Question about phototransintors

Discussion in 'General Electronics Chat' started by cumesoftware, Jul 13, 2007.

  1. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    I'm projecting a light follower robot, and I planning to use two voltage dividers, each one composed by two phototransistors and two protective resistors in series (in a sequence R - Pt - Sensing node - R - Pt). The middle node is to be connected to two comparators. The objective is to compare the light hitting each phototransistor. I'm using two dividers, one to compare front/back and one to compare left/right. Additional logic will be included. Originally I was using LDRs, but the guys from E.U. are taking them from the market (a stupid decision).

    So, my question is, what is the maximum current that a phototransistor can handle for safety and maximum lifespan? Should I consider the same for a LED (20mA)?
    I'm asking this because I pointed a phototransistor in direct sunlight, and the current reached a stunning value of 46mA. The phototranstor heated noticeably. I just don't want my project to auto-smoke in the sunlight like a vampire.

    I'm using L-7113P3C from kingbright (available in Europe):
  2. John Luciani

    Active Member

    Apr 3, 2007
    What was the Vce when the current reached 46mA? The *absolute maximum* for that
    part is 100mW. So if your Vce > 2.17V you have exceeded the maximum rating.
    I try to use devices at the conditions specified in the characteristics
    table. For the 7113 that would be Vce = 5V and currents in the 1-2mA range.
    At Vce=5V 20mA puts you at the *absolute maximum*.

    To determine life expectancy you would need to get the reliability data from Kingbright.
    If Kingbright does not provide reliability data at your operating condition you could use the Arrhenius equation to extrapolate.

    (* jcl *)
  3. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    It seems a good advice. So I will consider 2mA maximum.