Need simple ~ -10f to + 230 F 0 - 10 V converter

Discussion in 'The Projects Forum' started by tcmtech, Feb 7, 2014.

  1. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    As a few of you may have seen in the off topic area I designed and built a multi fuel boiler system for my brother a few weeks ago.

    Any way the control system is intended to use a common 0 - 10 VDC input for the temperature and what I deal with is largely off the shelf industrial stuff and not so much DIY electronics any more.

    So with that does anyone have a reasonably simple and cheap to make temperature to voltage converter circuit design that can be customised to work with a input sensing range of around -10 F to around +230F and put out a fairly linear 0 - 10 VDC reference voltage for that range?

    Preferably using non surface mount based components.

    Thanks! :)
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  3. crutschow

    Expert

    Mar 14, 2008
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    An AD590 which generates an output of 1μA/K (.555μA/°F) with an output of 298μA @ 25°C (77°F) and has a maximum temperature range of -55°C to 150°C (-48°F to 302°F) should work. The current can be converted to the desired voltage range with a suitable op amp circuit. If you want to go that route I can suggest an appropriate op amp offset and gain circuit.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You can use either the LM34 ( degree F) or LM35 (C) sensor and a gain stage after it. You just need to add DC offset to the gain stage so the output will be zero V at the temp you want and adjust the gain set resistor for full scale voltage. Very cheap and simple.

    Here's the one I use for my thermometer.

    On mine, I just "floated" the meter on a reference voltage so that the min temp reading displayed corresponded to the minimum on the meter scale. You could also use an offset diff amp gain stage. The charge pump ckt is to make the negative rail voltage to run the op amp.
     
    Last edited: Feb 8, 2014
  5. tcmtech

    Thread Starter Well-Known Member

    Nov 4, 2013
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    The LM34 based circuit would be perfect!

    The PLR system runs on a 24 VDC system voltage but takes 0 - 10 VDC on its analog inputs and the boiler will normally be working in the 100 - 200 F range on average.

    So what would be involved in redesigning the op amp circuit to have the 0 - 10 VDC output match fairly close to a 250 degree F temperature range (-20 to +230 F)? :)
     
  6. bountyhunter

    Well-Known Member

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    I think the LM34 is 10mV per deg F? Might check the data sheet, I designed my thermometer about 20 years ago so it's not fresh in my mind. A 250F range would be 2.5V total change, so your differential amplifier gain stage would be 4X gain.

    At -20F, the LM34 O/p will be -0.200V so you will need a Vref node of -0.2V to connect the diff amp negative input resistor (outboard end) to so the output of the amp will read 0V at -20F.


    You will need a negative rail to make the sensor it swing negative (see data sheet)

    http://www.ti.com.cn/cn/lit/ds/symlink/lm34.pdf

    I think this is right but it's all from memory.
     
  7. tcmtech

    Thread Starter Well-Known Member

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    If it needs a negative supply voltage to get the last -20 F of bottom range its not really necessary.

    I can live with it having anything close to 0 volts near 0 F for the bottom end limit.

    It's a fresh water system without antifreeze so if it ever gets below 32 f it the system will be froze solid anyway!

    More importantly I need the top end to be in the 8 - 9 volts output range at 200 F.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I think all you need is a non inverting op amp gain stage with gain a little over 4 amplifying the LM34 output, could add a trim pot in series with the FB resistor to tweak gain if desired.. With no negative rail, the op amp would need to be rail-to-rail input and output so it can handle voltages to ground.
     
  9. tcmtech

    Thread Starter Well-Known Member

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    Sounds easy enough. I can handle that! :)
     
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