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heat switching relay

Discussion in 'General Electronics Discussion' started by Akalian, Aug 22, 2012.

  1. Akalian

    Akalian

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    Aug 22, 2012
    Heya all.

    I have a question about modifying a circuit to do something "I" (its all about me ^^ ) want it to do.
    I want to try modify this one because I can buy them standard and it would be a lot easier than just developing the thing from scratch, I understand if it cant be done.
    Attached is the diagram of the circuit and also the board with all the reference numbers of where the pieces fit in. I'm not that great at all this electronics stuff but I'm trying to learn.
    basically what I'm trying to do is get a system where there are two thermistors (lets say A and B) A would be installed into a solar hot water panel and B into a hot water holder. If temp at thermistor A is hotter than temp at B the relay should click in and if not then should click back off.

    anyhow, thanks for the help in advance.
    Gordon
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You do something very similar to this except in you duplicate the thermistor/resistor combination on both inputs.

    If the thermistor connected to the inverting input (-) gets hotter than the other one, the relay will switch on.

    Note that this presumes NTC thermistors (most are) and that they are reasonably closely matched. You could replace one fixes resistor with a slightly lower value one in series with a lowish value trimpot to balance things out. (you might try a 8k2 resistor in series with a 5k trimpot
     
  3. duke37

    duke37

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    Jan 9, 2011
    Could you just replace R1 with Th2?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yeah, good idea. But there might be a problem with high current drain and possibly even self-heating of the thermistors if they thermistors are NTC and they get hot.
    I was going to suggest Steve's idea too, but with a single-supply op-amp or comparator, in case the thermistors get so hot that the inputs go too close to ground for the 741.

    Probably either way will work. If you want to be sure, give us the part number for the thermistors, and your working temperature range.
     
    Last edited: Aug 22, 2012
  5. Akalian

    Akalian

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    Aug 22, 2012
    Hmm, both thermistors would need to take temps anywhere between 0 - 100 degrees C
    Not sure on the part number is, I know the one thats in there is a 15K one. If you have suggestions on changing it and what I can add where I would appreciate it a lot.
    Thanks for the advice so far guys, you've been really helpful.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    "15K" isn't enough information. That's its resistance at a particular temperature, probably 25 degrees Celsius. It doesn't tell us how low its resistance will go at 100 degrees Celsius.

    Most likely its resistance won't drop too low at 100 degrees Celsius and there won't be a problem with either approach. Actually we don't even know if it's NTC or PTC. If it's PTC, its resistance will be higher at 100 degC which is fine.

    You would probably find it easier to use duke37's suggestion because it needs less changes.
     
  7. Akalian

    Akalian

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    Aug 22, 2012
    I'm going to give that a shot. but I'll try find out more information about it and let you guys know.
    i see the updated kit has two pots near R2 and R3 to cut out chatter but i'll see if it'll help.
    Thanks for the help again
     
  8. Akalian

    Akalian

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    Aug 22, 2012
    well i stopped at the place that sells the kit. the thermistors that are supplied on it are ntc. 15k at 25 degrees. they also only have 1 and 2 k ptc thermistors. so im not sure if theres some plan i can make there or just get another 15k ntc one and try it out anyhow
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK. You'll probably be fine. I would recommend duke37's suggestion of replacing R1 with the second thermistor. You then have the two thermistors in series across the power rails. At 25 degrees Celsius, each one will have a resistance of 15 kilohms so the total resistance is 30 kilohms, which at 12V causes only 0.4 mA of current (from Ohm's law, I = V / R), which is 2.4 mW power dissipation in each thermistor (from the power law, P = V I), which won't cause any problems (e.g. self-heating in the thermistors).

    But those thermistors are NTC, which means their resistance decreases with increasing temperature, so at 100 degrees C their resistance will be less than 15 kilohms. If it's A LOT less, current will be higher and there could be a problem with self-heating. That's why I asked if you had a part number for the thermistor, so we could see their resistance at 100 degrees C (this will be shown on a graph on the data sheet).

    But I doubt that their resistance will be that much lower at 100 degrees C. Even if it drops from 15 kilohms to 6 kilohms, that's still only 1 mA current, which is only 6 mW power dissipation per thermistor, which is still very small and won't cause a problem.

    The hysteresis that was added in the new model kitset could be a problem, because the deadband it introduces will not translate to a fixed number of degrees of temperature difference. The actual deadband in degrees Celsius depends on the circuit design (how the hysteresis is implemented), and the actual temperatures being measured.

    This inconsistency will be less noticeable if you use a fairly narrow deadband. For example, a deadband of +/- 1 or 2 degrees at 25 degrees. That would mean the relay might switch relatively often if the temperatures are close. Will there be a problem if the relay switches on a small deadband?

    Most likely there will be no problem at all. I'd say try it and see, and set the deadband quite small.

    A schematic of the new kitset would be helpful, and if you can get a part number for the thermistors, I or someone else can give you an idea of how the deadband will vary with the actual temperatures measured.

    I hope this isn't too much detail. Most likely you will be fine just replacing R1 with the second thermistor.

    Edit: If you can't get a part number for the thermistor and buy a second one, and you don't want to buy two kitsets, you could buy two identical thermistors, either NTC or PTC but with a fairly high resistance like 15 kilohms, and use them to replace R1 and the thermistor that came with the kitset.

    Edit: Do the two thermistors need to be placed very far from the kitset board? If so, use twin screened cable with the screen grounded at the kitset board end, to minimise noise pickup.
     
    Last edited: Aug 23, 2012
  10. Akalian

    Akalian

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    0
    Aug 22, 2012
    I have a diagram of the kit. It's a bit late now but i'll scan it and post it up for the hell of it anyhow. yeah they do have to be quite far apart but I have the exact cable you're talking about so i'm set there =)
    I can check the catalogue of the thermistor and get it for you. Just need to find a chance to get there again, but I'll try this first. The kit is cheap so even if I make a mini nuke it wouldnt bother me ^^
     
    Last edited by a moderator: Aug 24, 2012
  11. Akalian

    Akalian

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    Aug 22, 2012
    Hmm, question. if I replaced the R1 with TH2....what purpose would the Var-pot serve in terms of the systems function?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It would act to trim out any difference in the thermistors, or it would allow switching when the temperatures were other than exactly the same.

    Another issue you'll come across is that when the temperatures are very close, the relay will buzz on and off very quickly.

    You need to add some hysteresis so that the temperature needs to be (say) 1/2 a degree higher or lower to switch (what lappens is that is wont switch one way until the temperature is (say) 1/2 a degree higher, then it won't switch back until it's half a degree lower).
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    He says the new version of the kitset includes hysteresis. I've asked him to provide a schematic so we can see how the hysteresis is implemented and whether the mod will cause problems by its interaction with it.
     
  14. Akalian

    Akalian

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    0
    Aug 22, 2012
    well I tested it out. it works quite well actually. Thanks for all the help ^^
    BTW where and what would I add exactly to cut out the chatter if it becomes a problem?
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Using the original kitset design or the new kitset design?
    The basic idea is to add a small amount of positive feedback. That is, a high-value resistor from the output back to the non-inverting ("+") input (pin 3). If you would obtain and post the circuit diagram for the revised kitset, you would see something like that. But the details of how that positive feedback is implemented, and how (if) it interacts with the absolute resistance of the thermistors, are not obvious. That's why I asked to see the schematic.

    Without hysteresis, I also would expect the relay to chatter when both thermistors are around the same temperature (after the trimpot has been adjusted so the changeover occurs when the temperatures are the same).

    http://en.wikipedia.org/wiki/Hysteresis
    http://en.wikipedia.org/wiki/Positive_feedback
     
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