Thermostat and battery safeguard, help needed.

Discussion in 'The Projects Forum' started by Jacob J, Feb 6, 2011.

  1. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Hello all

    I just bought a boat and I am trying to figure out all the things that needs to be done before the fishing season begins. I have two 12V batteries in my boat, with 65Amps on them. I have an electrical cooling box, wich can hold my beer cold during a hot summerday on the water, but it doesnt have a thermostat. So what better way to get more into electronics, than just building one myself.

    Ive found this (the very top one) circuit for the thermostat, which is simple and seems to work for the designer:

    http://www.craig.copperleife.com/tech/thermo/

    I am not sure that I understand all what he discripes, but I hope that one of you guys can help me out there. The only thing I want the circuit to do, is to be able to adjust from 0 degres celcius to 10 degres celcius.

    Now that I am building such a device, I would like to put in a battery safeguard, so I dont drain all the power of the battery.

    I was thinking of using a LM311, for both the thermostat and the safeguard. The question is now, how do I get the reference voltage, that has to come into PLUS on the LM311? The voltage IN that goes to the MINUS of the LM311 is comming from the boat battery I guess?

    /Jacob

    PS:

    Am I right, that I can swap the VR1 with a normal resistor as soon as I have figured out, where 0 degree celsius triggers the circuit?

    The VR2 is the sensitivity right? It deturments how accurate the circuit is going to be. I have read that the relay will jigger if it is put too sensitive. Will I also be able to replace the potentiometer VR2 with a normal resistor as soon as I have figured out the sensitivity the circuit has to be at?
     
    Last edited: Feb 7, 2011
  2. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    I have now made a schematics of the thermostat and the safeguard.

    The things I am not sure about is:

    1. Can I just change VR1 and VR2 (in craigs schematics) with normal resistors, as soon as I have found the right value for both of them?

    2. I have a 6A fuse just after the battery. My cooling box runs at 5A, is a 6A fuse good enough for safety sakes?

    3. Do I have to put in some resistors before the thermostat circuit and the voltage comparator circuit? The boat battery has 65Amps, but both of the circuits will only take as much amper as needed right? So no resistors needed or have I misunderstood things?

    4. VR3 in my schematic (VR2 in craigs schematics), is it connected right? Or do I have to put the other pin on the potentiometer up to the diode?

    5. I am right in the assumption that this is dangerous powers we are dealing with in this circuit? Only 12V, but the boat battery delievers 65Amps, so it has the potential to kill right? Can I somehow prevent that? I am thinking if in some case one of the wires get riped out of the unit I am going to build and you touch this wire, will you then not be in risk of getting fried?

    Lots of questions, but I hope some of you can help me out here. I dont want to make anything unsafe.

    /Jacob

    ps. sorry for the bad quality of the image.
     
  3. wayneh

    Expert

    Sep 9, 2010
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    Yeah, it's pretty tough to see. Please try again. On the upside, I think we can help you get a better circuit than the linked one. I've recently built a TEC-based cooler with a thermostat.
     
  4. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Is that circuit you have build just as simple as this one? I dont need spot on 5 degrees celsius, but within +/-1 degree would be nice.

    I will try to upload it again in better quality if I can figure out how.

    Do you think this one would work? just for fun, it would be nice to see how far I have come in putting schematics together.
     
  5. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Here are some better images. They are in two parts, the thermo_part goes on top of the comparator_part.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Take a look at the circuit I'm using, attached. It includes an op-amp to reduce hysteresis to less than 0.1°C, and that's overkill for your needs. So you could eliminate the LM358 op-amp and put the LM35 thermometer IC output directly into the LM339 comparator. You'll need to change R9 and R10 to give you a reference of 0-0.3V or so, which will correspond to 0-30°C.

    Notice the use of the 7805 voltage reference. Since I needed the reference and had it on board anyway, I went ahead and powered my ICs with that to protect them from whatever else might be going on with the main supply. I think this would be useful in any automotive application, as I hear that auto electrical can be very noisy.

    Also notice the use of a MOSFET to switch the load. This I highly recommend over a relay, which, being mechanical, would wear out.
     
  7. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    The LM35 is a very expensive sensor I can see. Could it be substituded with a normal NTC 10K thermistor?

    How is the 7805 voltage regulator attached? And what is the A symbol at the LM358 and where does it go?

    Is it correct that you have two power supplies or how do you get the 5V @ 6A's? I really need to power this circuit with what I have, so a dedicated supply for this thermostat is not possible (maybe a battery pack could do, but else...).

    What does the LED do? Does it just light up when there is output on the LM339 and the cooling device is turned on?
     
  8. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Okay. Ive found this circuit:

    http://web.engr.oregonstate.edu/~traylor/ece112/lectures/comparator_ckts.pdf

    If I swap the + and - inputs on that comparator, then I would have a device that would turn of the LED (or an relay which I will use) when the voltage gets below 11 volts. Ill have to change the top resistor of the voltage divider I guess to suit it my voltage, but else it would work as a battery guard right?

    I have looked at the second circuit in this link (http://www.craig.copperleife.com/tech/thermo) and it seems to be more simple than yours, but maybe I just dont understand all the components function in yours. The circuit linked is also using a 10k thermistor, which I think I will have to make a 1k thermistor to get into the temperature range I need it to be in.

    But would those two circuits (the second in the craig link and the comparator one linked here) work or would I be better off using your circuit (then I would have to get a more deeply explanation of it, if you could be so kind)?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Gosh, I got 10 of them for $12 delivered.
    Let's start using this simpler drawing.
    Picture 1.png

    Yes, in my case I was using a computer PSU to get the amps required for the cooler. It would just fine to use a 12v supply for the cooler. Just increase the resistor limiting current to D2.

    Yup, I like using indicator LEDs for just about everything, but especially for things I can't see easily otherwise, like the TEC.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    In concept it's the same. The linked circuit doesn't show any voltage regulator, or the load, so might appear simpler. But it's really pretty much the same. And it's fine to use a thermistor instead of the LM35 if that's what your comfortable with.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    The current configuration lights the LED, by opening a path to ground, under a low voltage condition. Just FYI, the comparator cannot supply current, only sink it. And the LED is about the most it can handle. If you want to trip a relay or larger load, you should add a transistor (eg. MOSFET). Like the schematic I posted.
     
  12. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Okay, I can see the LM35 has 3 pins, +, - and output. The thermistor only has a + and -. DOes this mean, that I put the - to ground and + to the comparators input?

    I will notice that with the sink and the limited current that can flow.

    How does the 7805 connect? It has 3 pins. On the circuit you have I cant see where the pins go and I am not so expirienced, that I can think of the solution myself.
     
  13. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    As far as I can see after looking around, is that pin 1 goes to the 12 volt supply (the top of the 7805 block you have placed in the schematics). Pin 2 goes to the green line that goes to ground and Pin 3 goes to the line that goes out in the buttom of the 7805 block. The green and the brown line isnt connected behind the 7805 block.

    Have I understood this correct?

    What about the A symbol? What does that represent? Is that going to ground?

    What is the function of capasitor C2 in the simpler drawing?

    The +5 on the LM339 goes to somewhere after the voltage regulater and the 0 goes to ground right?

    Can you also tell me why R8 is not needed in the simple version of your circuit?

    I have been looking at the thermistor and how to use that instead of the LM35, but I can't see how it has to be connected. Do I have to put it between 12V and then the point where the LM35 has its output?
     
    Last edited: May 1, 2011
  14. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Okay I just found this circuit too: http://www.youtube.com/watch?v=RIBjLycP0uk&feature=BFa&list=ULMz8qKqe2kJE&index=3

    I think he uses another approche or am I wrong?

    He uses two 741 opamps to do something to the signals that go into the LM339 he uses. Impedance matching he is calling it and I have googled that and it seems to be a way of reducing noise, but I may be wrong.

    I think I will stick to your circuit wayneh (I will try the big one), because there seems to be so many ways to do this, so I think I will be better off, just sticking to one thing. But still if you could explain the things I asked in the previous post, I would be very glad and thankfull.

    I have put all the components (except the LM35) into the basket, so as soon as I have understood how to put it together ill post the order.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Not quite. The thing to always do for any part is to download the datasheet, which will show you the pinout and example circuits. The 7805 has 3 pins; input (unregulated, higher voltage), ground, and output (regulated voltage) from left to right when the part is held upright, tab to the back. The two capacitors both connect to ground, which passes behind the 7805 in my drawing. I should have put C4 on the same side as C3 for simplicity.

    No, in my drawing it goes to another circuit that's not relevant to your needs. Ignore it.

    It's to reduce noise. Any fast-moving voltage is passed to ground, but any slow DC voltage is seen at the comparator. It's optional, something to try with and without to see if it cleans up switch chatter, if that's a problem.

    correct, same for the other ICs.

    You caught me. It's a remnant from the drawing I extracted this from, and it's not needed. Draining charge of the gate of a MOSFET, which acts like a capacitor, IS a concern but the 339 can provide that in this circuit. The 100k resistor does nothing.

    I'm no expert on thermistors, so I don't want to advise on that except to point at the circuit you posted earlier. That circuit is designed to put a biased voltage onto the pin of the comparator. You then have to make sure the reference voltage applied to the other pin is in the right range. That's true using either approach, LM35 or thermistor, but the voltage you need is likely different.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    It's very similar in concept.
    I'm not sure they're needed, especially in a voltage follower configuration. Perhaps he is using them to amplify the voltage. That's what my more complex circuit with the op-amp does. But if you aren't amplifying, you might as well just connect directly to the comparator, as in my simpler circuit. The input impedance of the comparator is high enough (I think) to not distort the input voltage, and anyway it's probably as good as the (ancient) 741 op-amp.
    I think you may not need the precision of the more complicated circuit, which I designed to hold temperature within <0.1°C. The simpler circuit will give you ~±0.5°C, and for most things that's good enough.

    I was going to suggest that you may also be able to get away without the voltage regulator, but since this is a marine application, keep it. If voltage were steady and not too noisy, you could eliminate it. You'd need to adjust the current limiting resistors for the LEDs, and adjust the resistor values for the voltage divider (R9 and R10). BUT, the system voltage on your boat is likely NOT quiet or steady, so you're better off to keep the regulator in place.
     
  17. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Okay +/-0,5 degress C is more than enough, so the simpler circuit it is.

    I think I get the part with the voltage regulator. In fact, if I put in such a voltage regulator in the circuit by Craig, then I would have a whole circuit with the thermistor in place. Its practically doing the same, altho your circuit is using a LM339 and he is using a LM741, but I have placed a order for them both. Another thing is the C2 you have in your circuit, but that I can try to put in from pin 3 on the LM741 and put it to ground if there are any problems.
     
  18. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Am I right, that if I place two 10K NTC thermistors in parallel, then it would be the same as just having a 5K NTC thermistor? Its because I have read that I need a 1K or a 4,7K thermistor at these low temperatures I want and the place I am ordering from doesnt have 4,7K thermistors.
     
  19. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    Hmm I think I will stick with the simple circuit of yours. I can't figure out how to add the voltage regulator to Craigs circuit.

    Can anyone tell me how to replace the LM35 with a normal NTC thermistor in wayneh's simple circuit? Do I just put the thermistor from the point where the LM35 has its output pin and then to 12V?
     
  20. wayneh

    Expert

    Sep 9, 2010
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    All you need to do is, with the specs for the thermistor in hand, is use the thermistor as part of a voltage divider, as shown in Craig's circuit. I think I'd aim to have the target temperature give you a voltage dead in the middle of the voltage range, eg. 2.5v if you use the 5v regulator. You'll need to choose the appropriate resistors to make that happen. Ohm's law is your friend.

    Then, again using Ohm's law, you can choose the resistors for setting the reference voltage on the other pin of the comparator. I'd choose a fixed resistor of 5K so that the variable 10K resistor set at its center is also 5K, and the voltage of the divider is 2.5v at the comparator.

    One thing I don't know is how much the thermistor changes, and how that compares to the 10K pot. Maybe someone here can advise. I'd hate to have a situation where even a slight tweak of the pot causes a wide swing in temperature. It's possible you need a way to get finer tuning.
     
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