latch circuit

Discussion in 'General Electronics Chat' started by TAKYMOUNIR, Nov 9, 2012.

  1. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    when v1 is high v2 will be 0 v and the circuit will latch
    in this circuit i was missing r2 and the circuit was laching all the time becuase vbe =.7 of q2 so why this happen ?
    second to do reset can i just short vbe of q2 by using switch?
    thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    Without R2 the circuit was likely turning on due to leakage current from Q2.

    You can reset the latch (which is basically a transistor version of a Gate Turn-off SCR) by shorting the base to emitter of either Q1 or Q2, whichever is easier.
     
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  3. TAKYMOUNIR

    Thread Starter Active Member

    Jun 23, 2008
    351
    1
    but q2 is off so how this leakage current come from and why can you please explain it to me AND HOW CAN I PICK RIGHT VALUE FOR R2(R2 SHOULD BE GREATER THAN RBE TO BE ABLE TO SINK MOST OF THE CURRENT)
    and thanks very much for your respond
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    All transistors have a small leakage current flowing through them when off which is proportional to temperature. It's typical of all reverse-biased semiconductor junctions. For example, the data sheet for the MPASA55 shows a leakage of 0.1μA at room temperature.

    The value of R2 is not critical to sink this leakage current since the current is very small. A value of 10kΩ to 50kΩ or even higher should work, depending upon how high the ambient temperature will get.
     
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