Schmitt Trigger oscillator help

Discussion in 'General Electronics Chat' started by Robin Mitchell, Jul 3, 2011.

  1. Robin Mitchell

    Thread Starter Well-Known Member

    Oct 25, 2009

    Hi everyone,

    The link above says that for high freq oscillators using a schmitt trigger the resistor should be > than 10K. Today i used a 5K resistor with no problems. Why should it be greater than 10K ???

  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    I would think that this is related to how much current the gate output can deliver without the output voltages being too much degraded.

    That will depend on what type of gate you have, and perhaps on the supply voltage. The figures on the web page may allow for a bit of margin, and note that the performance of different devices even of the same type will be subject to production spreads.
  3. Audioguru

    New Member

    Dec 20, 2007
    The datasheets of Texas Instruments Cmos gates show graphs of typical and minimum output current when the supply voltage is 5V, 10V and 15V. With a 5V supply the output voltage into a 10k resistor is typically 4.5V, not 5V. The minimum output voltage with a 5V supply is about only 4.0V.
  4. SgtWookie


    Jul 17, 2007
    There is a mistake on that talkingelectronics page; although 74C14's can be used with up to ~15v (depending on manufacturer), the 74HC and HCT series are generally limited to ~7v or less as absolute maximums. If one attempts to use a higher Vdd, the IC may be permanently damaged.

    To avoid confusion, one can use a 4000-series CMOS 4106 hex Schmitt-trigger inverter instead. The pinouts are the same.

    The purpose of using the 10k resistor is to indeed limit the maximum output source/sink current. 10k is a pretty standard value to use for pull-up and pull-down resistors; if you used a lower value in an oscillator you might prevent the output signal from reaching a valid logic level, particularly if the output load elsewhere is significant.