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Relay acvitation on multiple Temperatures of an Object??

Discussion in 'General Electronics Discussion' started by Z2TT, Feb 8, 2013.

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  1. Z2TT


    Jul 6, 2009

    Basically I have a Alluminium pipe that is small, and it varies temperature from about 40 Celcius to say 150 degrees Celsius.

    What I want to do is have some sort of sensor that monitors the temperature of this pipe and switches a relay at two different temperature points that i desire, so for example to give out an earth that I can use to trigger relays.

    Example :

    Trigger 1 ON ---> 100C or anywhere above
    Trigger 1 OFF ----> When pipe reaches down to 70C ...

    and so on, when it heats up back to 100C, It will trigger the relay, and switch off at 70C or any temperature I want.

    So basically I need some sort of device that can firmly attach to the wall of the pipe without being affected by ambient temperature, and some sort of controller or relay that I can customize.
    The pipe does get hot, so the way of attaching the sensor to the pipe must ensure it wont come off with heat etc, e.g adhesive tape might not hold up in the long run depending on the type.

    Simplicity is the key as that is the only function I require.

    Any Ideas?

    Last edited: Feb 8, 2013
  2. Miguel Lopez

    Miguel Lopez

    Jan 25, 2012
    Can you get an RTD?

    You could attach it to the pipe and sense its resistance change and with a couple of opamp's do the job.

    A thermocouple could be nice too but maybe it would require more circuitry.
  3. duke37


    Jan 9, 2011
    I would think that epoxy resin (Araldite) would fix the thermistor to the pipe. There may be problems with differential thermal expansion.

    A wooden block held with a screw clamp would also do.

    There is a type of tape, usually yellow, (I forget the name) which is used to hold the windings of transformers. This is baked to make it permanent.
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    What accuracy do you need? A simple bimetal switch (2 switches, one for each temperature) does the job and may even be powerful enough that you can forgo the extra relay.
  5. Z2TT


    Jul 6, 2009
    Bimetal switch is too Bulky to Mount onto a small pipe (pipe has diameter of 7mm or so.

    Accuracy does not need to be perfect, Even 5 Degrees Celsius variance is OK, but more accurate the better.

    I was hoping there was something readily available that is user adjustable with many options and ranges.

    Only thing I can think of is a Thermostatic Switch such as on a Fridge, however the Cut in and out points are usually only 7 degrees apart or so and the
    capillary tube makes it a bit difficult to mount and bulky as the tube is quite long.
    Last edited: Feb 11, 2013
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Something like this?
    You may need 2 of them, one for each trigger point.
  7. Z2TT


    Jul 6, 2009
    Something like that looks good, but would like to do it with only one.

    Is there anyway I can easily build and mass produce a module that runs in a small box that has a variable resistance metal probe sticking out that i can clamp to my metal object....... and then have the CUT-IN Temperature at say 65 Celcius, and Cut-out temperature at 40 Celcius, or be able to vary the range with a pot, e.g 30 and 50, 20 and 40 etc etc cut in - cut / out?

    Basically the goal was to find something that is already produced but the thermister needs to be a metal probe that generates variable resistance so it is suited for direct contact with an object, rather than a thermister like in that picture you showed me Harold.

    I'm still trying to find something.

    Perhaps I can use that kit that you showed me Harold but change the Thermister to a probe type, e.g solder another one on?


    Really in the end I want something even more simple built for me that is smaller than possible, to do only one function, switch at two different temperatures.
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