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

Erich Camerling e.camerling at tiscali.nl
Tue Nov 26 10:26:10 UTC 2013

```          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

```