heater control circuit help

Discussion in 'The Projects Forum' started by dtak, Dec 21, 2010.

  1. dtak

    Thread Starter New Member

    Jan 11, 2010
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    Hi,

    I would like to build a heater control circuit. I am planning on using a Type K thermocouple interfaced with a Futuredesign FCD-C21 temperature PID process controller that outputs 0-10V. However, I am unsure of how to convert this 0-10V into something that can drive my heater. The heater is supposed to be run at 4.35 V at something around 2A. It is a very small low resistance heater for heating up a very small Oxygen sensor.

    Thanks for any help!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I assume the controller cannot output enough current on its own, so you'll need a power supply for the heater. A used power supply out of an old computer could be good choice, since it will have a 5v supply capable of more than enough current, and should be free.

    If you just want to turn the heater on and off, like your furnace, you can use the 10v output from the controller to drive the gate of a MOSFET such as IRF540. You'll want to send only 0 or 10v to the MOSFET, nothing in between, to avoid heat in the MOSFET from being less than fully on. In fact it would be worth thinking about adding another transistor and using the 12v power of the PSU to power the gate. Using a logic level MOSFET might be a better choice.
     
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  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Maybe something like this.
     
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  4. dtak

    Thread Starter New Member

    Jan 11, 2010
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    Thanks! Those are good ideas. I will give them a shot.

    I need to keep the temperature as constant as I can. Do you think an on/off approach introduce too much variation?

    Thanks again.
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Not to be a smart aleck, but it depends on how large the heater is, how often you turn the heater on and off, how much the ambient temperature changes, how large an area is being heated, how well the area is insulated, how well the air/water/whatever is circulated within the area, and how much thermal mass is used. It's a complicated issue, but under ideal conditions, the temperature can be stabilized within .5 degree F using a switched heater. If you supply some details of what you are trying to do, you will get better answers. :)
     
    Last edited: Dec 22, 2010
  6. Otaku

    Active Member

    Nov 19, 2008
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    Good questions, Tracecom. Dtak, if you're trying to heat just a small area to less than say, 80 °C, you could try using a TO-220 packaged resistor and pumping around 12 - 18VDC through it. I've used this arrangement several times for controlling heat to a small area. Use a 100K or 1M pot to dial in the temp. This method doesn't use PID to maintain the temperature, but it is pretty stable.
    Depending on how much effort you want to put into building this system, it may be more effective to get a used Watlow 925 heater controller and a SSR.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    All good points. I've recently built a temperature-controlled chamber (PMC, poor man's calorimeter) and I achieved on-off hysteresis of less than 0.1°C. It's still a simple circuit, but I definitely had challenges along the way in getting to that level. I only later realized that PWM control, although more complex, might have been a better strategy to start with.

    One key to reducing dead band is to agitate. The electronics can't respond to something they can't "see".
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    LM35 + an op amp and a linear driver which will of course waste a lot of heat but you can make a circuit this way to keep the heater at a constantly adjusting level. Same thing can be done with a simple PWM driver which in most cases can be made from a simple 555.

    You can always get more complicated but as in the questions above please describe the physical environment you're using it in and any other details you wish to share.

    http://www.circuitlake.com/precision-temperature-controller.html
    http://www.atmsite.org/contrib/Clement/newdewheater/
     
  9. wayneh

    Expert

    Sep 9, 2010
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    LM35 + op-amp + comparator + MOSFET switch...

    ... is what I used to achieve <0.1°C hysteresis without power loss from a linear control. The op-amp gain is ~20, and that was a compromise between allowing a useful range (more gain is bad) and mitigating comparator hysteresis of ~5mV (more gain is good).
     
  10. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have a datasheet on your PID controller? Google didn't tell me anything about it.

    Second: is there a reason you are using that particular heating element? If you have lots of them on hand, I understand, but if it was all that was handy, there may be better options once the application is known.
     
  11. marshallf3

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    Jul 26, 2010
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    Wayne - We still don't know the environment in which this is going to be used but it's outstanding you could achieve such close control on your circuit. Ever document it?
     
  12. wayneh

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    Sep 9, 2010
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    I'm planning to write it up as a finished project, but I'm still working on actually using it to collect experimental data. It's sitting next to my computer with a LabJack data acquisition device wired to it with 7 or 8 leads to watch temperature, cooler duty cycle, load power draw, and more. (I LOVE my LabJack). I thought it would be prudent to get that perspective on its performance before writing the final report. So far so good. Maybe after the holidays. ;)
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I understand totally. My being ill most of the year then all these holidays have put a damper on a lot of things I need to do &/or finish, not to mention the fact that I'm way behind in my projects down at the office building but I'll catch up eventually.
     
  14. wayneh

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    Sep 9, 2010
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    Not a bad epitaph. ;)
     
  15. dtak

    Thread Starter New Member

    Jan 11, 2010
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    Thanks for all of the suggestions! They are extremely helpful. As for more details, the heater is an integral part of an O2 sensor GMS10RVS which needs to be kept at a constant high temperature on the order of 700 degrees C. The mass of the heater + sensor is very small. I have attached the documentation I have for the controller and the sensor.
     
  16. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Way outside of my experience! I see now why you are using a thermocouple.
     
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