Thermocouple Amplifier and Logic Control Device

Discussion in 'The Projects Forum' started by meestahelectro, Sep 20, 2010.

  1. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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    I have a car that runs a bosch motronic fuel injection ECU system. It's a pretty neat system and I am focussed on understanding/adapting one certain aspect of it that relates to measuring the exhaust temperature.

    The system utilizes a k-type thermocuple probe. This probe connects into a blackbox device that interprets the temperature and then sends out a signal to the ECU. based on my service manual, if the temp is 920 the blackbox sends out 0vdc to the ECU, it temp rises above 960 degrees the blackbox sends out 13vdc to the ECU. below 920 the I am not sure what the signal is (if there is one), however below 920 the system interprets this as a state in which the exhaust is at an okay temperature.

    Also, the higher temperatures there is also a signal to the dash panel to illuminate a bulb to tell the driver there is a problem.

    So, I think I can make use of the AD595 IC, but am not sure how to switch from 0vds to 13dc and not sure what goes on when the temp is below 920.

    Any help would be greatly appreciated. thx, Marc
     
  2. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    We are pretty iffy about poking at or messing with emissions signals to a car's ECU.

    A thermocouple will output a voltage that depends on the temperature of the junction. The switch from 0 volts to 13 volts is probably from a voltage comparator.

    What use do you wish to make of an instrumentation amplifier?
     
  4. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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    hi, just to clarify - this is not an O2 sensor (oxygen sensor). this does not control emissions at all.

    all this does is tell the car if the catalytic converters are running too hot. 900+ degrees. If it is too hot, the car will literally shut half the engine down so as not to catch on fire. since this black box of sorts is very unreliable many car owners just disconnect he box and the car runs fine. I won't do that. I want to know if by some unlucy event I am running too hot.

    I thought I needed an amplifier to increase the thermocouple voltage signal to make use of it. I could be wrong.
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
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    What is it you intend to achieve - a temperature measurement of the exhaust? If so, it may be worth adding your own thermocouple, that way you can keep the systems somewhat isolated so in case of a fault it won't let the engine overheat as you mentioned.
     
  6. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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    yes, I want to measure the exhaust temp with the thermocouple and translate this into a 0v or 13v output signal based on a pre-specified temp (it need not be adjustable), so it can communicate back to the fuel injection ECU. I'm not looking to change performance or anything, just trying to understand and recreate the original design (which is encased in a hard black rubber box).

    the black box has a two pin connector (goes to thermcouple) and a 6 pin connector. I think 3 of the 6 are ground pins, one goes to a dash light to signal a problem, one goes back to the ECU (with 0v or 13v) and one must bring in power. I'm going to post more details when I get home and even upload the schematic and circuitry surrounding this black box.

    I'm not really sure how a voltage comparator would be utilized. I just have the one voltage signal coming in from the thermocouple.
     
  7. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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  8. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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    Here is a little more detail about what I am trying to achieve...

    the input is the K-type thermocouple.

    This is what I need the circuit to do:

    At 980 celcius (+/- 20 degrees), I need it to send 13vdc output. under 980 degrees, the output on this should be 0vdc.

    Also, there is another circuit v out that illuminates a small dash panel bulb. above 900 (+/- 20), the bulb should flash. above 940 (+/- 20) the bulb should stay on permanantly.

    that's it!!! LOL.
     
  9. tom66

    Senior Member

    May 9, 2009
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    If you know the voltage at 980 degrees C, then you can use this in a comparator circuit.
     
  10. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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    Tom

    yes I know that at 980 cel it should kick on the 13v. can you give me a little more detail on the comparative ciruit you mention? I will google it now. much appreciated.

    do you think it also uses a comparative circuit for the dash bulb too???
     
  11. meestahelectro

    Thread Starter Member

    Aug 10, 2006
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