LM35 opamp LM358 relay temp CTRL´d for INCUBATOR

Discussion in 'The Projects Forum' started by ricky diaz, Feb 23, 2015.

  1. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Hi guys, this time i´m working making an incubator, i found many schems about LM35 temp sensor but few to make what i want, this time the circuit has to keep close a realy wich turn on a lamp that will warm the incubator inside, once the inner space reach the desired temperature it has to turn off the bulb opening the relay and coming back once the temp reach a minimun setting, till now i´ve found a schem that in practice does what i want and with very few components wich make me doubt some times, working togheter beside the LM35 is the LM358 opamp, but i have a problem, i´m trying to run the circuit with 6volt the relay is 6volt too, but the drained current is to small i guess to close the relay right, i mean i dont hear a nice clic in the relay, i´ve did try with 12 volts and still the same, probbably there is something wrong in the circuit or am i using wrong pieces, attache is the schem for evaluation, i´ll appreciate any advice or help you guys can give me, thanks in advance anyone, cheers.

    i´m using a 2n3906 instead tip122.

    ThermoBase.JPG
     
  2. wayneh

    Expert

    Sep 9, 2010
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    R2 might be too large. How much current does the relay coil need? Whatever that is, divide by ten and size R2 to pass that much current.

    Keep an eye on the max output current rating of the LM358 - it should be enough but you don't want to cook it.

    You have plenty of voltage, so I'd use a MOSFET instead of a power transistor. A MOSFET doesn't need any base current.

    Oh hey, you said 2N3906? That's a PNP and is not a suitable choice for this. The original TIP122 is a Darlington, so the 1K value for R2 is probably fine for that.
     
    Last edited: Feb 23, 2015
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  3. ScottWang

    Moderator

    Aug 23, 2012
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    The values of RV1 is very strange, if the V(-) pin needs a lower voltage than V(+) then the RV1 should using a lower value as 200Ω or 500Ω, otherwise just take the R4 away and only using RV1 to adjust the voltage.

    The below is another circuit also using LM35 to control the Fan.

    Fan control temperature using sensor LM35.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    R4 may be misdrawn. Putting a 200 ohm load on the LM35 is recommended, so I think R4 should go to the other op-amp pin.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    One excellent trick with relays is to use ALL their spec range, here we care about "pull in voltage."

    A 6V relay may work just fine (and within it's manufacturer's spec) down to 5V. See what the next higher voltage rated device is: it is possible there is a 9V or so device that is spec'ed to work at 6V, and a 9V coil working at 6V draws far less current than a 6V running 6V.
     
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  7. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Hi wayneh, wich MOSFET will you recomend me? just give spec not branding or label then i have a wide choice to find any in that range, interesting suggestion about the mosfet, if a use that no resistor neede? for now the circuit keep the signal on while bulb is warming then shut off as the temperature gets to the set point, i´ll check the current for the relay and apply the corrections, in other circuit i notice people use both sides of the opamp wich is not happening here, so strange.
     
  8. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Thx Scott but the operation in the circuit i need is opposite, i mean to maintain warm like a water heater, did u read my post? cheers.
     
  9. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Nice circuit Dodgydave, but i need to do opposite, like a watter heater to maintain warm and incubator and the hysteresis must be very tight you can get eggs cold into the incubator, the temperature must be constant 37.5/38 celcius, in the post i do explain very well, cheers.
     
  10. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Friend, very nice remark, i didn´t notice that, in fact 9V relay drawn less current aabout 30% less in my case but i´ve allready have this one, spec doesn´t say anything about max volt range, just diferent models in volt range, in fact there is 9v model, i´ll look that one next time in the shop, for now i will try this one to make the circuit works fine, cheers.
     
  11. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    ok, i´ve checked the relay spec and current in this one is 120 mA max, 100mA rated, so it gave me 12, now is this number in Ohm?, thanks again man.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You can set the hysteresis with with the preset that i mentioned, in post #5 ,0.5C is 5mV difference in temperature levels,it wont work without hysteresis as it will chatter.
     
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  13. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Remember guys, this is an INCUBATOR so the main goal is to maintain warm temperature like a water heater, i don´t need to cooldown nothing, circuits recall for now doesn´t acomplished the task, they are cooling circuits, cheers.
     
  14. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    That´s true, i´ll check that, Thx friend.
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    You need to feed the Lm35 into pin 2 of the Lm358, and the temperature reference on pin 3, using an npn transistor on pin 1, then as the temperature rises above the level, the output will turn off the relay, and cool down to the lower level, then switch on, voltage levels are 375mV on 380mV off,

    the hysteresis is only 5mV.

    thats. 37.5C to 38C
     
    Last edited: Feb 24, 2015
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  16. wayneh

    Expert

    Sep 9, 2010
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    If you are using an NPN transistor as a switch to operate the relay, you would want to ensure 10-12mA onto the base pin of the transistor. This would ensure solid switching. To get to ohms, use ohms law: ∆V=I•R. Your ∆V is the voltage available at the base pin, minus the drop of 0.7V that will exist from the base to the emitter. The current is 0.01A (10mA). Solve for R.

    The MOSFET option eliminates the need for that base current. It needs only a voltage on its gate pin. (A small amount of current is required to charge and discharge the gate, which behaves like a capacitor, but this is not a concern at slow switching frequencies such as in thermostats.)

    A Darlington is two transistor stages, so you divide by ten twice, meaning you'd want 1mA on the base to ensure good switching. Your op-amp can provide that.

    I didn't mention hysteresis earlier because there may be enough "built in", and you may want to test how it goes without anything added. But I agree you need want some hysteresis to prevent chatter.
     
  17. ScottWang

    Moderator

    Aug 23, 2012
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    The circuit I linked just wanted you to check the structure of the circuit, you just change the input pins of op amp, and also change the pnp to npn bjt.
     
  18. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    ok i´ve allready have the LM358 opamp meaning that wich pin config u think should be? i´m not specialist man i´m learning, thanks.
     
  19. ricky diaz

    Thread Starter Member

    Feb 16, 2015
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    Awesome Wayneh, truly i need to understand and manage better the Ohms law since i´m not a professional but a hobbyist or enthusiast but using what i´m trying to do, this is not for playing games, i´ll try to find a MOSFET now to test what you advice, now wich pin config you think i should use from the sensor to opamp (LM358) in my case, cause most project use LM741 or any other, i´ve allreqady have this one and don´t wanna waste it, any other advice wi´ll be welcome.
     
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use pin 2 for the sensor input,and pin 3 for the reference as i said in previous posts!!
     
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