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What are R5 and D1 used for?

Discussion in 'General Electronics Discussion' started by holex, Aug 31, 2015.

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  1. holex

    holex

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    Jun 2, 2014
    Hi, what are D1 and R5 used for in this circuit?
     

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    No type is given for the IC, so one can only guess. It is probably a comparator (not an operational amplifier as one might think). Comparators often have open drain outputs, they can actively drive only low output voltage. To get a high oputput voltage a pull-up resistor is required. That is what R5 is for.
    The diode will change the feedback for low output voltages only. The change in feedback is for acheivung some hysteresis.
     
    Arouse1973 likes this.
  3. holex

    holex

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    Jun 2, 2014
    It is LM358P.
    I think R4 is used for hysteresis, not D1. Or am I wrong?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    LM358P is an operational amplifier with a push-pull output.
    R5 may have been inserted to increase Vout-high (the LM358 is specified with Vout-max=Vcc-1.5V @Rl >=2kΩ). The necessity for R5 is not obvious from this circuit alone. The designer of the circuit probably had some special load configuration in mind. The circuit will work without R5 in most cases.

    R4 is for hysteresis, but D1 will pass only low output voltage to R4. Therefore the "+" input of teh OpAmp will be lowered when the output of the opamp is low, but not increase when the output is high. Hysteresis is therefore not centered around a midpoint. D1 could be omitted, inthat case R2, R3 and R4 will have to be re-designed to achieve the same thresholds.
     
  5. holex

    holex

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    Jun 2, 2014
    Oh, I see. Thank you very much!
     
  6. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Without D1, changing either the trip point setting or the amount of hysteresis is a very messy calculation because the two are linked. With diode D1 in there, the trip point for increasing temperature has no hysteresis applied so it is set by R2 and R3 only.

    ak
     
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