shown in fig1 is a prior art current source driver for led and other loads . the load current in this circuit is independent of supply voltage changes but not for ambient temperature variations . in the circuit both transistors are the p - channel mosfet type with source , gate and drain terminals as depicted . these transistors can also be substituted with the pnp type of transistors with emitter , base and collector terminals being the mosfet source , gate and drain terminals respectively . ac supply vcc 1 is the positive side of a dc voltage source or the positive side of an unsmoothed rectified ac voltage source . the negative side of voltage source vcc 1 is connected to ground gnd 1 . the source terminal of transistor 100 is connected to supply vcc 1 by way of resistor 109 . the drain terminal of transistor 100 is returned to ground gnd 1 via load led 1 . resistor 107 is connecting the gate terminal of transistor 100 to node 111 on the bias string comprising diode 101 , diode connected transistor 103 and resistor 105 . the gate - to - source voltage of transistor 100 is compensated with the gate - to - source voltage of transistor 103 . hence , the load led 1 current is primarily set with the forward voltage of diode 101 appearing across resistor 109 . now since the diode 101 forward voltage hardly changes with supply vcc 1 voltage variations or loading the load led 1 current stays constant . with ambient temperature changes the gate - to - source voltage variation of transistor 100 can easily be compensated with a similar variation of the gate - to - source voltage of transistor 103 . this is fine but the variation with temperature of the forward voltage of diode 101 , responsible for setting the load led 1 current , is a problem . if diode 101 is bipolar with a temperature coefficient of − 2 mv / deg . c . the load led 1 current can vary by as much as 30 % with a change of 100 deg . c . in ambient temperature . furthermore and in order to minimize the voltage drop across resistor 109 if diode 101 is a low forward voltage schottky diode with a temperature coefficient of − 1 . 2 mv / deg . c . the load led 1 current can vary by as much as 50 % for a 100 deg . c . change in ambient temperature . shown in fig2 is a prior art current source driver for led and other loads . the load current in this circuit is independent of ambient temperature variations but not for supply voltage fluctuations . in the circuit both transistors are the p - channel mosfet type with source , gate and drain terminals as shown . these transistors can also be substituted with the pnp type of transistors with emitter , base and collector terminals being the mosfet source , gate and drain terminals respectively . ac supply vcc 2 is the positive side of a dc voltage source or the positive side of an unsmoothed rectified ac voltage source . the negative side of voltage source vcc 2 is connected to ground gnd 2 . the source terminal of transistor 200 is connected to vcc 2 by way of resistor 209 . the drain terminal of transistor 200 is returned to ground gnd 2 via load led 2 . resistor 207 is connecting the gate terminal of transistor 200 to node 211 on the bias string comprising diode 201 , diode connected transistor 203 and resistor 205 . the gate - to - source voltage of transistor 200 is compensated with the gate - to - source voltage of transistor 203 . hence , the load led 2 current is set with the voltage across resistor 201 also appearing across resistor 209 . in this circuit the load led 2 current is independent of ambient temperature variations . the reason is that transistors 200 and 203 by being properly matched have similar temperature characteristics . any variation with ambient temperature in the gate - to - source voltage of transistor 200 would be compensated with a similar variation in the gate - to - source voltage of transistor 203 in such a way that the bias at node 211 would not be affected . however , compensation for changes in the load led 2 current with supply vcc 2 voltage variations or loading is not expected to be the case in this circuit . any change in the supply vcc 2 voltage would affect the voltage across resistor 209 and hence the load led 2 current . for a 20 % change in the supply vcc 2 voltage there would be a 30 % change in the load led 2 current . according to at least one embodiment of the present invention depicted in fig3 is a schematic diagram of a thyristor switched current source driver for led and other loads . in this circuit the load current is independent of supply voltage and ambient temperature variations . the circuit is driven by an unsmoothed rectified ac voltage source having positive and negative sides connected to supply vcc 3 and ground gnd 3 terminals respectively . thyristor 310 having input , output and control terminals is driven from supply vcc 3 shown connected to thyristor 310 input terminal . the output terminal of thyristor 310 is returned to ground gnd 3 via resistors 311 and 313 in series connection . thyristor 310 control terminal is connected to supply vcc 3 by way of a trigger circuit comprising a series combination of resistor 305 and zener diode 303 operable in reverse or breakdown mode . included in the trigger circuit of thyristor 310 is capacitor 301 connected between the thyristor 310 control terminal and ground gnd 3 . transistor 300 is a p - chnnel mosfet having source , gate and drain terminals as depicted in a conventional diagram for a mosfet . transistor 300 can also be a pnp transistor with emitter , base and collector terminals being the mosfet source , gate and drain terminals respectively . the source terminal of transistor 300 is connected to supply vcc 3 by way of resistor 309 . the transistor 300 drain terminal is returned to ground gnd 3 via load led 3 . resistor 307 is connecting the gate terminal of transistor 300 to the voltage divider common point of resistors 311 and 313 . the reason for this divider is to obtain the correct bias for the transistor 300 gate terminal without allowing a large initial voltage drop across thyristor 310 . another reason is that by utilizing resistor 311 dimming can be introduced in the leds of load led 3 . by adjusting lower resistor 311 the bias at the gate terminal of transistor 300 is increased and this would cause the voltage across resistor 309 and the load led 3 current to be lowered . another way of introducing dimming in the leds of load led 3 is by adjusting lower resistor 305 . this time the voltage across thyristor 310 is reduced and this would cause the bias at the gate and source terminals of transistor 300 to go higher and the load led 3 current to go lower . half - cycle rectified ac oscilloscope voltage versus time waveforms for thyristor 310 triggering and load led 3 are shown in fig5 a , 5b and 5c for supply vcc 3 voltage changes form high to nominal to low respectively . any change in the supply vcc 3 voltage would affect the voltage across resistor 309 and hence the load led 3 current . what actually happens is that a change in supply vcc 3 voltage , as a result of shifts in the thyristor 310 trigger angle , would affect the voltage across thyristor 310 as well as the bias on the divider of resistors 311 and 313 and through resistor 307 the bias of the transistor 300 gate and source terminals . as shown in the thyristor 310 voltage waveforms of fig5 a , 5b and 5c , triggering on the ac supply vcc 3 voltage waveform occurs at the same voltage point vp 1 which , for example , may be taken to be about 15 volts . by properly selecting the triggering point vp 1 on the supply vcc 3 voltage waveform and keeping it constant for supply vcc 3 voltage variations , the correct bias is obtained at the gate and source terminals of transistor 300 to prevent the load led 3 current from changing . thyristor 310 by controlling the voltage on the bias string of resistors 311 and 313 enables the load led 3 current to stay constant for ambient temperature variations as well . this would not have been the case with conventional biasing schemes . transistor 300 gate terminal is returned to ground gnd 3 by way of resistors 307 and 313 . this would enable transistor 300 to be operable even before any triggering of thyristor 310 as shown in fig5 a , 5b and 5c load led 3 voltage waveforms . also indicated in fig5 a , 5b and 5c is that all load led 3 voltage waveforms are of the same amplitude , sinusoidal and in phase with the supply vcc 3 voltage waveforms . capacitor 301 can be omitted if thyristor 310 triggering is not needed beyond 90 electrical degrees . as an example , in fig5 a , 5b and 5c thyristor 310 may be looked at as triggering at about 40 , 50 , and 60 electrical degrees respectively . for sensitive thyristor 310 the control terminal input current is low and being the only current through resistor 305 requires for resistor 305 have a large value . it is actually more desirable for resistor 305 to have a lower value in which case triggering desensitization of thyristor 310 can be employed . this is done by connecting a small value resistor between the control and output terminals of thyristor 310 . temperature compensation must be carried out for variations in the load led 3 current as well as the voltage in the thyristor 310 load of resistors 311 and 313 . this compensation can be carried out by utilizing the positive temperature coefficient of the operable in reverse or breakdown mode zener diode 303 voltage to offset temperature related changes due to negative temperature coefficients of the thyristor 310 control terminal input current and control terminal to output terminal voltage . the change in the thyristor 310 control terminal input current is manifested as a voltage change across resistor 305 . compensation must also be carried out for the transistor 300 source - to - gate voltage variation with temperature . this again can be done by utilizing the variations with temperature of the zener diode 303 voltage and the thyristor 310 control terminal input current . however with mosfet devices the temperature coefficient of the gate - to - source voltage can be both positive and negative and cancel each other out at some value of drain current . therefore , if transistor 300 is set to operate at the zero temperature coefficient point for the gate - to - source voltage then temperature compensation would reestablish the selected triggering point vp 1 on the ac supply vcc 3 voltage waveform and the load led 3 current would be constant for ambient temperature changes as well as supply vcc 3 voltage variations . in the event of having variation with temperature in the gate - to - source voltage of transistor 300 temperature compensation can still be achieved but with a shift in the triggering point vp 1 on the ac supply vcc 3 voltage waveform . however , if the components used are selected with the proper electrical characteristics , the shift with temperature of the triggering point vp 1 on the ac supply vcc 3 voltage waveform , would have only a minor effect on the change of the load led 3 current with supply vcc 3 voltage variations . also the triggering point vp 1 on the ac supply vcc 3 voltage waveform can always be adjusted to accommodate the variation with temperature of the transistor 300 gate - to - source voltage . with a sensitive thyristor 310 the control terminal input current is very low and being the only current through resistor 305 requires for the value of resistor 305 to be large . actually in this case and for less troublesome temperature compensation it would be better if the value of resistor 305 is not very large . one way of doing this would be to desensitize the triggering of thyristor 310 . this can be done by connecting a small value resistor between the thyristor 310 control terminal and output terminal . the result would be additional current in resistor 305 and this would require for resistor 305 to have a lower value . for example , a resistor of 1 kohm would increase the current in resistor 305 by 0 . 65 ma if the thyristor 310 control terminal to output terminal voltage is 0 . 65 volts . excessive shifts with temperature of the predetermined triggering point vp 1 on the ac supply vcc 3 voltage waveform can still be realized if a large change in the load led 3 current is required . for example , for the triggering point vp 1 to shift lower for reduction in the load led 3 current at higher temperatures may require for the zener diode 303 voltage to be lower or the zener diode 303 be replaced with a negative temperature coefficient resistor . according to another embodiment the circuit depicted in fig4 is the n - chnnel mosfet version of the circuit in fig3 . it is a thyristor switched current source driver for led and other loads . in this circuit the load current is also independent of supply voltage and ambient temperature variations . the circuit is driven by an unsmoothed rectified ac voltage source having positive and negative sides connected to supply vcc 4 and ground gnd 4 terminals respectively . thyristor 410 having input , output and control terminals is driven from supply vcc 4 shown connected to thyristor 410 input terminal by way of resistor 413 . the output terminal of thyristor 410 is returned to ground gnd 4 via resistor 411 . thyristor 410 control terminal is connected to supply vcc 4 by way of a trigger circuit comprising a series combination of resistor 405 and zener diode 403 operable in reverse or breakdown mode . included in the trigger circuit of thyristor 410 is capacitor 401 connected between the thyristor 410 control terminal and ground gnd 4 . the reason for the thyristor load of resistors 411 and 413 being split up is for the gate terminal of transistor 400 to have the proper bias without a large voltage drop across thyristor 410 . this is no different form the way it is done in the circuit of fig3 where the bias at the gate terminal of transistor 300 is set with the voltage drop across thyristor 310 and resistor 311 . another reason is that with resistor 411 dimming can be introduces in the leds of load led 4 . by adjusting lower resistor 411 the bias at the gate terminal of transistor 400 is lowered and this would cause the voltage across resistor 409 and the load led 4 current to be lowered . another way of introducing dimming in the leds of load led 4 is by adjusting lower resistor 405 . this time it is the reduction of the voltage across thyristor 410 that would cause the bias at the gate terminal of transistor 400 to go lower . transistor 400 is an n - chnnel mosfet having source , gate and drain terminals as depicted in a conventional diagram for this type of a mosfet . transistor 400 can be an npn transistor with emitter , base and collector terminals being the mosfet source , gate and drain terminals respectively . transistor 400 can also be an integrated base bipolar transistor ( igbt ) with emitter , base and collector terminals being the mosfet source , gate and drain terminals respectively . the source terminal of transistor 400 is connected to ground gnd 4 by way of resistor 409 . the transistor 400 drain terminal is returned to supply vcc 4 via load led 4 . resistor 407 is connecting the gate terminal of transistor 400 to thyristor 410 input terminal . the reason for that is for the gate of transistor 400 to be biased from supply vcc 4 through resistors 407 and 413 in order for transistor 400 to be operable even before any triggering of thyristor 410 . the operation of the circuit in fig4 is similar to the p - channel mosfet circuit of fig3 in spite of the fact that some components are rearranged with respect to supply vcc 4 and ground gnd 4 . the load led 4 current is set with resistor 409 which this time is returning the source terminal of transistor 400 to ground gnd 4 . load led 4 is now connected between supply vcc 4 and the drain terminal of transistor 400 . in this circuit the load led 4 voltage waveform is referenced to supply vcc 4 . it is also sinusoidal and in phase with the ac supply vcc 4 voltage waveform . again by properly selecting a triggering point vp 2 on the supply vcc 4 voltage waveform and keeping it constant with supply vcc 4 voltage variations the correct bias is obtained at the gate and source terminals of transistor 400 to prevent the load led 4 current from changing . it is actually the variation of the voltage across thyristor 410 that enables the biasing scheme used to work properly , that is , to prevent the load led 4 current from changing not only for supply vcc 4 voltage fluctuations but also for ambient temperature variations . capacitor 401 can be omitted if no thyristor 410 triggering is needed beyond 90 electrical degrees . half - cycle rectified ac oscilloscope voltage versus time waveforms for thyristor 410 triggering and load led 4 are shown in fig6 a , 6b and 6c for supply vcc 4 voltage changes form high to nominal to low respectively . compensation for temperature related changes in the current of load led 4 and the voltage in the thyristor 410 load of resistors 411 and 413 can be carried out the same way as with the circuit of fig3 . this compensation can be done by utilizing the positive temperature coefficient of the operable in reverse or breakdown mode zener diode 403 voltage to offset temperature related changes in the thyristor 410 control terminal input current and control terminal to output terminal voltage and the transistor 400 gate - to - source voltage . the change in the thyristor 410 control terminal input current can be looked at as voltage change across resistor 405 . resistors with positive or negative temperature coefficients can also be utilized in place of zener diode 403 to obtain large changes in the load led 4 current . also with a sensitive thyristor 410 having low control terminal input current it may be necessary to desensitize the triggering of thyristor 410 in order to increase the current through resistor 405 . this can be done by connecting a small value resistor between the control terminal and output terminal of thyristor 410 . the result would be a higher current through resistor 405 which would enable resistor 405 to have a lower value and make compensation for temperature related changes in the load led 4 current easy to achieve . it could be apparent to those skilled in the art that modifications and variations can be made to the preferred embodiments of this invention without departing from the scope or spirit of the invention as defined by the appended claims . one very obvious modification would be to duplicate the preferred embodiments of this invention by referring to the actual components of a thyristor equivalent circuit . another modification would be to have the thyristor circuit incorporated in the same substrate as the mosfet .