[Alt-photo] 365 nm. UV-Densitometer. Part 3a of 7..

[Erich Camerling]

          UV-LED driver


             Power supply : + 10 Volt is enough for energizing the 
UV-LED till 625 mA.
             The  average current is ~ 60 mA at a duty cycle of 10 %

             Incoming signal : time ~ 10 msec. , repetition frequency : 
128 msec.
             Voltage is + 5 Volt , stabilized.
             This voltage is divided by R6/R7  to + 2.5 V on the TLC271P 
pin 3.
             That means that through R3 must run a current who delivers 
2.5 V.
             When R3 = 4 ohm the max current is 625 mA and that current 
also runs trough the UV-LED
             R3 = 4 ohm  --->  625 mA
                   = 5 ohm --->  500  mA
                  = 10 ohm ---> 250  mA
             For R1 you can use a NTC. Reason : when the surroundings 
temperature is rising the UV-LED delivers less light at the same current 
and  you can
             compensate   that.
             Compensating the drop in amount of light at a rising 
temperature is fairly complicated , particularly when using R1,Rx and R10.
             I myself started with only R3 = 10 ohm (250 mA pulse)
             and ended with R3 = 14,3
                                         R10 = 18,1
                                          Rx  = 49,9 and
                                          R1 = 22,0 ohm   (250 mA pulse)
                       OR                R3 = 7.5  (2x 15 parallel)
                                          R10 = 10
                                          Rx = 14,7 and
                                          R1 = 9.37 ohm (500 mA pulse)

             In between I used other resistors with different values but 
the last values gave the best result  (for me !!)
             At the end of these UV-Densitometer series I can put an 
Excel-file on internet which you can use to fill in your own R data and 
see the result

             To be continued at the end of this week