74LS04 inverter, maximum positive? and negative peak voltages that should be applied?

Discussion in 'Homework Help' started by ammanamu, Jun 4, 2013.

  1. ammanamu

    Thread Starter New Member

    Jun 4, 2013
    i am having problems with this question mentioned below.

    if applying an input waveform to the 74LS04 inverter, what are the maximum positive and negative peak voltages that should be applied?

    please help.
  2. MrChips


    Oct 2, 2009
    I wouldn't go above 5.5V and below -0.5V.

    You can limit the input current by putting a resistor in series with the input.
    Last edited: Jun 4, 2013
  3. WBahn


    Mar 31, 2012
    I'm assuming this is a homework question (since it's in the Homework Help forum) so the intent is probably that you show that you understand where to get this information and not what the actual answer happens to be.

    If this is correct, the most obvious place to start is the data sheet for the specific part. Have you found that datasheet? Keep in mind that you aren't looking for the 7404 data sheet, but the 74LS04 data sheet. World of difference in this context!

    Once you have the datasheet, you need to figure out where the specific information you want lives -- and sometimes that's not too obvious. Look the datasheet over and then post what you think the right spec is to use (or list several candidates if you aren't positive).
    screen1988 likes this.
  4. Papabravo


    Feb 24, 2006
    The datasheet is the place to find the authoritative answer for a specific part. In general you want to limit the inputs to ANY device to lie between the power supply rails. Some devices allow inputs outside this range but you have to find an explicit statement to that effect, you guessed it, in the datasheet. Many digital devices with a Vcc of 3.3V have inputs that are 5V tolerant.

    For digital devices the power supply rails will be Vcc and GND. For an opamp or comparator the lower rail might be below GND.

    Most digital devices include protection diodes from the input to Vcc and from the input to GND or Vee. This is why you will see specifications that allow you to go a few tenths of a volt outside the rails. The risk is that those diodes can draw excessive current which will heat up and destroy the device. That is why a current limiting series resistor is often employed on inputs.