Temperature switch with thermocouple to drive motor

Discussion in 'The Projects Forum' started by pro_steve, Mar 20, 2011.

  1. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    Hi everyone, I am new here, and am after a bit of advice.

    I will explain the situation. I am tuning my car and want to run water injection in the intake manifold to reduce the intake temps. I already have a K type thermocouple in the intake.
    I would like for a water pump to turn on when the temperature reaches 60 degrees Celsius (2.436mV)

    I have found that a comparator would be a good component to use to switch on a relay, and have drawn a circuit diagram with a potential divider to set the voltage going into the comparator.

    Please take a look and let me know if this would actually work! I will use a variable resistor in real life so that I could adjust it.

    [​IMG]

    Thanks,

    Steve
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    WELCOME

    Ur circuit is not bad I doubt tht you cud drive a relay from the comparator output. You will need a transistor to handle the relay. furthermore you should use decoupling capacitors to eliminate ignition noise as such
     
  3. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    I saw these in another circuit, but it seemed to me like it was over complicating it. Would it actually need them?

    Also when it turns on I don't want it to turn off again until it reaches 40 degrees for example. Otherwise it would be a bit unstable if it was right on 60....What could I use for this? Could i use the same chip maybe with some clever wiring?

    Steve
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Yes you can. To do tht U have to give hysteresis.

    It's like the output will be active when certain input is reached but will not deactivate until it fall down a certain reference

    Give us the thermocouple data and ur requirements and the detail of the environment you will use it and it's power source
     
    Last edited: Mar 20, 2011
  5. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    OK, well voltage output at 60degrees C is 2.436 mV, voltage output at 40 degrees is 1.612mV.

    The voltage source will be ignition live and that would be at something like 14 volts when the car is running maybe a bit more. It doesnt really need to be completely precise at the minute, as I can put trim pots where the resistors are to adjust it.

    How do I give hysterisis?

    Steve
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Our aim is to let you learn while you build.
    Search the AAC ebooks or do a Google search for hysteresis
     
  7. ifixit

    Distinguished Member

    Nov 20, 2008
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    Hi pro-steve,



    Here are some important things to consider...
    1. Comparator output is open collector, so the other end of the load must go to the battery.
    2. Protect the comparator from the relay coil back emf with a diode across the coil (reverse polarity).
    3. Use a transistor to buffer the output if required.
    4. How much current is needed to operate the relay and how much current can the LM339 deliver?
    5. Use a proper thermalcouple interface with cold junction compensation for accurate, repeatable measurements. Use google.
    6. Your reference voltage needs to come from a regulator, because the battery voltage varies too much.
    7. Protect the circuit from battery transients with capactor filtering.
    8. Check that the relay contacts can handle the motor worstcase current.
    Above all... have fun and work safely,
    Ifixit
     
  8. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    Ok, thanks for all the comments. I Will go to the labs at Uni tomorrow and try and play around with some components to see if I can get the circuit to work. The only problem is I am not an electrical engineer so I shouldn't really be in there, but I will pretend its for my project! lol

    Steve
     
  9. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    If I were to read about Schmitt triggers would I be heading in the right direction?
     
  10. ifixit

    Distinguished Member

    Nov 20, 2008
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    Yes, read about Schmitt triggers.

    To get hysterisis, you feedback some of the output voltage to the reference voltage such that the reference Voltage will be lowered to the 40°C level. Then when the output switches back, the feedback will return the reference V to the 60°C level. Use ohms law and resistors to figure it out.

    Good Luck,
    Ifixit
     
  11. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    I think I have figured out a way of doing it. This will use two of the comparators in the chip. When the thermocouple reaches the right voltage it switches on the comparator. I can also make it switch on another comparator which in turn switches on a transistor that creates a circuit with another resistor in parallel to the second one in the potential divider. Then this reduces the voltage needed to make the switch come on.

    I will put up a circuit diagram if it works :)
     
  12. pro_steve

    Thread Starter New Member

    Mar 20, 2011
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    Ok, so it didn't work at all, so I started from scratch.

    I can get a comparator to turn on an LED but not a transistor and LED.

    The transistor will stay on constantly.

    I have a friend at work who is going to help me out tonight, and he has a bread board, so hopefully we will get some kind of working circuit :)

    Apparently we have the equipment at work to make a PCB so we can shrink it down and make it look neat any tidy too.

    If I ever get it working then I will add my circuitry onto here!

    Steve
     
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