What are these resistors for?

Discussion in 'General Electronics Chat' started by m1ch43l, Jan 21, 2013.

  1. m1ch43l

    Thread Starter Active Member

    Aug 16, 2012
    Hello chaps,
    I've posted a schematic of a lm339 astable multivibrator driving a external circuit.
    What I'ld like to know is what the resistors R3 and R5 are for.
    I already know that it won't work without them.:D
    Secondly, can anyone please be kind enough to post a schematic of a triangular wave generator using the lm339 chip? I've tried 2 schematics on proteus, one with a square wave generator feeding a integrator, but none give me a clean symmetrical triangle wave.:rolleyes:

  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    R3 gives hysteresis so you have clean transistioning and no ringing as you get close to the very narrow "linear region" of the comparitor. Comparitors are high speed devices and, unlike op amps, they have lower input impedance. That lower input impedance means that the drive requirements on the input signal can differ whether the output is high or low. You can get some rapid on/off oscillations of the comparitor as the two inputs are very close in voltage (comparitor draws more from input signal to cause output to go low again and repeat until the two inputs are significantly different).

    R5 is a pull up resistor. High-speed comparitors a use an open collector output for higher speed (that is, no push/pull output). Therefore the collector of the output transistor should be connected to the positive voltage rail to complete the circuit.
    Last edited: Jan 21, 2013
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