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What Is Wrong With Me ??

Discussion in 'Electronic Basics' started by ahonda55, Oct 17, 2005.

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  1. ahonda55

    ahonda55 Guest

    Hi all , i have made this circuit ..

    it is a speed switch using LM290
    all of the circuit supply is 12
    And i had used a 100K variable resistor for R and adjusted it to abou
    C is .074uF and .01uF for the other capacitor
    and i have used a red LED as loa
    and i provided a very high frequency as Fin , so the LED is O
    but,, when i remove the Fin it always on
    what is wrong ?? what is wrong what is wrong ???? lo

    i want to let the switch On at about 120Hz , what are R and C value
    should i use ??
    and what is wrong with my work
    Plz help

    Thanx a lot :
  2. ehsjr

    ehsjr Guest

    You have not mentioned a resistor in series with the LED load,
    and your diagram does not show one. You need to limit the
    current through the LED with that resistor. Assuming a 12V
    supply, a 1K resistor will limit it to 12 mA, which is fine -
    you'll get a healthy glow from the LED. You could use a smaller
    resistor for more current - up to say 30 mA, but there is no
    need. In any even, it may be too late, now. The output
    transistor in the chip could already be shorted, which would
    give you the condition you have - LED always on.

    Rule of thumb: Unless you have a good reason not to, always
    use a current limiter with LEDs.

  3. DM

    DM Guest

    I don't believe you've hurt the output transistor since a shorted output
    would permanantly short the LED causeing the LED to blow.

    Make sure you are pulling down the input pin to ground through a proper size
    resistor (eg >1K). Otherwise noise and/or a dc voltage on that high
    resistance pin could be interpreted as a high frequency input.

    Also the "other" capacitor, in most cases, should be larger than the timing
    capacitor since its performing an integrating function.

    Let me know how it goes.

  4. ahonda55

    ahonda55 Guest

    Hi all
    first , there was a problem and i have fixed it, the ground connectio
    in pin 8 was not connected , now it is o
    second , i forgot to tell you that i already use a 1K resistor wit
    the LE

    The LED now is always OFF , whatever i change R1 , it is Off

    the IC is working good, i know that because i test it every short tim
    in my LED tachometer circuit

    The LED is off, but the LED itself is working good , i tested it

    note , i use the 14 pin IC see it here.

    What do you think friend
  5. ahonda55

    ahonda55 Guest

    Hi , the circuit now is alive !
    i was connecting the ground to pin 11 and 12, now i separated th
    ground and connected it to pin 12 only (GND

    but the circuit acts like it is mirrored , i mean , the LED is OFF bu
    when the frequency goes low the LED becomes ON .
    as i read in the data sheet (load is energized if Fin => 1/2RC) bu
    now something strange happens

    any ideas ?

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