Photodiode VS Phototransistor

Discussion in 'The Projects Forum' started by electro_2015, Feb 15, 2015.

  1. electro_2015

    Thread Starter New Member

    Jan 1, 2015
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    Hello,

    I've got an infra-red emitter and detector pair for a simple project. Both have 2 terminals. The emitter is dark blue and the detector is clear and transparent.

    What I need to know is whether the detector is a photodiode or a phototransistor? Or are these 2 the same?

    Thanks in advance
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Emitter is clear, detector is dark blue.

    Photo-transistor is slower than photodiode (not normally operated much above 40k Hz) and response is much higher. It is normally installed with forward bias (transistor's emitter more negative than collector pin).

    Photodiode can be operated in forward or reverse bias but also with a resistor in series and put the resistor and cathode of diode to ground. Then it is kind of a solar cell - high frequencies but you definately have to amplify the signal.
     
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  3. electro_2015

    Thread Starter New Member

    Jan 1, 2015
    10
    1
    According to the circuit i've got (yeah i'm not designing anything, just trying to understand and apply), I would say that the detector I have is a phototransistor because its positive terminal (the collector terminal i suppose) is connected to supply via a large resistor and the negative terminal (the emitter i suppose) directly to ground.

    Now I need to know how to calculate the current through the circuit given the supply voltage and the resistor value?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    If you check the devices using a ohmmeter with a diode check mode, a photodiode will conduct as a diode in one direction but a phototransistor will not.

    The current through the photodiode/phototransistor depends upon the amount of illumination it is seeing.
     
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  5. electro_2015

    Thread Starter New Member

    Jan 1, 2015
    10
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    But then there must be some way of estimating the amount of illumination it is seeing. Maybe by considering the current through the emitter LED?


    And what is the equation relating the amount of illumination and the collector current?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You cannot tell the difference from looking at it. They both can look the same.
    Usually when you buy them they will have a part number that you can look up to get their specifications. Once you have that you can estimate their performance vs light as it will be in the specification, but can vary a lot depending on what part you have. There are thousands of different ones, so one size does not fit all. Sorry.
    But having said that if you test it, as mentioned above, and discover it is a photo transistor or photo diode it may be possible to run some tests that may tell you more about it.
     
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The easiest way to tell a photodiode from photo transistor is to connect one end to ground, connect other end to a large value resists and connect the other end of the resistor back to ground. Then measure the voltage across the resistor. Then shine the IR emitter on it and measure again. If you get nothing, flip the photo-device and test again. If it is a diode, voltage will change in one of the directions. If a transistor, Voltage will not change in either configuration.

    Good luck.
     
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