trigger a relay with differing voltage

Discussion in 'The Projects Forum' started by jasonx, Oct 22, 2011.

  1. jasonx

    Thread Starter New Member

    Sep 13, 2008
    2
    0
    Hi guys I need a shove in the right direction.
    I need to switch a relay but the voltage does not drop to low
    It goes from 5v to 2.39v
    I bought some lm311n comparators but I can't figure out how to use them :confused:
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    please explain:

    1. what voltage do you want the relay to activate with?
    2. what's the relay model , voltage rating?
    3. what are the comparators for?

    if you post a diagram it gets easier for us
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    To use a comparator to make voltage decisions, a reference level is needed. If you have a stable power supply voltage, then a fraction of that may be used, by means of some resistors (a potential divider).

    If the supply voltage is not stable, for instance a battery that may run down, or if you need a very accurate threshold, a reference voltage device may be needed. In the past, Zener diodes were often used for this, nowadays special reference ICs or voltage regulators are more usual.

    Edit: The LM311 data sheet may help. Note that there are some pins best left alone unless special functions like offset trim, faster response or output strobe are needed. In particular, do not ground the strobe pin or you will shut it off. Don't forget to return the output ground though if using the main output to drive a relay etc. though, or it won't work.

    http://www.national.com/ds/LM/LM111.pdf
     
    Last edited: Oct 22, 2011
  4. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    wheres the input voltage from?
     
  5. jasonx

    Thread Starter New Member

    Sep 13, 2008
    2
    0
    Hi guys
    This is to switch on a extractor fan for a laser engraver
    I don't have a diagram
    I want to use a 5v relay
    The machine has a reference 5v a gnd and a trigger output which should drop from 5v to 0v but only drops to 2.39v :mad:

    I just presumed a voltage comparator would be the way to go but I am open to ideas.

    Thank you for the link Adjuster , that lots more example diagrams than the one I have.
     
  6. chrisw1990

    Active Member

    Oct 22, 2011
    543
    41
    perhaps you should find out why it doesnt drop to ground first?
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    A voltage comparator should work, but the output only dropping to 2.39V could be damaged, and so not reliable.

    If you measured this voltage while trying to drive a relay coil connected to 5V, you may have overloaded the output. Try measuring straight into a voltmeter with no load. A weak pull-down resistor (100kohm?) might help.

    Also, note that the relay coil should always have an anti-back emf diode, or expect grief when the coil turns off.
     
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