced. 
     To increase the desirability of a corded converter module over the choices of additional battery packs or a separate corded power device, it is necessary to provide the desired output power capability at the lowest possible cost while maintaining a high quality device. The cost of a corded converter module is strongly related to the output power capability of the converter module. The higher the output power capability, the higher the cost. Therefore, it is desirable to design the output power capability of the corded converter module to be comparable to the output power capability of the corresponding battery pack. In addition, the maximum envelope of a corded converter module must conform to the envelope of the battery pack with which it is interchangeable. With the introduction of cordless tools of 24 volts and greater, the envelope of a conventional corded converter module is adequate for supporting the power output levels required to drive power devices such as hand held power tools. In tool voltages of 18 volts and below, the smaller battery pack sizes pose a challenge to the designer of an equally powered chopper circuit. Therefore, the main constraints on the output power capability of a corded converter module are the goals of minimizing size and cost and increasing reliability. Previously, attempts to minimize the cost of corded converter modules have concentrated on matching the output power capability of the converter module to a given power tool power requirement and then minimizing the cost of the resulting converter module components. By designing the converter module for the minimum output power required to satisfactorily drive the power tool, lower cost electronic components can be chosen for the converter. However, merely selecting the lowest cost devices that will attain the desired output power capability typically only results in marginal cost savings. 
     To obtain significant cost savings it is generally necessary to eliminate components from the design of the corded converter module. In a previously filed application, the power transformer that is used in a conventional corded converter module to meet government safety requirements was eliminated (see U.S. application Ser. No. 09/458,285). Instead of using the power transformer to meet the safety requirements, a double insulated case was relied upon. Generally, the power magnetics including power transformers and power inductors are amongst the more costly components within a corded converter module. Typically, conventional corded converter modules use a power inductor in combination with an output capacitor to filter voltage that is applied to the power tool motor. The power inductor is typically a custom designed device that is bulky and expensive in comparison with the other components of the corded converter module. The filtered voltage from the power inductor is applied to the motor, which has an inductance that is inherent in the construction of the motor. Optimizing the design of the power inductor to match the desired output power capability merely provides marginal cost savings. 
     Operators of cordless power tools already faced with the cost of battery packs and battery chargers must also invest in expensive corded converter modules for their power tools. As an alternative many purchase a corded power tool to use in lieu of the cordless tool when an AC power source is nearby. Attempts to minimize the cost of corded conversion modules have been constrained by the cost of using power inductors in combination with an output capacitor to filter the voltage supplied to the motor. Obtaining further cost reductions by reducing the output power level of a corded converter module would result in under-powered power devices. While the prior art can be used to provide corded converter modules for a handheld power tool, it has not proven capable of providing low cost modules that are convenient to use. 
     SUMMARY OF THE INVENTION 
     The present invention decreases costs by filtering the motor drive voltage in a unique manner. The invention uses the inductance of the power tool motor windings rather than employing a discrete output filter. Eliminating the power inductor and output capacitor from the corded converter module significantly reduces the cost and weight of the module. A low cost corded power module provides operators of cordless power tools the low cost option of using a corded power module when AC power sources are available. This eliminates the cost of purchasing a separate corded power device as well as reducing the number of battery packs that must be purchased. 
     Corded power modules designed without output filters are substantially less expensive than modules designed with output filters. Additionally, eliminating the output filter decreases the weight of the module resulting in improved operator comfort. 
     For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a three-dimensional view partially showing the manner of connecting a battery power module to the power device; 
     FIG. 2 is a three-dimensional view partially showing the manner of connecting a line power module to the power device; 
     FIG. 3 is a block diagram of a first configuration of a tool power system constructed in accordance with the teachings of the invention; 
     FIG. 4 is signal diagram showing the voltage and current waveforms associated with the chopper module; 
     FIG. 5 is a block diagram of a second configuration of a power system for the power device; 
     FIG. 6 is a detailed block diagram of a first embodiment of the second configuration of a power system for the power device; 
     FIG. 7 is a detailed block diagram of a second embodiment of the second configuration of a power system for the power device; 
     FIG. 8A is a detailed block diagram of presently preferred embodiment of a power system for the power device; 
     FIG. 8B is a block diagram of a third configuration of a power system for the power device; 
     FIG. 9 is a three-dimensional exploded view of the battery power module of the presently preferred embodiment of the invention; 
     FIG. 10 is a three-dimensional exploded view of the line power module of the presently preferred embodiment of the invention; 
     FIG. 11 is an end view of the battery power module illustrating an attached terminal block; 
     FIG. 12 is a three-dimensional view of the power tool terminal block that mates to both the battery power module terminal block and the converter power module terminal block; 
     FIG. 13 is a two-dimensional view of the interface between the battery power module terminal block and the power tool terminal block; and 
     FIG. 14 is a two-dimensional view of the interface between the line power module and the power tool terminal block. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a dual-mode portable power tool  12  according to the present invention is shown. While the present invention is shown and described with a reciprocating saw  12 , it will be appreciated that the particular tool is merely exemplary and could be a circular saw, a drill, a sander, or any other similar portable power tool constructed in accordance with the teachings of the present invention. 
     The power tool  12  includes a tool interface (not shown) which is driven through a gear train (not shown) by a DC motor  11 . The motor  11  is mounted within a housing  91  that includes a handle  92  extending therefrom. A trigger switch  93  is mounted in the handle  92  behind the motor  11 . The DC motor  11  is adapted in the preferred embodiment to be powered by a  24  volt DC source, although other DC voltage systems, such as 18 volts or 100 volts, could be used. In a first operating mode shown in FIG. 1, the power tool  12  is powered by a removable battery power module  14 . Alternatively, as shown in FIG. 2, the power tool  12  may be powered from common 115 volt AC line power via a line power module  16  which is adapted to be plugged into the power tool in place of the battery power module  14 . Additionally, the power tool  12  may be powered from 100 VAC, or 240 VAC, as well as from a DC generator (not shown) via the line power module  16 . Following the description of the electrical circuitry, a more detailed description of the mechanical configuration of the power tool  12  is provided. 
     Referring to FIG. 3, a first embodiment of a tool power system constructed in accordance with the teachings of the invention is illustrated. The power system includes a chopper module  21   a  that converts a rectified AC or DC input voltage to a chopped output voltage that is applied to the motor  11 . In the presently preferred embodiment, the chopper module  21   a  uses a transformerless configuration, however it is within the scope of the invention to use a transformer isolated configuration such as a forward, half-bridge, and flyback. In addition, although the present embodiment operates at a fixed frequency of 25 kHz, it is envisioned that the power system can be operated at higher or lower operating frequencies as will be explained later in this specification. A line power conditioner  18  coupled to common 115 volt AC provides line power. The line power conditioner  18  includes an EMI filter  15  to attenuate high frequency conducted emissions that are conducted from the chopper module  21   a  onto the AC line. A full wave bridge rectifier (FWB)  17  connected to the EMI filter  15  rectifies the line voltage. The chopper module  21   a  chops the rectified voltage and supplies the chopped voltage to the DC motor  11 , thereby providing controlled power for the power tool  12 . Output filtering, such as a power inductor and output capacitor, is not used to attenuate the switching frequency AC components. Instead, the inductance  22  of the motor  11  is relied upon to filter the chopped voltage. FIG. 4 illustrates the chopped voltage, Vm, applied to the motor  11 , and the motor current, Im, that results from the averaging effect of the motor inductance  22 . Relying on the inductance  22  of the motor  11  to filter the output voltage negates the need for an output filter within the chopper module  21   a , thereby decreasing the cost of the power tool  12  and requiring less volume for the line power conditioner  18 . The selected switching frequency of the chopper module is strongly affected by the value of the inductance  22 . The switching frequency of the chopper module is preferably chosen so that the magnitude of current flowing through the motor inductance  22  varies by less than approximately 10% of the average current at the switching frequency. However, it is within the scope of the invention to select a switching frequency so that the magnitude of current flowing through the motor inductance  22  varies by less than approximately 40%. A control input  19  provides a signal for setting the duty cycle of the chopper module  21   a , so that the average DC voltage applied to the DC motor  11  is maintained within the operating range of the motor  11 . In the embodiment, the control input  19  is an open loop signal provided by a trigger switch on the power tool, however it is within the scope of the invention to provide closed loop control of the power tool by monitoring power tool parameters such as motor current, back EMF voltage and motor speed. Sensors for monitoring the power tool parameters include tachometers, motor back EMF voltage monitors, motor current monitors, motor average voltage monitors, and DSPs of motor current commutation. 
     The chopper module  21   a  is alternatively powered by the battery power module  14 , which supplies a DC voltage from a battery pack (not shown). The battery power module  14  connects to the chopper module  21   a  in lieu of the line power conditioner  18 . In operation, DC voltage from the battery power module  14  is chopped by the chopper module  21   a  in response to the control input  19  and supplied to the DC motor  11 . Similar to operation from the AC line, the chopped voltage is filtered across the internal inductance  22  of the motor  11 , providing an average DC voltage within the operating range of the motor  11 . 
     Illustrated in FIG. 5 is a block diagram of a presently preferred embodiment of a power subsystem for the power tool  12 , constructed in accordance with the principles of the invention. The power subsystem includes a line power module  16   a  that converts line power to a chopped voltage that is supplied to a tool module  20 . Alternatively, a battery power module  14  provides a DC voltage to the tool module  20  from a battery pack (not shown). The tool module  20  has two operating modes. When connected to the line power module  16 , the tool module  20  supplies the chopped voltage from the line power module  16  to the DC motor  11 . In addition, the tool module  20  receives a control input  19  corresponding to a trigger switch position (not shown) and sends a corresponding PWM control signal  24  to the line power module  16  to control the duty cycle of the chopped voltage. When connected to the battery power module  14 , the tool module  20  chops the DC voltage from the battery pack and supplies the chopped voltage to the DC motor  11 . The duty cycle of the chopped voltage is regulated by the control input  19 . 
     Illustrated in FIG. 6 is a detailed diagram of a first embodiment of a power tool power subsystem conforming to the principles of the invention. The power subsystem includes a line power module  16   a  that converts line power to a chopped voltage that is supplied to a tool module  20   a . Alternatively, a battery power module (not shown) provides a DC voltage to the tool module  20   a  from a battery pack (not shown). 
     The line power module  16   a  includes an EMI filter (not shown) and full wave bridge rectifier (FWB)  17  for filtering and rectifying input line power. A series switch  26  for repetitively chopping the input power is connected in series with the output of the FWB  22  and the motor  11 . A chopper controller  28  supplies a drive signal to control the operation of the series switch. An interface circuit  30  is connected from the tool module  20   a  to the chopper controller  28 . Although in the present embodiment the interface circuit is an optocoupler, the scope of the invention includes other interface circuits such as differential amplifiers and signal transformers. The interface circuit  30  receives a duty cycle signal  24  from the tool module  20   a  for controlling the duty cycle of the series switch  26 . A voltage source  32  with a series resistor  33  provide a current path for the duty cycle signal  24  from the tool module  20   a . A free-wheeling diode  38  supplies a current path for the motor current during time periods when the series switch  26  is not conducting. 
     Continuing to refer to FIG. 6, the tool module  20   a  has two operating modes. When connected to the line power module  16   a , the tool module  20   a  supplies the chopped voltage from the line power module  16   a  to the DC motor  11 . In addition, the tool module  20   a  receives a control input  19  corresponding to a trigger switch position (not shown), and sends the corresponding duty cycle signal  24  to the line power module  16   a  to control the duty cycle of the chopped voltage. When connected to the battery power module  14  (FIG.  3 ), the tool module  20   a  chops the DC voltage from the battery pack and supplies the chopped voltage to the DC motor  11 . The duty cycle of the chopped voltage is regulated by a control input  19 . 
     The tool module  20   a  includes a series switch  34  for sending the duty cycle signal  24  to the line power module  16   a  and for chopping the DC voltage from the battery power module (not shown). A tool controller  36  controls the tool module series switch  34  in response to the control input  19 . A free-wheeling diode  40  connects to the battery power module to provide a current path for freewheeling motor current when the battery power module (not shown) is connected to the tool module  20   a . The diode  40  is selected to be a Schottky diode or other fast recovery diode to reduce conduction losses. 
     Referring to FIG. 6, the operation of the illustrated embodiment during line power mode is as follows. The operator adjusts the trigger switch position to provide a desired control input  19  to the tool module controller  36 . In response to the control input, the tool module controller controls the operation of the tool module series switch  34  that alternately provides a short and an open across the voltage source  32  and series resistor  33 . When an open is applied, current flows through the voltage source  32 , the series resistor  33 , and the input to the interface circuit  30 . The output signal from the interface circuit is averaged by the chopper controller  28  and used to control the pulse width of the line power module series switch  26 . As will be recognized by those skilled in the art, it is within the scope of the invention to synchronize the chopper control  28  to the output signal from the interface circuit and use the pulse width of the signal superimposed on the chopper pulse to drive the series switch  26 . In response to the series switch  26  turning on, current flows from the FWB  17  through the internal inductance of the motor  11 , and the line power module series switch  26  before returning to the FWB  17 . When the series switch  26  turns off, the current that was ramping up through the internal inductance  22 , begins to ramp down as it flows from the internal inductance  22  through the motor  11  and then back through the line power module free-wheeling diode  38 . 
     During the battery power mode, the line power module  16   a  is replaced by a battery power module (not shown). Once again, the operator adjusts the trigger switch position to provide a desired control input  19  to the tool module controller  36 . In response to the control input, the tool module controller controls the operation of the tool module series switch  34  that alternately turns on and off. Pulse width modulation is employed in the embodiment, however it is within the scope of the invention to use other modulation methods such as frequency modulation. When the switch  34  is on, current flows from the battery power module, through the internal inductance and the motor  11 , and the tool module series switch  34  before returning to the battery power module. When the series switch  34  turns off, the current that was ramping up through the internal inductance  22 , begins to ramp down as it flows from the internal inductance  22 , through the motor  11 , through the tool module free-wheeling diode  40 , and then through the battery power module jumper wire  101 . This jumper wire  101  is a key solution to the conflicting ratings of low voltage diode  40  and high voltage diode  38 . 
     Referring to FIG. 7, a second embodiment of a power tool power system conforming to the principles of the invention is illustrated. The second embodiment includes a chopper module  16   b  for chopping line power during line power mode, and a tool module  20   b  for chopping battery during battery power mode. The chopper module  16   b  comprises a FWB  17 , a chopper controller  42 , a free-wheeling diode  44 , and a series switch  46 . The FWB  17  rectifies 120 Vac, 60 Hz input power. The series switch  46  chops the rectified line power and couples the chopped signal through the tool module  20   b  to the motor  11 . The chopper controller  42  controls the series switch  46  in response to a duty cycle signal  48  from the tool module  20   b . The free-wheeling diode  44  provides a conduction path for current from the motor  11  when the series switch  46  is in the non-conducting state. 
     The tool module  20   b  comprises a series switch  50  for chopping battery power during the battery power mode. A low voltage MOSFET having a breakdown voltage slightly greater than the battery voltage is used as the series switch  50 . During line power mode the series switch  50  is turned on continuously to prevent the line voltage from overstressing the device Vds breakdown voltage. A gate resistor  52  is coupled between a tool controller  54  and the series switch  50 . The tool controller  54  supplies a pulse width modulated output that drives the tool module series switch  50  during battery power mode and supplies the duty cycle signal to the chopper module  16   b  during line power mode. A diode  58  is connected from a module power port  48  to the tool controller  54  for supplying circuit power. A signal diode  56  for providing the duty cycle signal to the chopper module  16   b  is connected from the module power port  48  to the input of the series switch  50 . A power diode  60  for supplying an alternate source of circuit power during battery power mode is connected from the motor power input to the tool controller  54 . A free-wheeling diode  62  is connected in parallel with the motor  11  through the battery power module jumper wire (not shown) for providing a current path for current from the motor  11  during battery power mode. 
     The operation of the second embodiment is similar to the operation of the first embodiment with the exception of the method of coupling the pulse width signal from the tool module  20   b  to the chopper module  16   b . During line power mode, the chopper module  16   b  forces the tool module series switch  50  on continuously and provides circuit power to the tool module  16   b  through the module power port  48 . When the output of the tool module controller  54  is in the high state, the signal diode  56  is reverse-biased preventing current from the chopper module  16   b  from flowing into the tool controller  54 . When the output of the tool module controller  54  is in the low state, the signal diode  56  is forward-biased permitting current to flow from the chopper module  16   b  into the tool controller  54 . The chopper controller  42  obtains the duty cycle information by sensing the change in current magnitude of the duty cycle signal  48 . 
     Referring to FIG. 8A, a presently preferred embodiment of a power tool power subsystem conforming to the principles of the invention is illustrated. The presently preferred embodiment includes a chopper module  16   c  for chopping line power during line power mode, and a tool module  20   c  for chopping battery power during battery power mode. Similar to the second embodiment, the chopper module  16   c  comprises a FWB  17 , a series switch  46 , and a free-wheeling diode  44 . In addition, the chopper module  16   c  includes a chopper controller  64 , an interface circuit  66  and a module power circuit  68 . The chopper controller  64  controls the series switch  46  in response to a duty cycle signal  72  from the tool module  20   c  that is transmitted through the interface circuit  66 . 
     Similar to the second embodiment, the tool module  20   c  comprises a series switch  50  for chopping battery power during the battery power mode, a gate resistor  52 , a tool controller  54 , a diode  58 , a free-wheeling diode  62 , and a power diode  60  for supplying an alternate source of circuit power. In addition, the tool module  20   c  includes a signal diode  70  for providing the duty cycle signal to the chopper module  16   c . The signal diode  70  is connected from the module power circuit  68  to the input of the series switch  50  to turn the switch  50  on continuously during line power mode. An output  74  of the tool controller  54  connects to the interface circuit  66  for providing a duty cycle signal  72 . 
     The operation of the presently preferred embodiment is similar to the operation of the first embodiment with the exception of the method of coupling the pulse width signal from the tool module  20   c  to the chopper module  16   c . During line power mode, the module power circuit  68  of the chopper module  16   c  forces the tool module series switch  50  on continuously and provides circuit power to the tool module  16   c . When the output  74  of the tool module controller  54  is in the high state, current from the output  74  flows through the interface circuit  66  of the chopper module  16   c  providing duty cycle information. The chopper controller  64  averages the pulsed duty cycle information and in response controls the chopper module series switch  46 . Averaging the duty cycle information permits the chopper controller  64  and the tool controller to be operated unsynchronized. 
     Referring to FIG. 8B, an alternative embodiment in accordance with the principles of the invention is illustrated. This embodiment differs from the previous embodiments in that a chopper module that is common to the power path of both the battery power module and the line power module is not included. Instead, a unique chopper module is included for each of the battery power module  14   d  and the line power module  16   d . This configuration provides higher efficiency during the line power operating mode by eliminating one MOSFET from the primary conduction path. However, placing a MOSFET within the battery power module increases the heat to which the battery assembly is subjected. 
     The battery power module  14   d  includes a battery assembly  80  for supplying battery power. A chopper module  82  converts the battery power to a series of voltage pulses that are coupled through a tool controller  84  to the motor  11 . A duty cycle signal  86  is coupled from the tool controller  84  to the chopper module  82  for controlling the duty cycle of the voltage pulses. The chopper module  82  includes a relatively low voltage MOSFET (not shown) for chopping the voltage supplied by the battery assembly  80 . 
     The line power module  16   d  includes an EMI filter  87  and FWB  88  for attenuating high frequency components and rectifying the line power. The output of the FWB  88  is coupled to a chopper module  90  that converts the rectified line power to a series of voltage pulses that are coupled through the tool controller  84  to the motor  11 . The duty cycle signal  16   d  from the tool controller  84  is coupled to the chopper module  90  of the line power module  78 . A relatively high voltage MOSFET (not shown) is employed in the chopper module  90  for chopping the rectified voltage supplied through the FWB  88 . 
     As illustrated in the previously described embodiments, the invention is preferably practiced with a non-transformer-isolated line power module, although it is within the scope of the invention to employ transformer isolation. The non-transformer-isolated technique is described in previously filed U.S. application Ser. No. 09/458,285. Eliminating the power transformer in addition to eliminating the output filter provides additional cost savings and an additional reduction in the circuit complexity and size. Therefore, the preferred embodiment of the invention is practiced with a non-isolated line power module  16 . As described more fully below, the elimination of transformer isolation impacts the type of housing and power interface that are employed in a power tool  12 . 
     Returning to FIG. 1, although the power tool motor  11  of the presently preferred embodiment is designed to be powered by a relatively low voltage DC power source (i.e., a DC source less than 42.4 volts), the housing  91  of the power tool  12  in the preferred embodiment is nonetheless double insulated from the electrical system of the tool. As is well known to those skilled in the art, power tools designed to be operated by a high voltage power source, such as a conventional AC or corded power tool, are typically constructed so that the housing of the tool is double insulated from the electrical system of the tool for safety reasons. In this manner, the operator of the tool is protected against electrical shock in the event of a short in the electrical system of the tool. Generally, cordless or DC powered tools are powered by low voltage power sources and therefore do not require such safety measures. Consequently, conventional DC powered tools do not insulate the housing from the electrical system of the tool. 
     There are of course, many DC powered portable devices that are alternatively powered from high voltage AC house current. To enable this alternative operation, however, AC/DC powered devices generally employ output filters in combination with transformers to provide a DC output that is isolated from the high voltage AC power source. The DC output provides the power required to operate the power device. 
     While this solution may be acceptable for relatively low powered devices, such as portable stereos, the power requirements of many portable power tools necessitates the use of large output filters and step-down transformers which are not only bulky, but also very heavy. Consequently, DC powered tools that can alternatively be powered from AC house current have rarely been offered commercially. 
     The present invention solves this dilemma by providing a relatively light weight converter that has a non-filtered output for driving the power device. By eliminating the output filter in the presently preferred chopper module  21   a  of FIG. 3, the output voltage supplied to the motor  11  of the power tool  12  is a series of voltage pulses. The inductance  22  of the motor  11  is used to filter the voltage pulses so that an average voltage that is compatible with the motor  11  is applied. In the preferred embodiment, the step-down transformer is eliminated in addition to eliminating the output filter. By eliminating transformer-isolation in the presently preferred chopper module  21   a , the output voltage supplied to the motor of the power tool  12  is referenced to the  115  volt AC input. Consequently, double insulation of the tool housing from the electrical system of the power tool is necessary. A double insulated housing is also necessary when a step-down transformer is employed that provides an output having a maximum voltage amplitude that is greater than 42.4 volts. 
     In addition, since the presently preferred embodiment does not employ a step-down transformer, the power interface is provided with male connectors uniquely adapted to make electrical contact with, and thereby receive electrical power from, specially recessed female connectors in the line power module  16 . Thus, despite the non-isolated construction of the line power module  16 , all applicable safety requirements for operating a power tool from a high voltage power source are satisfied. Following is a detailed description of the housing and power interface that is employed in the presently preferred embodiment of the invention. 
     With particular reference to FIGS. 9 and 11, the battery power module  14  of the present invention is illustrated to generally include a housing  118 , a battery  120  which in the exemplary embodiment illustrated is a  24  volt nickel-cadmium battery, and a battery pack terminal block  122 . To facilitate releasable attachment of the battery power supply module  14  to the tool  12 , the upper portion  125  of the housing  118  is formed to include a pair of guide rails  124 . The guide rails  124  are adapted to be slidably received into cooperating channels  113  (FIG. 1) formed in a handle  92  of the tool  12 . To further facilitate removable attachment of the battery power supply module  14  to the tool  12 , the upper portion  125  of the housing  118  further defines a recess  126 . The recess  126  is adapted to receive a latch (not shown) carried by the housing of the tool  12 . The latch is conventional in construction and operation and is spring biased to a downward position so as to engage the recess  126  upon insertion of the rechargeable battery power supply module  14 . Removal of the battery power supply module  14  is thereby prevented until the spring bias of the latch is overcome in a conventional manner insofar as the present invention is concerned. 
     With continued reference to FIGS. 9 and 11, the battery pack terminal block  122  comprises a main body portion  128  constructed of rigid plastic or other suitable material and a plurality of blade-type terminals  130 . In the exemplary embodiment illustrated, the battery pack terminal block  122  includes four blade terminals  130 . Two of the blade terminals  130  comprise the positive and negative terminals for the battery  120 . A third terminal  130  may be used to monitor the temperature of the battery  120  and a fourth terminal may be used to identify the battery type (e.g., 24 volt NiCad). As best shown in FIG. 11, a pair of holes  132  are formed in the two guide rails  124  in the upper portion  125  of the battery pack housing  118  on either side of the row of blade terminals  130 . The function of these holes is described below. 
     Turning now to FIG. 12, the terminal block  134  of the power tool  12  is shown. The main body of the tool terminal block  134  is also constructed of a rigid plastic material and is formed with a row of four U-shaped guideways  136  guiding the four corresponding blade terminals  130  of the battery power supply module  14  when the battery pack is inserted into the tool  12 . Located within the guideways  136  are female connectors  138  that are adapted to engage and make electrical contact with the blade terminals  130  of the battery power supply module  14 . Although the tool terminal block  134  shown is designed to accommodate four female connectors for each of the four battery pack blade terminals  130 , only two female connectors  138  adapted to engage the positive and negative blade terminals  130  of the battery power supply module  14  are used in the tool terminal block  134 , as the remaining two battery pack blade terminals  130  are only used when recharging the battery power supply module  14 . 
     Also connected to the positive and negative female terminals  138  in the tool terminal block  134  are positive and negative male terminals  140  that project through openings  142  in the terminal block on either side of the row of guideways  136 . As will subsequently be discussed below, the male positive and negative terminals  140  are used to electrically connect the tool  12  to the AC/DC converter module  16 . 
     With additional reference to FIG. 13, the interface between the battery terminal block  122  and the tool terminal block  134  is illustrated. As the guide rails  124  of the battery power supply module  14  are slid into the channels  113  in the tool housing, the battery pack terminal block  122  is guided into alignment with the tool terminal block  134  as shown. To further facilitate proper alignment between the two terminal blocks  122  and  134 , the main body portion of the tool terminal block  134  includes a pair of laterally spaced rails  144  that are adapted to be received within the grooves  146  provided in the battery pack housing  118  immediately below the guide rails  124 . Further insertion of the battery power module  14  into the tool  12  results in the positive and negative blade terminals  130  of the battery power module  14  passing through the openings in the U-shaped guideways  136  and engaging the female connectors  138  in the tool terminal block  134 . Note that the male positive and negative terminals  140  from the tool terminal block  134  simultaneously project into the openings  132  formed in the rails  124  on the upper portion  125  of the battery pack housing  118 , but do not make electrical contact with any terminals in the battery power module  14 . Similarly, the remaining two blade terminals  130  from the battery terminal block  122  project into empty guideways  136  in the tool terminal block  134 . 
     Returning to FIG. 2 with reference to FIG. 10, the line power module  16  according to the present invention is adapted to convert 115 volts AC house current to a pulsed output having an average DC voltage of  24  volts. The housing  148  of the converter module  16  in the preferred embodiment is configured to be substantially similar to the housing  118  of the battery power module  14 . In this regard, the housing  148  includes first and second clam shell halves joined at a longitudinally extending parting line. An upper portion  150  of the housing  148  includes a pair of guide rails  152  similar to those of the battery power supply module  14  for engaging the channels  113  in the tool housing. The upper portion  150  also defines a recess (not shown) which includes a latch (not shown) for preventing the inadvertent removal of the converter module  16 . The housing  148  also defines a recess  151  in which a fan  145  is adapted for providing cooling airflow to the converter module  16 . Attached to the fan  145  is a fan cover  147  for preventing foreign objects from impeding the operation of the fan  145 . Within the housing  148  several heatsinks  143  provide heat spreading and cooling for selected power converter components. 
     With additional reference to FIG. 14, the interface between the line power module  16  and tool terminal block  134  is shown. The line power module  16  includes a pair of female terminals  154  that are adapted to receive the male terminals  140  of the tool terminal block  134 . In a manner similar to that described above in connection with the installation of the battery power module  14  on the tool  12 , the guide rails  152  on the upper portion  150  of the converter housing  148  are adapted to engage the laterally spaced rails  144  on the tool terminal block  134  as the line power module  16  is installed on the tool  12  to ensure proper alignment between the female connectors  154  of the line power module  16  and the male connectors  140  of the tool  12 . 
     Due to the non-isolated nature of the line power module  16  in the presently preferred embodiment, the female terminals  154  are recessed within the upper portion  150  of the housing  148  of the line power module  16  to meet safety requirements. In the preferred embodiment, the female terminals  154  are recessed within the housing  148  of the line power module  16  by at least 8 mm. 115 volt AC power is converted to a pulsed voltage output by the line power module  16  and delivered to the tool  12  through the female terminals  154 . When the line power module  16  is operatively installed on the tool  12 , the female terminals  138  of the tool terminal block  134  are electrically inoperative. 
     The line power module  16  initially converts the low frequency AC input to a rectified voltage, then the chopper module  21   a  converts the rectified voltage to a high frequency pulsed voltage output that has an average DC level suitable for operating power tool  12 . The power tool  12  uses the inductance  22  of the motor  11  to filter the pulsed output of the chopper module  21   a  rather than including an output filter within the chopper module  21   a , thereby significantly reducing the cost and weight of the chopper module  21   a.    
     In addition, the power tool employs double insulation of the motor  11  rather than transformer isolation of the chopper module  21   a , thereby further reducing the cost and weight of the chopper module  21   a.    
     Additionally, the chopper module  21   a  is designed with a comparatively small number of components while providing an efficient conversion process. This further enhances the lightweight, compact features of the chopper module  21   a . The size of the chopper module  21   a  further permits the use of the line power module  16  in power-operated devices, such as the reciprocating saw  12 , which heretofore were too small to support and contain conversion units providing power in a range of at least 50 watts and higher. 
     Further, while the preferred embodiment of the chopper module  21   a  converts a low frequency, high voltage level to a pulse train having a DC voltage level suitable for operating the motor  11 , the chopper can be used to convert a high DC voltage level to a pulse train by applying the high DC level directly to a suitable power cord and plug that connects to the input of the line power module  16 . In this manner, the power tool  12  could be operated from the high DC voltage source instead of the low DC voltage of the cells and thereby conserve the charge life of the cells. 
     The chopper module  21   a  could be designed to operate from external AC power sources other than 120 volts at 60 Hz. Without departing from the spirit and scope of the invention, the chopper module  21   a  also could be designed to provide a voltage pulse output having a DC output voltage level in a range of 3.6 to 48 volts. In a particular example, the chopper module  21   a  could be adjusted to develop a pulse output having a DC voltage output level of 24 volts, derived from an external AC source of 220 volts at 50 Hz as applied to a suitable power plug and cord. The chopper module  21   a  could then be used to provide inexpensive dual mode capability for power-operated devices that operate at a DC voltage supply level of 24 volts. 
     The reciprocating saw  12  is merely illustrative of one example of many power-operated, cordless-mode devices that become more versatile because of the inventive cost efficient dual-mode capability. Other examples of power-operated cordless devices which are enhanced by the inventive concept include, but are not limited to, drills, screwdrivers, screwdriver-drills, hammer drills, jig saws, circular saws, hedge trimmers, grass shears, as well as battery-operated household products and the like. 
     Thus it will be appreciated from the above that as a result of the present invention, an inexpensive dual-mode corded/cordless system for power-operated devices is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.