the present invention relates to a voltage limiting regulator for use in conjunction with an ac alternator having dc - excited field . the terms alternator and generator have been used interchangeably herein . referring first to fig1 there is shown a schematic cross - sectional view of an alternator suited for use with the voltage limiting regulator of the invention , generally at reference number 100 . an armature 102 is located between at least two independent field producing elements 104 on either side of armature 102 . each field - producing element 104 is shown having a coil 106 which is connected to an external voltage source 107 a , 107 b . both field producing elements 104 may be powered either independently or in parallel as operating circumstances dictate . a shaft 108 extends beyond both distal ends of armature 102 . a pulley 110 on a first distal end of shaft 108 allows for the connection of alternator 100 to an external prime mover ( not shown ), a set of slip rings 112 at a second distal end of shaft 108 allows for the transfer of the generated ac voltage from armature 102 to an external ac bus 114 . any alternator configuration , many of which are well known to those of skill in the art , may be used with the regulator of the present invention . this assumes , of course , that the alternator &# 39 ; s field is produced from an externally - applied dc voltage 107 a , 107 b ( fig1 ). referring next to fig2 there is shown a schematic block diagram of a system including the inventive voltage limiting regulator and an alternator 100 ( fig1 ). an external ac load 122 is connected to ac bus 114 . an ac pickup 124 monitors the voltage and / or current of ac bus 114 . the function of ac pickup 124 may be performed by a number of devices well known to those of skill in the art . these devices may be either directly connectable to ac bus 114 or , in alternate embodiments , may be inductively coupled devices such as current probe , hall effect devices , or the like . the invention is not considered limited by the device chosen for implementing ac pickup 124 . ac pickup 124 is connected to an input of regulator 120 . an external battery 126 is also connected to regulator 120 . at least one output from regulator 120 is connected to alternator 100 and dc voltages 107 a and , optionally , 107 b are thereby supplied . referring next to fig3 there is shown a slightly more detailed schematic diagram on the regulator of the present invention in it operating environment . an ac sense voltage 114 related to the actual output of the alternator is clamped by a varistor 115 or other similar spike - suppressing device well know to those skilled in the art . a bridge rectifier 204 provides a time - varying dc voltage related to ac input voltage 114 . a dc set - point control potentiometer 218 , connected to a first input of a voltage comparitor circuit 221 , is used to adjust the desired ac output voltage from the alternator being controlled . a negative dc balance control 225 , feed from the negative dc output of bridge rectifier 204 is connected to a second input of comparitor circuit 221 , in effect providing a stable reference voltage to the comparitor 221 . the output of comparitor 221 is coupled through and optical isolator 224 to an output controller 225 . output controller 225 accepts a dc input voltage from a battery 126 ( fig1 ) or similar dc voltage source . output controller 225 then provides a time - varying dc output voltage responsive to the output of comparitor circuit 221 whereby the output of the controlled alternator is held substantially constant at a voltage determined by set point control 218 . a detailed description of one embodiment of the inventive regulator is provided hereinbelow . referring now to fig4 b and 4 c , there are shown two portions of an electrical schematic diagram of voltage - limiting regulator 120 ( fig2 ), generally at reference number 200 . ac bus 114 is bridged by a surge suppressor 202 such as a varistor or similar device , and then connected to a bridge rectifier arrangement 204 . a smoothing capacitor 206 is connected at the output of bridge rectifier 204 and a dc voltage bus 208 is established . the dc voltage on dc voltage bus 208 is typically in the range of 350 vdc . a zenar diode 210 , is connected to dc voltage bus 208 through voltage divider resistors 214 , to the base of a regulator transistor 212 . the regulated output voltage is typically in the range of 24 volts dc . a second voltage divider network is formed by resistors 216 , potentiometer 218 and resistor 220 is also connected to dc voltage bus 208 . the output from the second voltage divider network is typically in the range of 5 volts dc . a comparitor 222 has a first input connected to regulator transistor 212 and a second input connected to the pick - off point of the voltage divider 216 , 218 , 220 . in the embodiment chosen for purposes of disclosure , resistive element 218 is a potentiometer which allows adjustment of a voltage feed to comparitor 222 . this adjustable voltage ultimately controls the output voltage of the ac generator which is in the approximate range of 110 - 130 volts ac in the preferred embodiment . the output of comparitor 222 is connected to an isolator 224 , typically an optical isolator . a variety of devices suitable for use as an isolator are well known to those skilled in the circuit design art . a pair of output power devices 226 , 228 are connected to the output of isolator 224 . typically , devices 226 , 228 are mosfets . flyback diode 230 is connected to devices 226 , 228 . a second surge suppressor 232 is connected across dc output buses 234 a , 234 b which are connected to the field excitation inputs of alternator 100 ( fig2 ). in operation , an ac voltage in the range of either 120 - 130 or 220 - 240 volts , depending upon the specific configuration , is applied to the input of regulator circuit 200 from ac bus 114 . the ac voltage is dependant upon the alternator configuration selected . this applied ac voltage supplies an input sensing voltage , the exact voltage depending upon the configuration . surge suppressor 202 is designed to clamp down the input sensing voltage thereby removing spikes which may be present thereupon . the dc output from bridge rectifier 204 , appearing on dc voltage bus 208 , will typically be in the range of 180 volts dc for a 120 volt ac input and 350 volts dc when 220 volts ac is applied . the combination of resistors 214 , zenar diode 210 and regulator transistor 212 from a voltage regulator providing a fixed , stable dc voltage for use as a supply voltage for comparitor 220 . in the embodiment chosen for purposes of disclosure , approximately 24 volts dc has been chosen as the supply voltage . it should be obvious to those of skill in the art that other supply voltages could be chosen and obtained by varying component values . the second voltage divide network ( i . e ., resistors 216 , potentiometer 218 , etc .) are used to develop a reference voltage for comparitor 222 . the embodiment chosen for purposes of disclosure , the reference voltage is approximately 5 volts dc . potentiometer 218 may be used to adjust the ultimate nominal ac output voltage of alternator 100 . in the embodiment chosen for purposes of disclosure , potentiometer 218 can vary the nominal output voltage in a range of approximately 110 - 130 volts ac , assuming that the 120 volt configuration has been selected . a similar range of output voltage adjustment is also available in the 220 volt configuration . it will also be obvious to those of skill in the circuit design art that other reference voltage values could be chosen to meet a particular operating requirement . comparitor 222 is supplied a fixed , reference voltage at a first input and a portion of the rectified dc sense voltage at a second input . when the sense input drops below the reference voltage ( e . g ., 5 volts dc ), the output of comparitor 222 shifts from its nominal “ high ” signal level to a “ low ” signal level . this causes a “ high ” signal to be applied to output transistors 226 , 228 . the output of comparitor 222 is isolated from the remainder of the output circuitry by isolator 224 . this prevents the relatively high output voltages from reaching the low - voltage portions of the regulator circuitry in the event of circuit failure . in addition , isolator 224 minimizes the possibility of electrical shock to the user . in the embodiment chosen for purposes of disclosure , the dc output voltage may assume a range of approximately 4 vdc to 14 . 4 vdc thereby allowing the output voltage of alternator 100 be held constant under a wide range of loads . it is to be understood that the present invention is not limited to the embodiments described above , but encompasses any and all embodiments within the scope of the following claims .