photodiode's output current

Discussion in 'General Electronics Chat' started by thomas_n, May 14, 2011.

  1. thomas_n

    Thread Starter New Member

    May 14, 2011
    Hi All,
    I am doing the experiment with photodiode and laser light.
    The laser source I used is 4.5mW infrared laser with OPV382 and the PD used is
    SFH206 with 0.6A/W intensity. I put the the laser source and the PD at 10cm apart. Then 1ohm series resistor is put at PD circut to measure the current.
    The current measured is only 10uA.
    My question is
    (1) does that 10uA current make sense? I am expecting minimum 100uA with that light source and that distance.
    (2) How can I improve the current output at PD?
  2. designnut


    Apr 21, 2011
    Try 1000 ohms or more, you get too small a voltage drop.
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Do not be so quick on ASSUMING the laser output is what the spec say it is.

    They are often lower and as most people do not have ways of measuring such things, they are quite content to provide 'specs' that say otherwise.
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    Where did you come up with 1 ohm anyway? I can only assume you have it in series with a load you're trying to drive.

    Also, the monochromatic output of the laser diode may not be within the peak operating area of the photodiode.
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    Yea, there's people on eBay selling lasers with something like 20,000 mW of output, if that were true they wouldn't be so cheap, they also couldn't go around selling them to the general public.

    Class I/1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players.
    Class II/2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers.
    Class IIIa/3R lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause damage to a spot on the retina.
    Class IIIb/3B can cause immediate eye damage upon exposure.
    Class IV/4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class.
  6. hgmjr


    Jan 28, 2005
    You can use a current mirror circuit such as the one shown in this post.

    Last edited: Feb 22, 2012
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    Several useful points have already been stated, perhaps worth re-capping:

    1) Is the laser really radiating 4.5mW in the first place - can you check it?
    2) It the 0.6A/W responsivity figure correct at the laser wavelength?
    (According to these data sheets, they do seem to match.)
    3) Can you get a good enough photo=current measurement with 1Ω series resistance / would a bigger resistor work better?

    I hope that the circuit you are using assures a reasonable bias for the detector at the expected level of illumination, for instance the total load resistance must not be so large that the detector could saturate.

    Apart from these issues, one big remaining question is the coupling ratio. If your figures are as you assume them, the current with perfect coupling would be 2.7mA. Your 10μA result suggests that only 0.37% of the IR igot to the detector (about 49dB loss). That's not much coupling: did you check the alignment was peaked up for maximum response? The data-sheet for the laser suggests a pretty narrow angle so this may be quite critical.