Recovery of the phototransistor after big IR pulse

Discussion in 'General Electronics Chat' started by kender, May 25, 2008.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007
    263
    0
    Colleagues,

    I’m working on a small device, whose purpose is to produce 8W IR pulses with IR LEDs. Each pulse is 100us long. The pulses are produced only when the device is in proximity of the target. The proximity is established by the IR proximity sensors* (SFH9201 made by Osram). The proximity sensor consists of a small IR LED, a phototransistor and a 20k pull-up connected to the collector. Part of the 8W IR reflects into the phototransistor and saturates it. The problem is that it takes the transistor more than 1ms to come out of saturation, after the 8W IR is turned off, and I can’t make proximity detection during this time. To deal with this problem, I’ve connected the pull-up to the digital output of the PIC and drove the pull-up low, when 8W IR was on, then drove it high to make a proximity detection. The phototransistor was still saturated for 1ms. Why does it take so long to recover? Why does it saturate when the collector is pulled to the ground?

    I can upload the schematics and waveform, if anyone wants to take a look.

    Thanks,
    - Nick

    * This part of the design was inherited. If the device will be redesigned, it will probably get a capacitive proximity sensor.
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Just like in a diode, it takes some time to set the barrier potential, so some electrons will biase the phototransistor during that time. Now to reply to your other question, a phototransistors is equivalent to transistor with a photodiode connecting the the base to its collector. In order to not saturate the transistor, I'm afraid you have to use a bigger collector resistor.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    How the photodiode is connected to your transistor?.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    It is a photo transistor. The example I've referred to is its equivalent circuit. I'm sorry if I complicated things a bit.
     
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    I'd need the partnumber to help any. I'm seeing several available that can do 7 - 50ns depending on load.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I see you have offered to post the schematic. I would recommend you go ahead and post it.

    hgmjr
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Did you give up on us, Kender?

    hgmjr
     
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