modify comparator circuit

Discussion in 'General Electronics Chat' started by simf14, May 3, 2011.

  1. simf14

    Thread Starter Active Member

    Jul 27, 2007
    43
    0
    Hello Everyone. I am a noob. This is a comparator circuit listed in the tutorials...I would like to modify it to run on 4.5 V instead of the 18V. Also curiously enough the pot resistance was not labeled. Would I need to add or change resistance or the comparator?

    Thank you.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Most comparators need a pull up resistor to Vcc. I suspect you are confusing a comparator with an op amp. Good examples of this include the LM393 and the LM339, a dual and quad example. I'm sure there are comparators that this is not true, but I don't know of any numbers off the top of my head.

    The other thing about these comparators is they have a 16ma limit. Plenty for a LED, but the more current you allow the slower they go. These are meant to be digital gates, a single byte D/A if you will, with an extremely fast switching speed. A simple op amp can't come close to this switching speeds, and are meant to be linear. A comparator only does digital, on/off.

    Here is an example how I boost the current limits of these little suckers...

    LEDs, 555s, Flashers, and Light Chasers

    Chapter 5 - The 555 and PWM

    [​IMG]

    Since these are a type of digital gate, it is a good idea to ground the unused inputs. Since you've grounded everything else, it doesn't hurt to ground the output pin (no pullup resistor).
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    To use the circuit as shown you could use a push-pull output comparator. There are plenty of them on the market. Parts like the LPV7215 have a push-pull CMOS output, as opposed to an open collector output, and can easily run off of the 4.5 volt supply.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Here is the data sheet...

    http://www.national.com/pf/LP/LPV7215.html#Overview

    It can handle 19ma at 5V.

    I prefer the LM393, probably because I bought over 20 of them last time I ordered.

    *********************************

    One negative, LM393 or LM339, 2V to 36VDC power supply voltage.

    LPV7215, 1.8V to 5.5V power supply voltage.

    A significant difference.
     
    Last edited: May 3, 2011
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    When you look under the hood there is definitely a significant difference and each has its applications. I have no monetary interest or preference in either, i.e. I don't have either on my work bench...
     
Loading...