comparator help

Discussion in 'The Projects Forum' started by tiny747, Jul 31, 2010.

  1. tiny747

    Thread Starter New Member

    Jul 31, 2010
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    Hopefully I have an easy question. I am trying to use a comparator to switch on or off in relationship to a reference voltage. I am using a TLC3702 comparator, aka digikey part number 296-12188-5-ND. One input is from an amploc 25 found here http://www.amploc.com/AMP--55a.pdf. And the other input is a resistor division network with 5 volt supply using a 1 megaohm variable resistor with a center tap going to comparator. The output goes directly to a Mosfet. So my question is this, do I need to have a resistor on the inputs so that it doesn't hurt the comparator (load up)? If it does need a resistor so that the inputs don't overload how do I determine the correct size? As far as I think the offset voltages don't concern me because I can adjust for that with the variable pot? Right? Thanks in advance for your help.
     
  2. tom66

    Senior Member

    May 9, 2009
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    I don't have an answer to your main question, but a word of advice on the 1 megohm resistor; you really should pick a higher value because the input current of your comparator will cause the voltage to deviate significantly. A 10k or 1k pot would be much better.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you notice the sensor has an offset of vcc/2 ?
    This means the output will vary around 2.5 Volts at a vcc of 5 Volts.

    Bertus
     
  4. tiny747

    Thread Starter New Member

    Jul 31, 2010
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    the amploc25 starts out at vcc/2 with no current being sensed going through it. But as current starts flowing through the hall effect sensor it goes up from there. Yes I see this. Hopefully I understand it correctly. I have built part of this circuit board and that is how it seems to work so far.

    I also will also look into getting a smaller value variable resistor as the last post suggests.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    Can you post a schematic of what you have upto now?
    That way we can see how to alter it to get it working.

    Bertus
     
  6. tiny747

    Thread Starter New Member

    Jul 31, 2010
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    You guys are really fast posters! I am working on drawing up a schematic but I am not very good at it. I'm not a tech or anything like that. I'll try and have it up with 20-30 min.
     
  7. tiny747

    Thread Starter New Member

    Jul 31, 2010
    16
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    Hopefully this worked better?
     
  8. tiny747

    Thread Starter New Member

    Jul 31, 2010
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    This circuit hasn't failed yet. (I haven't powered it up) I just wanted to make sure that I don't need resister in series with the inputs before I put it all together. I don't like smoke.
     
  9. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    You'll be OK. The input impedance of many comparators is at least 10 megohms, most are higher; adding a resistor will do little if anything to 'protect' your comparator.

    If you're worried about voltage spikes, put a zener on the input with a series resistor, that'll short out any voltage spikes.
     
    Last edited: Jul 31, 2010
  10. tiny747

    Thread Starter New Member

    Jul 31, 2010
    16
    0
    thanks for all the post. That is all I needed.
     
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