Version 4 SHEET 1 1572 680 WIRE 48 0 -32 0 WIRE 224 0 128 0 WIRE 336 0 224 0 WIRE 224 80 224 64 WIRE -32 96 -32 0 WIRE 336 128 336 0 WIRE 224 192 224 160 WIRE -32 304 -32 176 WIRE 224 304 224 272 WIRE 224 304 -32 304 WIRE 336 304 336 208 WIRE 336 304 224 304 WIRE 336 336 336 304 FLAG 336 336 0 SYMBOL res 144 -16 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 750 SYMATTR SpiceLine pwr=5W SYMBOL res 320 224 M180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R2 SYMATTR Value 250 SYMBOL diode 208 0 R0 SYMATTR InstName D1 SYMBOL voltage 224 176 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 124 Left 2 SYMATTR InstName V1 SYMATTR Value 4.4V SYMBOL voltage -32 80 R0 WINDOW 3 24 44 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 24 72 Left 2 SYMATTR InstName V2 SYMATTR Value 0 SYMATTR SpiceLine Rser=2 SYMBOL res 240 64 M0 SYMATTR InstName R3 SYMATTR Value 1.45 SYMATTR SpiceLine pwr=1W TEXT -66 360 Left 2 !.dc v2 0 50 .5 TEXT 408 96 Left 2 ;D1 R3 V1 Simulate a 5.1V 1W Zener, which wasn't in my library. TEXT -352 -80 Left 2 ;R1 is rated at 5 Watts. V2 is your source voltage and the simulation run is set to sweep it from 0V to 50V. R2 is your 20mA load. TEXT -360 -32 Left 2 ;Max source (V2) current is 55mA.