using an NTC to switch on or off a relay for a heater

Discussion in 'General Electronics Chat' started by dumle29, Jun 26, 2011.

  1. dumle29

    Thread Starter Member

    Jun 26, 2011
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    okay, so i have been tinkering a bit with putting up a home etching station, including, ardweeny timed UV exposure box, a heated air agrigated etcher and we now need to construct a crued method of keeping an home made "oven" at ~ 60° C

    so here is what we came up with:

    http://snuletek.com/ting/Unavngivet.jpg

    would this work?

    also we know it would work if the resistors were of the correct value (have worked with some LDR circuits, maybe some rearrangeing of the components would be needed)
     
  2. #12

    Expert

    Nov 30, 2010
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    That schematic is so crude it even has the transistor labeled wrong. Nobody could recommend it based on the information given.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I'm wondering why you are not using an Arduino to measure the resistance of the NTC resistor, and energizing the relay depending on the temperature?

    You must have some unused I/O pins from your previous Arduino project; this one could be done using just two pins and a small amount of CPU time.
     
  4. dumle29

    Thread Starter Member

    Jun 26, 2011
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    well the reason why i wouldnt use the arduino is because its in another box (the UV exposure box) and because, well i havent even programmed the timer on that one, (because i cant :/).

    i have made a better schematic (the other one was made in paint (and ugly)

    i couldnt remember the pnp and npn arrow placement (i was just trying to understand the circuit)

    i have made another better one in fritzing and its basicaly a voltage divider.

    http://snuletek.com/ting/Untitled Sketch 2_schem.jpg
     
  5. SgtWookie

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    Jul 17, 2007
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    That new schematic is still pretty rough.

    Have a look at this:

    [​IMG]

    The R1/R2 values are still up in the air, as I don't know what your NTC resistor will measure when it's around 60 degrees.

    Anyway, R1, R2 and NTC make up a voltage divider. When the voltage at Q2's base exceeds about 1v, Q1 will start to turn on, conducting a very small amount of current. By the time the voltage at Q2's base exceeds ~1.3v, Q1 will probably conducting enough current to engage the relay RLY2 and supplying power to the heater.

    I used a 2nd transistor to supply current to the base of Q1, as the voltage divider won't supply much current at all to the base, so the collector wouldn't sink enough current to turn on the relay. This way, Q2 is an emitter follower, and the small amount of current flowing through R1 & R2 is multiplied by the gain (hFE) of the transistor. R3 limits Q2's maximum collector current to ~17mA, and consequently Q1's maximum base current.

    R4 keeps Q1 turned off when Q2 is sourcing only small amounts of current.
    D1 is the reverse-EMF protection diode for the relay coil.
     
  6. dumle29

    Thread Starter Member

    Jun 26, 2011
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    okay so at 60 C the resistance of our 22k NTC is 6k

    we asked the guy in the shop (he´s an electro-engineer) and he sayd it should work.

    so the circuit is basically a voltage divider like circuit.

    5v
    |----------------------------|Collector
    |.......................................|
    7k.....................................|
    |.......................................|
    |................................==========
    |-10k trim---250R--|Base|BC547 NPN ||
    |................................==========
    |.......................................|-------|
    NTC...................................|....1N4007 protective diode
    |..................................relay coil.....|
    1k......................................|-------|
    |----------------------------|
    0V
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it won't be very stable over temperature, as the Vf of the base-emitter junction and the Vf of the 1n4007 will decrease as temp increases, which will throw off the trip point.

    You really need a circuit using a comparator.
    I started drawing one using an LM111/LM311, but don't have time to finish it.
    It's "sort of" there, except the LM111/LM311 is limited to <50mA current for a relay, and I don't know what you're planning on using as a power switch for the mains.
    An SSR (solid state relay) would work fine, but they are more expensive than electromechanical relays.
    If you're going to use an electromechanical relay, it'll either need to draw less than 50mA current at your supply voltage, or the comparator output will need a heavier-duty driver.

    What do you want to power this with as far as the low voltage, and do you have a relay or SSR in mind to do the switching - or if you have one already, what does it require?
     
  8. dumle29

    Thread Starter Member

    Jun 26, 2011
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    Well, the transistor and diodes wont be in the oven, ill just put the NTC on two wires and stick it in there.
    The oven only needs to be ~ 60 C since its for drying photo resist on PCB (the hotter the faster but it has to be below some ~80 C)
    I will be using a relay rated 10A, coil current < 100 mA (Im running another one with a BC547 and it hasnt popped on me, and doesnt get warm either)
    a wall to usb adaptor (so 5V 1A)
     
  9. SgtWookie

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    Jul 17, 2007
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    Here's what I've been working on - see the attached.
     
  10. #12

    Expert

    Nov 30, 2010
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    The drawing in post 6 shows a typical mistake of putting the relay coil on the emitter side. That will make a voltage follower and the relay will only get about 1.7 volts. Just pointing out a common mistake that you should be aware of and correct in later revisions.
     
    dumle29 likes this.
  11. dumle29

    Thread Starter Member

    Jun 26, 2011
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    #9

    looks great, however, its a little overkill for our application, as simple as possible will do it there

    oh and thanks #12 for avoiding a mistake for me :) im still only 16 and have a lot of learning to do :)
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Well, I tried simulating your proposed circuit. It's not promising.
    The 7k resistor caused the relay to engage too late.
    I increased it to 7.5k and it wouldn't engage.

    That transistor doesn't have enough gain for the circuit to work reliably.
     
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