op amp comparator values.

Discussion in 'General Electronics Chat' started by toffee_pie, Mar 26, 2013.

  1. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
    162
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    hi guys,

    can someone tell me if i have the right resistor values here to light up the leds according to the supply voltage, ie 1 led = < 3.5volts, 2 leds 3.5-4.2v, 3 leds = 5volts.

    thanks.!

    [​IMG]
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Most everything looks ok. The LEDs will only see 0.6 mAmp (assuming red LED) so they will not be very bright (barely a glow if they are older LED technology). Most comparitors can handle a bit more current (~5 mA). I would push 2 mAmps though them so a resistor in the range of 1500 ohms will work. Check the datasheet for your comparitor to find output drive (mAmps). Most LEDs can handle 20 mAmps (check that datasheet too - if you have one).
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,371
    5kΩ series resistors are too high in value. Try between 470Ω - 1kΩ.
    Your 2.5V Zener diode is pointing the wrong way.
     
  4. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
    162
    7
    thanks guys,


    i will have up to 1A flowing on the 5v rail, so used 5k values?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That 1 amp value is what is available for the total load the power supply can serve.

    Use ohms law for each path connecting your supply to ground. Use 5 volts, subtract 2 volts for the LED, you get three volts to drop with a resistor. If the target is 10 mA, then 3 volts divided by 0.010 amps yields 300 ohms (330 ohms is a nice standard value to use).

    Treat the 1 amp like a reservoir that is available, not like a flow.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It doesn't look OK to me. Remember that, when the LED switches, the voltages on your dividers will equal the reference voltage. See the attachment. Remember that LEDs 2 and 3 are on when the comparator output is low.
    Your zener is upside down, and unless you are actually using a reference IC (not a zener), it will not be stable as Vcc changes. Low voltage zeners are poor references.
    BTW, use a comparator IC, like an LM393, and not an op amp, unless it has rail-to-rail output swing.
    As the others said, you need to lower your LED current limiting resistor values.
     
  7. toffee_pie

    Thread Starter Active Member

    Oct 31, 2009
    162
    7
    i know about the zener, its corrected the op amp is a LM393 i got. still testing to room for corrections.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Just a nitpick - LM393 is a comparator. It is NOT an op amp.
     
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