Adjust heat dissipated by a power resistor - temperature control

Discussion in 'General Electronics Chat' started by aleca, Apr 13, 2016.

  1. aleca

    Thread Starter New Member

    Apr 13, 2016
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    I have currently two power resistors, 12 ohm, 25 W, this kind of product: http://fr.aliexpress.com/item/10-Oh...2502805027.html?spm=2114.44010308.4.35.tK9mXc

    I am trying to maintain the temperature inside an enclosure in a given range, say 37.5°C .. 38.2°C . It must never go higher than 38.5°C
    The problem I have with these resistors is that the inertia is too high. If I stop the current when the correct temperature is reached, the temperature keeps on climbing because some heat is still stored in the resistors heatsink, and I overshoot the target temperature by 2°C for a short amount of time. I dissipate the heat with a 12V PC fan . The problem also exists in the reverse direction. When the temperature reaches 37°C, it takes too much time to heat the resistors, and the temperature drops to 35°C.
    The heat is currently provided only by these two resistors.

    Can you suggest me a way to have a more fine grained control on the resistors temperature, and the temperature inside the enclosure in general ? Currently the Rs are wired in parallel to a 18V/65W DC power supply, and controlled by a temperature controller Willhi Wh 7016, but I intent to replace it with an Arduino.
     
  2. grahamed

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    Jul 23, 2012
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    Classic PID control problem
     
  3. shortbus

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  4. wayneh

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    One simple solution is to turn off one of the resistive heaters once you're within some degrees of your target. Similarly, you could add a smaller resistor for fine adjustment, turning off the big ones when you're within, say 2° of target. PID is the industrial strength solution but there are many other solutions that will be adequate.
     
  5. #12

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    Overshoot implies too much power. Try toning it down so you need a higher percentage of run time to maintain equilibrium.
     
  6. crutschow

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    You likely need some sort of a proportional control loop that generates a low-frequency PWM (duty-cycle-control) signal to control the power to the resistors.
    PID is the most common control loop but I like Fuzzy Logic control for a microcontroller, which seems easier to write the code for.
    Here's a article I wrote on that.
     
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  7. AnalogKid

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    First, what do we have to work with - How do you power the resistors now? Is it just on/off connections to a fixed power supply, or do you have a way to adjust the power being dissipated (either a variable DC voltage or PWM)?

    You can change to a resistor structure with a lower thermal mass, such as bare nichrome wire and a small fan, but *all* thermal management systems have the time delays you describe, and all solutions have one thing in common - change the power applied to the heating element *slowly*. There are simple and complex ways to do this, but the bottom line is that there is a time delay from a change in energy input to a change in perceived temperature. To prevent overshoot, your controller must apply power more slowly than this time constant.

    ak
     
  8. hp1729

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    Why go to the extremes of only on and off?
     
  9. #12

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    I was thinking about applying a wattage that would almost keep the temperature high enough, like 90% of the usual needs, and do the switching on an additional resistance that is good for 20% of the usual needs so you're flipping between 90% and 110% instead of flipping between 0% and 200%.
     
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  10. crutschow

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    That may be a simple way to do it as long as the ambient temperature outside the box doesn't vary too much (which would determine the maximum fixed power you could apply).
     
  11. Dodgydave

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    I would use an integrator pulsed temperature controlled circuit like in soldering irons, similar to this, it use ac but you can modify yours and use dc with a mosfet.
     
  12. aleca

    Thread Starter New Member

    Apr 13, 2016
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    Because that's how the hardware I bought is designed to work. However, I would be more interested in a circuit that would allow me to control more finely the amount of current flowing through the power resistor; what circuit using PWM, a fixed 18V DC power supply, two power resistors would allow me to fine grain control their dissipated heat ? Is this similar to designing an Arduino controlled variable DC power supply ?
     
  13. bertus

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  14. Tonyr1084

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    As a micro-section technician I used to accelerate the cure time of sample cups used for molding PCB samples in an epoxy. At first my boss gave me a toaster oven. But that also had the "Thermal Inertia" (or TI for short) you're experiencing. The heating element was on till the set temperature. Then the - still some 5,000˚F - heating element continued to raise the temperature well above where I needed it to cure the epoxy without melting the plastic sample cups. But once the thermal rise ceased and began its decrease to the point where it again switched on - that TI took a minute to begin to warm the oven again. That oven didn't last long before I put it into storage.

    I came up with a different solution (similar to yours). I used one of those long filament light bulbs mounted in an aluminum enclosure. Wired in a light dimmer and set up a thermocouple and digital thermometer. When the temperature got close to where I needed I'd just turn down the dimmer. Of course, this was a manual operation, but it solved the problem of curing the epoxy in 15 minutes as opposed to air curing which took up to 6 hours.

    I know nothing about PID controllers, but now that I know OF them I think I'll be looking into that for temperature control problems I may tackle in months (or years) to come. But yes, I HATE that TI.
     
  15. wayneh

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    If you don't care about the warm-up period, you can get perfectly good and very precise temperature control with just an on/off thermostat. Yes, it will overshoot when you first turn it on. Then the heat goes off and stays off until it falls back to target. After that, the response time becomes much shorter and that means the temperature overshoot will be much smaller.
     
  16. cmartinez

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    If you're going to use an MCU, then keeping historical data of over and undershoot temperatures should be fairly simple, and adjusting for that inertia a pretty straight forward problem. That is a basic form of PID that should also account for temperature variations outside of the enclosure.
     
  17. crutschow

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    You can simulate an integrator based controller (the I in PID), such as Dodgydave suggested, with the Arduino to generate a PWM output to control the heater.
    Are you familiar enough with programming the Arduino to generate code for that?
    I can also help you with trying Fuzzy Logic control using the Arduino if you are interested in that (it consists basically of a series of If-Then-Else statements).
     
  18. RichardO

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    When I click on your link a box comes up claiming to be Google. they want me to agree to privacy stuff. There is _not_ a way to close the box! I have to close the browser tab.

    I don't know if this is legit but I don't like it in the slightest.:eek:
     
  19. Dodgydave

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    temp.gif Here you are..note the drawing is incomplete, the temperature sensing is a thermocouple not thermistor.
     
    Last edited: Apr 15, 2016
  20. cmartinez

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