Abstract:
A method and apparatus for providing a welding current is disclosed. The power source is capable of receiving any input voltage over a wide range of input voltages and includes an input rectifier that rectifies the ac input into a dc signal. A dc voltage stage converts the dc signal to a desired dc voltage and an inverter inverts the dc signal into a second ac signal. An output transformer receives the second ac signal and provides a third ac signal that has a current magnitude suitable for welding. The welding current may be rectified and smoothed by an output inductor and an output rectifier. A controller provides control signals to the inverter and an auxiliary power controller that can receive a range of input voltages and provide a control power signal to the controller.

Description:
This is a continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 09/827,440, filed Apr. 6, 2001, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding Power Source, which is continuation of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 09/200,058, filed Nov. 25, 1998, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding Power Source, which issued on May 29, 2001, as U.S. Pat. No. 6,239,407, which is a continuation of U.S. patent application Ser. No. 08/779,044, filed Jan. 6, 1997, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding Power Source, which issued on Dec. 14, 1999 as U.S. Pat. No. 6,002,103, which is a continuation of Ser. No. 08/342,378 filed Nov. 18, 1994, entitled Method And Apparatus For Receiving A Universal Input Voltage In A Welding Power Source, which issued on Feb. 11, 1997, as Pat. No. 5,601,741. 

   FIELD OF THE INVENTION 
   This invention generally relates to power sources. More particularly, this invention relates to inverter power sources employed in welding, cutting and heating applications. 
   Power sources typically convert a power input to a necessary or desirable power output tailored for a specific application. In welding applications, power sources typically receive a high voltage alternating current (VAC) signal and provide a high current output welding signal. Around the world, utility power supplies (sinusoidal line voltages) may be 200/208V, 230/240V, 380/415V, 460/480V, 500V and 575V. These supplies may be either single-phase or three-phase and either 50 or 60 Hz. Welding power sources receive such inputs and produce an approximately 10–40 volt dc high current welding output. 
   Welding is an art wherein large amounts of power are delivered to a welding arc which generates heat sufficient to melt metal and to create a weld. There are many types of welding power sources that provide power suitable for welding. Some prior art welding sources are resonant converter power sources that deliver a sinusoidal output. Other welding power sources provide a squarewave output. Yet another type of welding power source is an inverter-type power source. 
   Inverter-type power sources are particularly well suited for welding applications. An inverter power source can provide an ac square wave or a dc output. Inverter power sources also provide for a relatively high frequency stage, which provides a fast response in the welding output to changes in the control signals. 
   Generally speaking, an inverter-type power source receives a sinusoidal line input, rectifies the sinusoidal line input to provide a dc bus, and inverts the dc bus and may rectify the inverted signal to provide a dc welding output. It is desirable to provide a generally flat, i.e. very little ripple, dc bus. Accordingly, it is not sufficient to simply rectify the sinusoidal input; rather, it is necessary to also smooth, and in many cases alter the voltage of, the input power. This is called preprocessing of the input power. 
   There are several types of inverter power sources that are suitable for welding. These include boost power sources, buck power sources, and boost-buck power sources, which are well known in the art. 
   Generally, a welding power source is designed for a specific power input. In other words, the power source cannot provide essentially the same output over the various input voltages. Further, components which operate safely at a particular input power level are often damaged when operating at an alternative input power level. Therefore, power sources in the prior art have provided for these various inputs by employing circuits which can be manually adjusted to accommodate a variety of inputs. These circuits generally may be adjusted by changing the transformer turns ratio, changing the impedance of particular circuits in the power source or arranging tank circuits to be in series or in parallel. In these prior art devices, the operator was required to identify the voltage of the input and then manually adjust the circuit for the particular input. 
   Generally, adapting to the various voltage inputs in the prior art requires that the power source be opened and cables be adjusted to accommodate the particular voltage input. Thus, the operator was required to manually link the power source so that the appropriate output voltage was generated. Operating an improperly linked power source could result in personal injury, power source failure or insufficient power. 
   Prior art devices accommodated this problem by configuring the power source to operate at two different VAC input levels. For example, U.S. Pat. No. 4,845,607, issued to Nakao, et al. on Jul. 4, 1989, discloses a power source which is equipped with voltage doubling circuits that are automatically activated when the input is on the order of 115 VAC, and which is deactivated when the input is on the order of 230 VAC. Such sources are designed to operate at the higher voltage level, with the voltage doubling circuit providing the required voltage when the input voltage is at the lower level. This type of source, which uses a voltage doubling circuit, must use transistors or switching devices as well as other components capable of withstanding impractical high power levels to implement the voltage doubling circuit. Further, the circuitry associated with the voltage doubling circuit inherently involves heat dissipation problems. Also, the voltage doubling circuit type of power source is not fully effective for use in welding applications. Thus, there exists a long felt need for a power source for use in welding applications which can automatically be configured for various VAC input levels. 
   Welding power sources are generally known which receive a high VAC signal and generate a high current dc signal. As particularly effective type of the power source for welding applications which avoids certain disadvantages of the voltage doubling circuit type of power source generally relies on a high frequency power inverter. Inverter power sources convert high voltage dc power into high voltage AC power. The AC power is provided to a transformer which produces a high current output. 
   Power inverters for use over input voltage ranges ale generally known in the art. For example, a power inverter which is capable of using two input voltage levels is disclosed in U.S. Pat. No. 3,815,009, issued to Berger on Jun. 4, 1974. The power inverter of that patent utilizes two switching circuits; the two switching circuits are connected serially when connected to the higher input voltage, but are connected in parallel to account for the lower input voltage. The switching circuits are coupled to each other by means of lead wires. This inverter is susceptible to operator errors in configuring the switching circuits for the appropriate voltage level, which can result in power source malfunction or human injury. 
   Other prior art welding sources that improved upon manual linking provided an automatic linkage. For example, the Miller Electric AutoLink is one such power source and is described in U.S. Pat. No. 5,319,533 incorporated herein by reference. Such power sources test the input voltage when they are first connected and automatically set the proper linkage for the input voltage sensed. Such welding power sources, if portable, are generally inverter-type power sources, and the method by which linking is accomplished is by operating the welding power source as two inverters. The inverters may be connected in parallel (for 230V, for example) or in series (e.g., for 460V). Such arrangements generally allow for two voltage connection possibilities. However, the higher voltage must be twice the lower voltage. Thus, such a power source cannot be connected to supplies ranging from 230V–460V to 380V–415V or 575V. 
   A 50/60 Hz transformer could be used to provide multiple paths for various input voltages. It would, however, have the disadvantage of being heavy and bulky compared to an inverter-type welding power source of the same capacity. In addition, if it was automatically linked a in the Miller AutoLink example given above, it would have to have link apparatus for each voltage. Such an automatic linkage would be complicated and probably uneconomical for the range of voltages contemplated by this invention. Thus, it is unlikely that prior art power sources that automatically select the proper of two input voltage settings will accommodate the full range of worldwide electrical input power. This shortcoming may be significant in that many welding power sources are purchase to be transportable from site to site. The ability to automatically adapt to a number of input power voltage magnitudes is thus advantageous. 
   It is, therefore, one object of this invention to provide a welding power source that receives any of the above-mentioned input voltages, or any other input voltage, without the need of any linkages, whether manual or automatic. Additionally, it is desirable to have such a welding power source that incorporates inverter technology and without using high power 50/60 Hz transformers. 
   SUMMARY OF THE INVENTION 
   The present invention is a power source that is capable of receiving any input voltage over a wide range of input voltages. The power source includes an input rectifier that rectifies the ac input into a dc signal. A dc voltage stage converts the dc signal to a desired dc voltage and an inverter inverts the dc signal into a second ac signal. An output transformer receives the second ac signal and provides a third ac signal that has a desired current magnitude. Although not necessary, the output current may be rectified and smoothed by an output inductor and an output rectifier. A controller provides control signals to the inverter and an auxiliary power controller is capable of receiving a range of input voltages and provides a control power signal to the controller. 
   A method for providing a welding current includes rectifying an ac input and providing a first dc signal. The first dc signal is then converted into a second ac signal. Then the second ac signal is converted into a third ac signal that has a current magnitude suitable for welding. The welding current may then be rectifies and smoothed to provide a dc welding current and an auxiliary power signal is supplied at a preselected control power signal voltage, regardless of the magnitude of the ac input signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the preferred embodiment of the present invention; 
       FIG. 2  is a detailed diagram of the input rectifier of  FIG. 1 ; 
       FIG. 3  is a detailed diagram of the boost circuit of  FIG. 1 ; 
       FIG. 4  is a detailed diagram of the pulse width modulator of  FIG. 1 ; 
       FIG. 5  is a control circuit for the auxiliary power controller of the present invention; and 
       FIG. 6  is a block diagram of an alternative embodiment in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , the welding power source  100  includes an input rectifier  101 , a boost circuit  102 , a pulse-width modulator  103 , a controller  104 , an auxiliary power controller  105 , a pair of storage capacitors C 3  and C 7 , and their associated protective resistors R 4  and R 10 , an output transformer T 3 , an output inductor L 4 , feedback current transformers T 4  and T 6 , feedback capacitors and resistors C 13 , C 14 , R 12  and R 13 , and output diodes D 12  and D 13  to provide a welding output current on welding output terminals  108 . A cooling fan  110 , a front panel  111 , and a remote connector  112  are also shown schematically. 
   In operation, power source  100  receives a three-phase line voltage on input lines  107 . The three-phase input is provided to input rectifier  101 . Input rectifier  101  rectifies the three-phase input to provide a generally dc signal. A 10 microfarad capacitor C 4  is provided for high frequency decoupling of the boost circuit. The dc signal has a magnitude of approximately 1.35 times the magnitude of the three-phase input. The decoupled dc bus is provided to boost circuit  102 . As will be described in greater detail below, boost circuit  102  processes the dc bus provided by input rectifier  101  to provide a dc output voltage having a controllable magnitude. In the preferred embodiment the output of boost circuit  102  will be approximately 800 volts, regardless of the input voltage. 
   The output of boost circuit  102  is provided to pulse-width modulator  103 , where the dc bus is inverted and pulse-width modulated to provide a controllable signal suitable for transforming into a welding output. Controller  104  is a main control board such as that found in many inverter-type welding power sources. The main control board provides the control signals to pulse-width modulator  103 , to control the frequency and pulse-width of pulse-width modulator  103 . Input rectifier  101 , pulse-width modulator  103 , controller  104  and output transformer T 3  are well known in the art. 
   The output of pulse-width modulator  103  is provided to an output transformer T 3 , which, transforms the output of PWM  103  to provide a voltage and current suitable for welding. Transformer T 3  has a center tap secondary and is provided with a turns ratio of 32 turns on the primary to 5 turns on each half for the center tap secondary. Of course, other transformers may be used. The alternating output of transformer T 3  is rectified and smoothed by an output inductor L 4  and output diodes D 12  and D 13 . Inductor L 4  has an inductance sufficient to provide desirable welding characteristics, such as, for example, in a range of 50–150 microhenrys. 
   Auxiliary power controller  105  receives the input line voltage and converts that voltage to a 18 volt dc control signal. The 18 volt control signal is created regardless of the input voltage, and is provided to boost circuit  102 . Boost circuit  102  uses the 18 volt control signal to control its switching frequency and the magnitude of its output. Auxiliary power controller  105  also provides a 48 volt center tap ac power signal to controller  104 . 
   Front panel  104  is shown schematically and is used to convey operating status to the user, as well as receive inputs as to operating parameters. Similarly, remote connector  112  is shown schematically and is used to receive inputs as to operating parameters. 
   Generally speaking, at power-up a three phase input is provided on input lines  107 . A plurality of initially open contactors  115  isolates the input power from input rectifier  101 . However, the input power is provided to auxiliary power controller  105 . As will be described in greater detail below, auxiliary power controller  105  determines the magnitude of the input power, and opens or closes a number of contacts to provide a 48 volt center tap ac output to controller  104 , regardless of the input. The contacts are closed and opened in such a way as to provide safeguards against underestimating the magnitude of the input voltage, and thus protecting the circuit components. Also, auxiliary power controller  105  provides an 18 volt dc control signal to boost circuit  102 , regardless of the magnitude of the input. 
   After the voltage level has been properly determined by closing the proper contacts controller  104  causes contacts  115  to be closed, thus providing power to input rectifier  101 . Input rectifier  101  includes a precharge circuit to prevent a resonant overcharge from harming capacitors C 3  and C 7  and to avoid excessively loading of the input source. A signal received by input rectifier  101  from a tap on transformer T 3  turns on an SCR (described in more detail below). The conducting SCR bypasses input current around the precharge resistors. 
   The output of input rectifier  101  is provided to boost circuit  102 . Boost circuit  102  is well known in the art and integrated circuit controllers for boost circuits may be purchased commercially. In operation boost circuit  102  senses the voltage at its inputs and its outputs. As will be described in more detail later and IGBT (or other switching element) is switched on and off at a frequency and duty cycle (or pulse width) to obtain a desired output voltage. In the preferred embodiment the desired output voltage is approximately 800 volts. 
   Boost circuit  102  thus provides an output of about 800 volts to 800 microfarad electrolytic capacitors C 3  and C 7 , which have 45K ohm bleeder and balancing resistors R 4  and R 7  associated therewith. Capacitors C 3  and C 7  thus acts as a dc link for PWM  103 . 
   PWM  103  receives a generally constant 800 dc signal and modulates it to provide, after transformation, rectification and smoothing, a welding output at a user selected magnitude. PWM  103  modulates its input in accordance with control signals received from controller  104 . PWM  103  also receives a 25 volt dc power signal from controller  104 . Such a PWM is well known and PWM  103  may be purchased commercially as a single module. 
   The output of PWM  103  is provided to output transformer T 3  and which transforms the relatively high voltage, low current signal to a voltage suitable for use in welding. The output of transformer T 3  is rectified by diodes D 1  and D 13 , and smoothed by output inductor L 4 . Thus, a generally constant magnitude dc welding output is provided on welding outputs  108 . 
   Current transformers T 4  and T 5 , provide feedback signals to controller  104 , snubber capacitors C 13  (0.1 microfarads) and C 14  (0.022 microfarads), and snubber resistors R 12  (12 ohms) and R 13  (47 ohms) suppress voltage transients associated with recovery of D 12  and D 13 . Controller  104  compares the feedback signals to the desired welding current, and appropriately controls PWM  103  to adjust its switching pulse width if necessary. 
   Referring now to  FIG. 2 , the preferred embodiment for input rectifier  101  is shown in detail and includes a full wave bridge comprised of diodes D 4 , D 5 , D 6 , D 9 , D 10  and D 11 . The bridge rectifies the three phase input to provide a signal having a magnitude of about 1.35 times the input voltage magnitude. A pair of 50 ohm resistors R 1  and R 2  are provided to precharge capacitors C 4 , C 3  and C 7  (shown in  FIG. 1 ) upon start up. This prevents a sudden surge of current from being dumped into capacitors C 4 , C 3  and C 7 . 
   After the precharge is completed an SCR Q 1  is turned on via a signal from a tap on output transformer T 3  (also in  FIG. 1 ). The signal from transformer T 3  is provided to the gate of SCR Q 1  via a current limiting resistor R 6  and capacitor C 6 . A recovery diode D 7  and snubber resistor R 5  are provided across the gate of SCR Q 1 . SCR Q 1  shunts the resistors and allows the maximum current flow to inductor L 2  of boost circuit  102 . 
   A plurality of varistors RV 1 –RV 3  are provided to suppress line spikes. Additional varistors (not shown) may be provided between D 9 –D 11  and ground to further suppress spikes. 
   As one skilled in the art will readily recognize, other circuits and circuit elements will accomplish the function of input rectifier  101 . 
   Referring now to  FIG. 3 , the details of one embodiment of boost circuit  102 , which operates in a manner well known in the art, is shown. Generally speaking, boost circuit  102  provides an output voltage that is equal to the input voltage divided by one minus the duty cycle of a switch IGBT 1  in boost circuit  102 . 
   Thus, if the switch IGBT 1  is off 100% of the time the output voltage (the dc link voltage) is equal to the input voltage (from capacitor C 4  and input rectifier  101 ). In one embodiment the lowest input is about 200 volts, and the desired output (dc link voltage) is 800 volts, thus the upper limit for the “boost” is about 400%, and requires a duty cycle of about 75%. 
   The operation of a boost circuit should be well known in the art and will be briefly described herein. When switch IGBT 1  is turned on, current flows through an inductor L 2  to the negative voltage bus, thus storing energy in inductor L 2 . When switch IGBT 1  is subsequently turned off, the power is returned from inductor L 2  through a diode D 1  and a 14 microhenry saturable reactor L 1  to the dc link. The amount of energy stored versus returned is controlled by controlling the duty cycle in accordance with the formula stated above. In order for the boost circuit to operate properly inductor L 2  must have continuous current, therefore inductor L 2  should be chosen to have a large enough inductance to have a continuance current over the range of duty cycles. In one embodiment inductor L 2  is a 3 millihenry inductor. The remaining elements of boost circuit  102  include a 0.0033 microfarad capacitor C 1 , a diode D 3 , a 1 ohm resistor R 1 , a 50 ohm resistor R 6 , a diode D 8 , a 50 ohm resistor R 7  and a 0.1 microfarad capacitor C 8  which are primarily snubbers and help the diode recover when switch IGBT 1  is turned on. 
   Boost circuit  102  includes an IGBT driver  301  that controls the duty cycle of switch IGBT 1 . Driver  301  receives feedback signals indicative of the output voltage and the input current, and utilizes this information to drive switch IGBT 1  at a duty cycle sufficient to produce the desired output voltage. 
   In one embodiment, boost circuit  102  includes a shunt S 1  (shown on  FIG. 1 ). Shunt S 1  provides a feedback signal that is the current flowing in the positive and negative buses. A Unitrode power factor correction chip is used to implement boost circuit  102  in the preferred embodiment and requires average current flow as an input. In response to this information and the dc link voltage, driver  301  turns switch IGBT 1  on and off. 
   As one skilled in the art will readily recognize, other circuits and circuit elements will accomplish the function of boost circuit  102 . 
   As stated above, the output of boost circuit  102  is provided to capacitors C 3  and C 7  ( FIG. 1 ) and is the dc link voltage. In one embodiment the dc link voltage is 800 volts, as determined by the switching of switch IGBT 1 . In the preferred embodiment, using the component values described herein the dynamic regulation of the dc link voltage is 80 volts from full load to no load. Static regulation is about a +/−2 volts, with a ripple of about +/−20 volts. 
   The dc link voltage is provided to pulse width modulator  103 . PWM  103  is a standard pulse with modulator and provides a quasi-square wave output having a magnitude equal to the magnitude of the input, as would any other PWMs. Thus, the output of PWM  103  is about +400 volts to −400 volts for an 800 volt peak to peak centered about zero. 
   PWM  103  includes a pair of switches Q 3  and Q 4  (preferably IGBTs) and a pulse width driver  401 . Driver  401  receives feedback from current transformers T 1  and T 2 , and receives control inputs from controller  104 . In response to these inputs driver  401  provides gate signals to switches Q 3  and Q 4 , thereby modulating the input signal. A capacitor C 2  (4 microfarad) a capacitor C 9  (4 microfarad) are provided between the dc link and the output transformer T 3 . A capacitor C 5  (0.0022 microfarad), resistor R 11  (50K ohm) and resistor R 9  (50K ohm) are snubber circuits. 
   As one skilled in the art will readily recognize, other circuits and circuit elements will accomplish the function of PWM  103 . 
   The output of PWM  103  is provided to transformer  103 , and the current in transformer  103  is determined by the modulation of PWM  103 . As stated above, the output of transformer T 3  is rectified by diodes D 12  and D 13  and is smoothed by inductor L 4 . The dc output current is fairly flat; the ripple at full load (300 amps) is about 12 amps peak to peak. At full load the duty cycle of each switch Q 3  and Q 4  of PWM  103  would be about 20–35% (40–70% overall duty cycle). 
   In an alternative embodiment the output of PWM  103  may be rectified by other output rectifiers such as a synchronous rectifier (cycloconverter) that provides an ac output signal at a frequency less than or equal to the frequency of the output of PWM  103 . Other output circuits, including an inverter  601  (see  FIG. 6 ), that provide a welding current may also be used. 
   Referring again to  FIG. 1 , controller  104  is connected to current transformers T 4  and T 5 , which provide feedback information. Controller  104  receives power from auxiliary power controller  105  and provides as one of its output the driver control for the PWM driver. It also includes an over voltage protection sense which monitors, the voltage coming out of input rectifier  101 . If the voltage from input rectifier  101  is dangerously high controller  104  causes contactors  115  to open, to protect circuit components. According to one embodiment 930 volts dc is the cut off point for what is considered to a dangerously high voltage. 
   As may be seen from the above description, welding power source  100  receives an input voltage and provides a welding output. Regardless of the magnitude of the input voltage boost circuit  102  boosts the input voltage to a desired (800 volts e.g.) level. Then PWM  103  modulates the signal to provide an appropriate level of power, at 800 volts, to transformer T 3 . 
   The above arrangement is satisfactory for any input voltage, however, there must be some mechanism to provide control voltages at the proper level. As will be described below, auxiliary power controller  105  performs that function, and the embodiment thereof is shown schematically in  FIG. 5 . 
   With reference now to  FIG. 5 , a plurality of connectors J 1 , J 2 , J 3  and J 4  are shown. An 18 volt dc control voltage output is provided on connector J 1  to boost circuit  102  (shown on  FIG. 1 ). As will be described in greater detail below, the 18 volt dc control signal is provided regardless of the magnitude of the input voltage. Connector J 2  feeds power back to auxiliary power controller  105  for internal use. Connector J 3  connects the input ac voltage to appropriate taps on a transformer T 7  ( FIG. 1 ) to provide a 30 volt ac signal to remote connector  112  ( FIG. 1 ). Similarly, a 48 volt center tap ac signal is provided to controller  104 . Controller  104  uses the 48 volt center tap ac signal to generate dc control signals and to power fan  110 . Connector J 4  of auxiliary power controller  105  is connected via a user controlled on/off switch S 4  to the input power lines ( FIG. 1 ). 
   Auxiliary power controller  105  controls the connections to taps on the primary of an auxiliary power transformer T 7 . Transformer T 7  is a 200 VA transformer whose primaries are connected to auxiliary power controller  105  as described above with reference to connector J 2  and J 3 . Several taps on its secondary are connected to controller  104  and the remaining secondary taps are connected to remote connector  112 . 
   Referring again to  FIG. 5 , the taps on J 3  are associated with the following voltages: 575, 460, 380, 230 volts, and the return, beginning at the uppermost tap and proceeding downward. As will be described below, when auxiliary power controller  105  selects the appropriate tap for a given input voltage, transformer T 7  will provide a 48 volt center tap ac signal on its secondary for use by controller  104 . 
   As may be seen on  FIG. 5 , the ac input is received on connector J 4  and provided (via a fuse F 1 , and a pair of 4.7 ohm resistors R 18  and R 19 ) to a series of relays K 2 B, K 1 B, K 3 C and K 3 B that determine the tap on connector J 3  selected for the output. When 575 volts are present at the input relays K 2 B and K 3 C should be to the right. Then the input is connected across the upper and lower most taps on connector J 3 . These taps are connected to the appropriate taps on transformer T 7  such that the output of transformer T 7  that is provided to controller  104  is approximately 48 volts center tap when 575 volts are provided to the primary of transformer T 7 . 
   When 460 volts are present at the input relay K 2 B should be to the left, and relay K 1 B should be to the right. This connects the ac input to the second uppermost and the lowest taps on connector J 3 . The remaining voltages are similarly accommodated. A pair 0.15 microfarad capacitors C 13  and C 14  are provided for snubbing and spike suppression as the primaries of transformer T 7  are switched. 
   In operation the circuitry on the left side of  FIG. 5  determines the input voltage, and sets the relays for that voltage. At start up the relays are as shown in  FIG. 5  and are suitable for an input voltage of 575 volts. Because this is the highest possible input voltage, all components will be protected, i.e. either the voltage is properly selected, or the input voltage is less than the component design capabilities. If auxiliary power controller  105  determines that 575 volts are in fact present, the relays will remain as shown. However, if auxiliary power controller  105  determines that less than 575 volts are present, the state of relay K 2 B will be changed (to be to the left), so that the output is appropriate for a 460 volt input. 
   This process is repeated, always stepping down to the next highest voltage, until the appropriate input voltage is sensed. In this manner the components in controller  104  will be protected from a dangerously high voltage being applied to controller  104 . 
   The voltage for sensing is provided to auxiliary power controller  105  via connector J 2 , which is connected to secondary taps on transformer T 7 . Thus, if the tap selected on connector J 3  was not correct, then the voltage on connector J 2  will be too low, and auxiliary power controller  105  will select the appropriate relay setting to step down to the next voltage level. As stated above, the stepping down continues until the proper voltage is sensed on connector J 2 . 
   The input from connector J 2  is provided to a rectifier comprised of diodes CR 1 , CR 2 , CR 3  and CR 4 . These diodes rectify the ac signal and provide it to a pair of 220 microfarad smoothing capacitors C 1  and C 2 . The rectified voltage is +/−18 volts dc if the proper tap on connector J 3  is selected. If the incorrect tap is selected the voltage will be less than +/−18 volts, but will be referred to as nominally +/−18 volts. The nominal +/−18 volt supply is provided at other locations throughout the auxiliary power controller  105  circuit, including to a 30 volt zener diode CR 7 , used to determine if the proper tap on connector J 3  has been selected. 
   Auxiliary power controller  105  determines if 575 volts is present on the input using the following components: zener diode CR 7 , a 10 microfarad capacitor C 9 , a pair of gates U 2 B and U 2 C configured as darlington drivers for a winding K 2 A of relay K 2 , a 10K ohm resistor RN 2 A, a 10K ohm resistor RN 2 B, a 820 ohm resistor R 9 , and a diode U 3 B. Gates U 2 B and U 2 C are also used as sensing devices and have a threshold of about 4 volts (relative to their reference voltages) on the input (pin  1 ) of gate U 2 B pin  1 . 
   Initially, gate U 2 B has a LOW output and is referenced to nominal −18 volts. Gate U 2 B will not switch states so long as the input is at least 4 volts greater than its reference voltage (nominally −18 volts relative to ground). In operation the nominal +18 volts will be provided to diode CR 7  and the nominal −18 volt signal is applied to a 10 microfarad capacitor C 9 . As a result of the 30 volt zener drop, the input to gate U 2 B will be at −12 volts (relative to ground) if the proper tap has been selected. If 575 volts are present at the input, there will be 6 volts relative to the reference voltage (−18 volts) at the input to op amp U 2 B, and the output state of gate U 2 B will remain low. So long as the output of U 2 B remains low the current will not flow in the winding of relay K 2  and relay K 2 B will remain as shown in  FIG. 5 . 
   However, if only 460 volts are present on the input and the relays are as shown in  FIG. 5  (as they will be at power up), then the nominal +/−18 volts will actually be +/−14.4 volts. Thus, 28.8 volts are applied across zener diode CR 7  and capacitor C 9 . Given the 30 volt zener drop, −14.4 volts will be applied to the input of gate U 2 B. Because this is also the reference voltage for gate U 2 B, the threshold is crossed, and the output of gate U 2 B will change states. Current will then flow in the winding of relay K 2  and relay K 2 B will change states, configuring the J 3  taps for 460 volts. If less than 460 volts is present at the input the same result will occur. 
   The sensing and stepping down to 380 volts and 230 volts occur in a similar manner using similar components. Referring to  FIG. 5 , the sense and step down circuit to 380 volts include a 100 ohm resistor R 17 , a pair of 10K ohm resistors RN 2 C and RN 2 D, an 820 ohm resistor R 8 , a diode U 3 C, a 10 microfarad capacitor C 6 , a pair of gates U 2 D and U 2 E, and a winding K 1 A for relay K 1 . A relay K 2 C is provided to prevent relay K 1  from changing states before the step down to 460 volts occurs. In the manner described above with respect to the step down to 160 volts, the current will be provided to winding K 1 A of relay K 1  if less than 460 volts is provided at the input. This will cause relay K 1 B to move to the left position and connect the tap on J 3  associated with a 380 volt input. 
   The circuitry associated with the step down to 230 volts includes a 100 ohm resistor R 16 , a pair of 10K ohm resistors RN 1 A and RN 1 B, an 820 ohm resistor R 11 , a diode U 3 E, a pair of gates U 2 F and U 2 G, a winding K 3 A for relay K 3 , relay K 1 C, diode CR 5  and zener diode CR 4 . A relay K 1 C is provided to prevent relay K 3  from changing states before the step down to 380 volts occurs. The step down to 230 volts operates in the same manner as the step down to 380 volts and 460 volts as described above. If less than 380 volts is applied on the connector J 4  inputs, gates U 2 F and U 2 G will cause current to flow through winding K 3 A of relay K 3 . This will cause relay K 3 B to move to the left and connect the tap on J 3  for 230 volts to the ac input. 
   Thus, as may be seen from the above description, the circuitry of auxiliary power controller  105  senses the ac input voltage and connects the appropriate tap on the auxiliary power transformer T 7  to the ac inputs voltage. As may be seen from the above discussion, this is done in a manner which protects components by assuming the voltage is, upon start up, the highest possible voltage. If the voltage is less than the highest possible voltage, the next lowest voltage will then be assumed. This process is repeated until the actual voltage is obtained. 
   In the event that the ac input is 230 volts, at start up there will not be sufficient power from the nominal +/−18 volt signal to drive the relays because the tap associated with 575 volts on connector J 3  is selected at start up. To compensate for this, circuitry that boosts the voltage supplied on connector J 2  is provided. This circuitry includes a 1 millihenry inductor L 1 , a switch Q 4 , a timer U 1 , a switch Q 2 , a switch Q 1 , and a switch TIP 120 . Also included are associated circuitry including a 22 ohm shunt resistor R 13 , a 1K resistor R 5 , a 10K resistor R 12 , a 10K resistor R 14 , a 2.2K resistor R 4 , a 1K resistor R 6 , a 1K resistor R 2 , a 20K resistor R 3 , a 220 ohm resistor R 7 , a 10K resistor RN 1 D, a 4.7K resistor R 10 , a 470 picofarad capacitor C 4 , a 0.001 microfarad capacitor C 3 , a 0.1 microfarad capacitor C 5 , a 220 microfarad capacitor C 11 , a 220 microfarad capacitor C 12 , a diode CR 12 , a diode CR 8 , a zener diode CR 10 , a diode CR 5 , and a zener diode CR 11 . 
   The boost power source circuitry operates as a typical boost circuit. The boost is provided by inductor L 1  and switch Q 4 . During the time switch Q 4  is ON, current flows through inductor L 1 , shunt resistor R 13  and switch Q 1  to the negative voltage supply. During this time, energy is stored in inductor L 1 . When switch Q 4  is OFF, the energy stored in inductor L 1  is returned to the positive voltage supply (+B) through diode CR 12 . By appropriate timing of the turning ON and OFF of switch Q 4 , a desired voltage may be obtained. Timer chip U 1  is used to provide the ON/OFF gate signals to switch Q 4  and is an LM555 timer. When the voltage on resistor R 13  becomes sufficiently high, it will trip the input on U 1 , which in turn will cause the output of timer U 1  to turn switch Q 1  OFF. 
   Initially, switch Q 4  is in the ON position and current increases and eventually reaches the point where the voltage on resistor R 13  is sufficiently high to trip the threshold on timer U 1  through resistor R 12 . Thus, switch Q 4  will remain ON for a length of time sufficient to build up enough energy to, when it is turned OFF, raise the nominal +/−18 volts to a level sufficient to drive the relays. 
   Switches Q 2  and Q 1  enable or disable timer U 1  when the taps on connector J 3  are such that the nominal +/−18 volt, signal is actually +/−18 volts. When switch Q 2  is turned OFF, timer U 1  is disabled through its VCC input. Also, switch TIP 120  is a linear regulator. When the nominal +18 volt supply is insufficient to drive the relay, switch TIP 120  will provide the boost source to drive the relays. When the nominal +18 voltage is sufficient to drive the relay, switch Q 2 , timer U 1  and switch Q 4  are turned off. The +18 volt supply is coupled through L 1  and CR 12  to regulator TIP 120 ; the +B boost supply is then fed directly by the sufficiently high +18 volt supply. The TIP 120  regulator regulates relay supply at 24 volt relative to the −18 volt supply. 
   In addition to the circuitry above, circuitry is provided that protects in the event of an overvoltage. This circuitry includes a switch Q 5 , a gate U 2 A, a 100 ohm resistor R 15 , a 10K ohm resistor RN 3 A, a 10K ohm resistor RN 3 B; a 10K ohm resistor RN 3 C, a 10 microfarad capacitor C 10 , diodes CR 14  and U 3 H, and 10 volt zener diode CR 13 . An overvoltage occurs when the tap selected on connector J 3  corresponds to a voltage less than the voltage at the ac input. This may occur when either the incorrect tap has been selected or when a temporarily high voltage is provided at the ac input. 
   In the event an overvoltage occurs, the voltage at the node common to diodes CR 13  and CR 7  will rise to a voltage greater than 14 volts with respect to the nominal −18 volt signal. This causes the low side of diode CR 13  to be greater than 4 volts with respect to the nominal −18 volt signal, and the input of U 2 A will change from an input low state to an input high state. When the input of U 2 A changes from low to high, the output will change from an output high state to an output low state. The output low state of U 2 A will bring the relay supply voltage to a virtual 0 through diodes U 3 H and CR 14 . This causes the relays to return to the state shown in  FIG. 2 , which accommodates the highest voltage possible (575 volts). At that time the previously described tap selection process stepping from the 575 to 460 to 380 to 230 taps begins again until the correct tap is selected to match the input voltage received on connector J 4 . Accordingly, the components of controller  104  will be protected. 
   Other modifications may be made in the design and arrangement of the elements discussed herein without departing from the spirit and scope of the invention as expressed in the appended claims.