op-amp or comparitor controlling relays

Discussion in 'The Projects Forum' started by eddie6775, Jan 12, 2013.

  1. eddie6775

    Thread Starter New Member

    Jan 6, 2013
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    Hello, Before I begin I just want to say this.. I have NO clue how to upload images from Eagle, other wise this would be a much easier question to ask so bear with me.

    I recently built a voltage comparator circuit out of Forest Mims's Basic electronics book (Page 83 for any one who has the book) and modified it slightly to work as a solar tracker. and would like to set the circuit up to control relays. This is what I don't understand: I can find relays that trigger at 5v, (my circuit is 6v) but none of those will handle the current draw from the 12v seat motor I want to control. And this circuit cant come close to triggering a 12v automotive relay, I thouht I would make the circuit 12v to match the motor but I'm not sure it can handle that.
    I know I'm missing something obvious, so I've come for advise and input.

    please take note*I'm just thrilled to have made this thing work at all!
    So to sum it up, tiny circuit, HUGE motor! Avoid smoke....:)

    Thank you to any one in advance who offers there two cents.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Try taking a screen shot and attaching it as a png.
     
  3. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    Screenshot.png

    Hope this works, Thank you!
     
  4. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    I uploaded the wrong schematic. The one i wanted is the same thing with added relays but in practice I just put solder pads so I can try different relays and motors. I hope my approach makes sense.

    Screenshot-1.png

    Ok, THIS is what I'm attempting to do. The schematic above is what on the breadboard so far.
     
    Last edited: Jan 12, 2013
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You need to replace D1 and D2 with current limiting resistors to limit transistor base current. (It works now because of the particular configuration but is not safe for the transistor in most cases)

    The more typical configuration is to swap the positions of the transistor and relay coil. (Transistor emitter to Gnd, Relay coil to V+.)

    Look up using a transistor as a switch.
    Is this some where in the MIMS book ?
     
  6. eddie6775

    Thread Starter New Member

    Jan 6, 2013
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    Yes it is. I'll have to go back and do some homework! I'll bread board what you suggested tonight. I'm not quite sure I fully understand swapping the positions of the transistor and relay coil but after I go out in the barn and try it I might. :confused:
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    Like This:
    [​IMG]
     
  8. eddie6775

    Thread Starter New Member

    Jan 6, 2013
    17
    0
    It took a minute going back and fourth between pictures but I see what might be happening now. The excess voltage in my circuit didn't have a place to go and the back e.m.f. would have killed my transistor? And now that I think about it and compare the two schematics I see how the resistors would work where as the diode now seems redundant.

    Think, think, think.

    I'll try it! Thank you! .........Give me a week, and I'll re invent the wheel if ya want,
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
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