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Switch circuit problems.

Discussion in 'Electronic Basics' started by Rubicon, Oct 16, 2004.

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  1. Rubicon

    Rubicon Guest


    I have an IR switch circuit composed of an IR LED, IR phototransistor
    to LM393 comparator with feedback to a 74HC74 positive edge flip-flop.
    The F/F had contolled a small relay via an NPN transistor and worked
    fine. When I decided to replace the relay with an NPN power
    transistor or NPN mosfet I found it wouldn't work correctly.

    If they switched appox 50mA then it would turn on and off but if they
    switched (350mA) it would turn on and stay on. I tried various
    different transistors and base resistor values to no avail. The mosfet
    I connected directly to the F/F output and then tried switching it
    with a small NPN transistor without success. However when I tried
    adding a resistor (over 10K) between the mosfet and the F/F output it
    worked. This I don't understand as I thought the mosfet was a voltage
    controlled device and as the F/F output swings nicely from almost 5VDC
    V+ to almost V- it stumps me.

    Does anyone have an explanation as to why this is?

    In addition I have another little IR phototransistor switch circuit
    with an NPN darlington pair switching a relay from their common
    collectors to V+ upon interruption of the IR beam. How can I
    incorporate the darlington pairs amplification into the first circuit
    for increased range? Each time I've tried this I've failed to get it
    to work.


  2. Brian

    Brian Guest

    What you should look at is the output of the F/F (when there is no
    resistor between the output of the F/F and the mosfet), to see if it is
    switching. Flip-flops can be fussy sometimes about the load they are driving
    (sometines it will keep them from switching). The resistor between the
    output of the F/F and mosfet, may give it the decoupling that it needs.
    Also, be sure to have a decoupling capacitor across the power suppy leads of
    the F/F.
  3. rayjking

    rayjking Guest

    semiconductors do not have.
    this is very common in control circuits. This gives a dead band and not
    simply on-off.

  4. Lord Garth

    Lord Garth Guest

    Don't forget an inverse parallel diode across the relay coil.
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