Why my Op-Amp is freezed? :)

Discussion in 'General Electronics Chat' started by booboo, Jun 7, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
    165
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    Hi everybody
    I'm trying to connect a thermocouple to an Op-Amp to amplify the output of a thermocouple. this is the circuit:

    [​IMG]
    The Op-Amp is LM358N. when I use a potentiometer for the input(HEATER+) to test and check the circuit, everything work correctly but when I connect the circuit to the heater(or thermocouple because it's a soldering iron), the Op-Amp get freezed! :) I mean it just show a little and constant amount in the output (while the input is changing). why? something must be wrong.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A soldering iron element (if that's what you're using as your "thermocouple") may show surprisingly little resistance variation with temperature.
     
    korchoi likes this.
  3. ian field

    Distinguished Member

    Oct 27, 2012
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    I was just wondering whether the TS breaks the supply to the element during the period when the sensor circuit takes a measurement?
     
  4. korchoi

    Member

    Jun 5, 2015
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    Any input voltage you give to the Op Amp will be clamped to 0.7V by the diodes.
    Thus, in the output, the voltage will always be a multiple of the diode junction voltage, and not the "thermocouple" voltage.
    seriously, what exactly are you using for a thermocouple? Soldering irons elements do not generate voltage.Metal Junctions do.
    The conversion of electricity to voltage is one-way only on soldering irons:Electricity becomes Heat, But the opposite doesn't happen.
     
  5. booboo

    Thread Starter Member

    Apr 25, 2015
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    I'm trying to make this project:

    http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=5264&start=180#p58264

    In the first post of this topic you can see some good information about this type of soldering irons:

    http://dangerousprototypes.com/forum/viewtopic.php?f=56&t=5264#p50627

    But your statements are way off.:rolleyes: this circuit should work. and also when I measure the positive of the Op-Amp, there is the voltage as I expected (no clamped voltage). take a look at the links that I mentioned above.
     
    absf likes this.
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, it shouldn't. The input clamp diodes are not the problem; with a 1 M bias resistor, just about anything will have a low enough source impedance to pull the input below 0.6 V. The problem is the gain. You indicate a minimum gain of 221. That's a lot for any opamp, especially for a relatively low open-loop gain device like the 358. It is one of my favorite parts, but it also is 44 years old and a child of its time. And it has a relatively high input offset voltage that could add 2 V of error to the output. Think about a capacitor between R18 and GND to reduce this error.

    Also, what is the sensor device? Thermocouple, thermistor, etc.? Manufacturer, vendor, part number ... anything? To design this circuit you need to know the exact characteristics of the sensor and the input voltage the opamp will be seeing.

    ak
     
    absf likes this.
  7. korchoi

    Member

    Jun 5, 2015
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    I don't know how this bias thing would work. Maybe if the non-inverting input was attached R17 instead of the diode, then it would be easier on my head.
    But i think i know what the diodes are supposed to do. If the input goes too high(0.7V is too high for thermocouples), then they clamp.
    But clamping mechanisms are usually implemented on the output. Maybe the output is supposed to take heavy loads?
     
  8. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Hi AK
    I have changed my Op-Amp to some other but I had the same problem. even with a perfect Op-Amp like AD8572ARZ-REEL7. it's an auto-zero drift rail to rail I/O Op-Amp and a good choice for such a job. I have changed 4 Op-Amp to amplify the output of thermo for this job but still the same problem!:(

    That guy that is the designer of this project has said:
    "T12 have thermocouple in series with the heater with a voltage of around 9mV at 450 degrees celsius, so the voltage to the heater must be turned off to measure the voltage of the thermocouple. Don't measure the resistance of T12. It is pointless to use it when a thermocouple is already available, and is far more precise than resistance.

    I have made a controller for T12 myself. Here's how it works:
    - the 24V AC from transfomer is rectified but not filtered
    - this rectified 24V AC is the power to the T12, and is switched on and off for a millisecond on every half period of the AC, using P channel MOSFET.
    - when the voltage is turned off, the voltage of the thermocouple is available at t12 terminals, so it can be measured by an ADC in MCU
    - the MCU controls the heating using slightly modified PID algothitm, regulating how many mains periods the heater is turned on, and how many mains periods it is turned off."

    Are these informations enough for figuring out this problem?
     
    absf likes this.
  9. booboo

    Thread Starter Member

    Apr 25, 2015
    165
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    The clamp diodes are supposed to protect the input of the Op-Amp when it goes too high because we have to turn the solder on while it's connected to the input of the Op-Amp and for turning on the solder iron we need to apply 24V. the output of the Op-Amp goes to input of ADC. it's not a heavy load, is it?
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I don't see how you can have a thermocouple in series with a soldering iron heating element. Please post a schematic of the entire setup.

    ak
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    There's too much missing from the TS's description to even guess at what's going on!
     
  12. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    I have changed the Op-Amp to MCP6V07. but still I have the same problem.:mad:
    Ok, this is the internal structure of T-12 tips:

    [​IMG]
    the sensor is the thermocouple.

    Please ask your questions.
    Any idae or suggestion would be appropriate.
     
  13. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    When I connect the input of the amplifier (+HEATER) to GND and or 3v3, it just show 50mv. What's the matter with amplifier?
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What is the power supply voltage to the opamp?
    Show us the circuit you are using to connect/disconnect the element in each half cycle.
     
  15. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    For positive rail, 3v3 and negative, GND.
    Is this enough?

    [​IMG]
    What do you mean "in each half cycle"?
     
  16. booboo

    Thread Starter Member

    Apr 25, 2015
    165
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    One strange point!!
    when I measure the +input of the Op-Amp, it's(for example) is 1.2 but -input of the Op-Amp is 50mv! like output.:confused:
     
  17. booboo

    Thread Starter Member

    Apr 25, 2015
    165
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    And another strange point!
    When I measure GND by DMM, it show me between 50-60mv. it must be 0v.
     
  18. booboo

    Thread Starter Member

    Apr 25, 2015
    165
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    WoW! I'm measuring the GND and it's different in each part of the circuit while all of them is connected together.
     
  19. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Half cycle = half period, as you stated :-
    Can you post the schematic of that part of your circuit?
    Are you sure you have the + and - inputs the right way round?
    With respect to what are you measuring GND? Normally GND is the reference point.
     
  20. AllVol

    AAC Fanatic!

    Nov 22, 2005
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    I believe if you dump the two rectifier diodes you'll see some results. I have just duplicated your circuit as near as possible and could get nothing in the way of output with the diodes in place.
     
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