Differential Temperature Controller

Discussion in 'The Projects Forum' started by mburman, Dec 9, 2010.

  1. mburman

    mburman Thread Starter New Member

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    Long ago in a life Far, Far Away...

    I took an electronics course... but that was High School... and now it's more than 20 years later...

    I'm trying to build a Differential Temperature Controller for a solar water heater. The idea is that the controller measures the temperature at the solar panel and at the storage tank. When the temperature at the panel is higher than in the tank, a relay closes and turns on a circulation pump. After the temperature is balanced the pump turns off. I found a schematic here... http://www.redrok.com/images/diftemp.gif and used the one on the right (the one with thermisters). From that I made the circuit diagram. I built it up on a board and it looks amaturish, but all the connections are good. Here's my problem... I'm not getting sufficient voltage from the op amp output to switch the relay. I measured with a Voltmeter and the output is showing the voltage thru the relay coil. When I disconnected the relay from the output I can measure the voltage and see it change with the temperature difference so it is working...

    Now... my question... How can I get the output high enough to switch the relay without being so high that the relay won't turn off when the temp calls for it to be off...?

    Thanks for all suggestions!!

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  2. bertus

    bertus Administrator Staff Member

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    Hello,

    The picture at the redrok site does not show.(cannot connect error).
    The picture you posted in the site is very tiny.

    Bertus
  3. mburman

    mburman Thread Starter New Member

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    Sorry... resized the image... if its too small still let me know... thanks for looking! I don't know why the link to the schematic didn't work... it works for me...?

    I appreciate your help!

    Attached Files:

  4. wayneh

    wayneh AAC Fanatic!

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    I think you need a transistor (MOSFET would be good) to amplify the current from the op-amp to something high enough to power the relay. Or, your relay is shot. Does it trigger when activated manually?

    The diagram is still confusing. Are there unshown connections to power the op-amp?
  5. someonesdad

    someonesdad Senior Member

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    You haven't given a schematic, at least one that shows the parts you're using and the connections. The question is the relay -- what is the current its coil requires and what load is it switching?

    If you're switching a line voltage load with the relay, a solid state relay could be used as they can usually interface directly with the output from an op amp or comparator. If not, then you'll need something like a transistor like wayneh suggested. Remember to put a diode around the coil to help clamp transient inductive switching spikes.
  6. mburman

    mburman Thread Starter New Member

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    The relay does work and all of my connections are on the diagram. The schematic i built it from does not show powering the op amp but I did temporarily try it. It did put out 12 volts when powered, but the output never changed when the thermisters were warmed. Without the power to the chip I get 6 to 8 volts depending on the temp difference between the thermisters. So the circuit seems to work... it just doesn't reach the 12 volts the relay needs to trigger. If I added a transistor (or mosfet) where/how would I need to connect it and what type would I need to buy?

    Thanks again for the help!
  7. iONic

    iONic Senior Member

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    Here is the schematic from the site:

    [​IMG]
  8. wayneh

    wayneh AAC Fanatic!

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    The op-amp MUST be powered for the circuit to have a chance. It's meant to act as a comparator, with the output swinging all the way to one rail (+12v) when the "+" input voltage exceeds the "-" input voltage, and to the opposite rail (ground) when the inputs reverse polarity. The datsheet will specify how much current it can source (acting at "+") or sync (at "-") "Hysteresis" is the middle ground, where switching would be chaotic back and forth if you don't have at least a little hysteresis to quiet it. A small feedback from the output gives a little hurdle the input has to overcome to make a switch.

    If you didn't see the output change, first make sure you're looking at the right pins (check the datasheet) and then look at the two inputs and the output. If it really isn't working, try one of the other op-amps available on the same IC.

    If the relay needs more current than the op-amp spec says it can handle, you'll need the transistor. Let's get the op-amp working first. Try using an LED with a 1K resistor in place of the relay, until the rest is sorted out.
  9. iONic

    iONic Senior Member

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    In other words try this:

    [​IMG]
  10. wayneh

    wayneh AAC Fanatic!

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    Sweet! What he said.
  11. mburman

    mburman Thread Starter New Member

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    My deepest of thanks to all!! I removed the op amp from the chip holder, balanced the voltage with the 20K pot for the inputs (+ and -) to the op amp. connected power to the op amp terminals and put the chip back in and Voilla! The relay switches!! Seems I needed to find the balance point on the pot! Now... I'm under the impression that the 200K pot is supposed to control how long the relay remains closed after the thermisters reach equal temperatures... am I wrong about that function? If I'm wrong... what's it supposed to do? I know I need to play with this some more before I put it into service...

    Thanks again for all your comments!! :)
  12. iONic

    iONic Senior Member

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    Hysteresis can be used to filter signals so that the output reacts slowly by taking recent history into account. For example, a thermostat controlling a heater may turn the heater on when the temperature drops below A degrees, but not turn it off until the temperature rises above B degrees. Thus the on/off output of the thermostat to the heater when the temperature is between A and B depends on the history of the temperature. This prevents rapid switching on and off as the temperature drifts around the set point.
  13. wayneh

    wayneh AAC Fanatic!

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    Sorry to be niggling, but hysteresis has nothing to do with history, in the sense of there being any memory of a previous state. Hysteresis is just a dead band range of the input voltage in which no switching of the output will occur. If that dead range is exceeded, switching will occur as quickly as the device is capable of.

    The pot to adjust hysteresis sets how much of the output voltage is fed back to create the hysteresis "hurdle". More feedback means the input has to swing farther to overcome that hurdle and trigger the switch. You generally want the hysteresis to eliminate the effects of NOISE while allowing a small SIGNAL to be communicated. It's touchy to pick the needle from the haystack, and that's why you need a pot instead of a fixed resistor.
  14. iONic

    iONic Senior Member

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    Guess I'll not quote Wikipedia too often then.
  15. tracecom

    tracecom Well-Known Member

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    History was not the best word choice, but it's not entirely wrong. Hysteresis in some thermostats is referred to as a "dead band" or "dead zone." The "history" is the state (on or off) that the thermostat is in until the temperature moves out of the "dead zone," at which time the thermostat will change state.

    My recommendation with regard to Wikipedia is to use the information there only after it has been verified by another source or two. After all, by design, anyone can alter any of the entries on Wikipedia. I have found Wikipedia to be right much more often than it's wrong, but if the info is important, I still want confirmation of everything I read there.

    ETA: I think we're all in agreement about hysteresis; it's just the word "history" used in the sense of memory that is troubling. :)
  16. wayneh

    wayneh AAC Fanatic!

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    I suppose any system that holds its state until forced into another state could be said to have a memory. Memory of the past is embodied in the state of the device itself. This is, after all, the basis for all electronic memory. I should have kept my niggling to myself.
  17. mburman

    mburman Thread Starter New Member

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    WOW... I'm amazed... It works!! Thanks again for all the comments and information / advice! As a follow on... is there any way to "read" the actual temperature each thermistor is sensing? I've seen examples of people using Digital volt meters to display temperature but I can't imagine how to do it without impacting the signal to the op amp. I also have NO idea of what components would be needed nor their connections... If I'm asking too much, I apologize... but it seems like a fun project and useful (at least for me :)).

    Thanks again for helping me get this working!
  18. wayneh

    wayneh AAC Fanatic!

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    I think you could certainly calibrate your readings, by measuring voltage against a known good thermometer, and preparing a chart to convert from voltage to temperature.

    But I use the LM35 IC instead of thermistors. It just needs power and it will output a voltage directly proportional to temperature, eg. 0.15v at 15°C and 0.25v at 25°C. It's linear (unlike a thermistor), reasonably precise and even accurate over quite a wide range.
  19. tracecom

    tracecom Well-Known Member

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    The farenheit version of the LM35 is an LM34DZ; both are three pin devices (power, signal out, and common ground.) You can supply a low voltage and then use a DMM to read the temperature directly (in millivolts) - LM35 in celsius and LM34 in farenheit.

    However, thermistors can be very accurate, especially over a narrow temperature range.
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