Request Assistance: Temperature Switch

Discussion in 'General Electronics Chat' started by phototron, Nov 28, 2013.

  1. phototron

    Thread Starter New Member

    Nov 22, 2013
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    Hello. I have been getting great help from members, particularly MikeML, in this thread to make a low-voltage cut-off circuit to protect a battery of 4 AA NiMH cells that will power a small 1.5-2W heater.

    The purpose of the heater is to ensure that a hummingbird feeder doesn't freeze when the temperature goes below zero C. Here in the Pacific Northwest (Victoria, BC) the Anna's Hummingbirds don't migrate. So I want to ensure one of their primary winter food sources is available.

    The circuit MikeML proposed looks good, and though I am still trying to fully understand it's operation, I have decided to give it a try. However, the battery pack is small and I know we only have freezing temperatures some of the time, so to increase the time span per charge, I would like to make the circuit turn off the load (heater) when the temperature is greater than 1 degree C and turn on again when the temperature is lower than 1 degree C.

    If this is a relatively possible thing given my limited experience, I would greatly appreciate the help. I was wondering about using a thermistor, but maybe a temperature sensor module would be best.

    In MikeML's circuit, the voltage reference TL431 has a 50kΩ adjustment to set the low-voltage cut-off. If I knew how the voltage reference component worked, I would know if an increase or a decrease in the adjustment resistance raises or lowers the trigger voltage. Then I might know if a thermistor would be able to reduce the trigger when the temperature is high so the load remains off until the temperature decreases. I will have an opportunity to learn more when I receive the parts and can get hands-on.

    Would it be better to have a second cut-off circuit in series with the low-voltage circuit? Maybe a similar circuit to MikeML's but with a temperature reference instead of voltage.

    Insight would be greatly appreciated. Also, happy American Thanksgiving!
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Low voltage cut-off and temperature-controlled cut-off are quite distinct and would need two references. The temperature-controlled cut-off would require hysteresis of a few degrees. Quite doable.
     
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  3. #12

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    Nov 30, 2010
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  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    phototron,

    do you have any thermistors at hand? Who do you buy from? Digikey?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Any reason to use battery instead of AC? Heaters just require so much current that batteries don't last long. A 7 - 15 watt lightbulb would be perfect to heat a small feeder if it is insulated.
     
  6. #12

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    ps, batteries freeze, too...just not at 0 centigrade.
    Been there, got caught doing it wrong.
     
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  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It has been my experience that thermistors can give great results in a thermostat...as good as, or better than an LM34 or LM35, if the covered temperature range is small. I have a design with an LM358 and a 10k thermistor that gives resolution down to .1°F in an incubator.
     
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  8. #12

    Expert

    Nov 30, 2010
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    So my 35 year old design is not all that bad? Thank you!
    (I was certainly a lot less educated 35 years ago.)

    That particular project kept my bedroom at the correct temperature last night.:p
     
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  9. phototron

    Thread Starter New Member

    Nov 22, 2013
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    Hi MikeML,

    I don't have any thermistors. I have bought from Canada.newark.com before, but would be able to use Digikey or Mouser or anyone.
     
  10. phototron

    Thread Starter New Member

    Nov 22, 2013
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    You're right GopherT, a 5W nightlight covered in aluminum foil works very well according to my experiences a few years ago, but where I live now there are no outdoor electrical outlets, amazingly! So battery is my only option. Although a more experienced fella could make an inductive power transfer from the basement through the single-pane window...

    I did consider getting a cheap 11.1v Lithium-ion Polymer B-Grade battery from HobbyKing.com, but I figure learning to make a low-voltage cut-off for 4.8v NiMH would be easier.

    Once I better understand the cut-off circuit for the NiMH, I'll likely be able to improve on the heater by taking advantage of the Lithium packs.
     
  11. phototron

    Thread Starter New Member

    Nov 22, 2013
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    That was the most depressing part about working with cheap electric bikes - they used Lead Acid batteries. In the winter they were terrible, even here in relatively mild Victoria.

    Actually, the most depressing thing was the poor build quality. But the batteries were pretty awful. Thankfully this is a new era and Lithium-ion cells of various types are affordable. Though for this project the cells are NiMH, they should be able to deliver enough energy.

    I think the cells will help to slightly increase the temperature of the feeder as they discharge.
     
  12. #12

    Expert

    Nov 30, 2010
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    @phototron
    A laymans explanation of the TL431:

    The TL431 is a sort of zener diode with an amplifier inside. If the voltage on Vref is more than 2.5 volts, the chip conducts current, up to as much as 100 ma. If the voltage at Vref is below 2.5 volts, the chip does not conduct current. The TL431 does not want its Vref pin to be higher than 2.5 volts. If the voltage is higher than 2.5 volts, the chip dumps current to ground until the voltage on Vref goes down to 2.5 volts. The arrangement of resistors from the output voltage to the Vref pin allows you to set the output voltage at places more than 2.5 volts.

    There is also a current through the Vref pin of about 4 microamps. That is designed to be very low so that the error of the resistor network times 4 microamps does not have much effect on the accuracy. The value of the zener voltage is thus adjustable with fixed resistors or adjustable resistors.

    The TL431 can be used as a switch or an amplifier, depending on how imaginative you are.

    Got it?
     
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  13. MikeML

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    Here is the beginnings of a 2degC thermostat, based on (you guessed it) TL431s.

    The temperature sensor R1 is a 10K glass bead thermistor, like this one. I modeled its resistance vs temperature, and adjusted the resistors for a trip point of 2degC. Note the use of the Beta value from the data sheet. Modeling the resistance of an NTC thermistor is discussed here.

    The first TL431 just regulates the battery voltage to 3.15V. The battery can vary from >5V down to 3.3V without the regulator dropping out.

    The second TL431 is the temperature switch. It switches when the voltage divider R2/R1 outputs 2.49V. R2 can be trimmed to set the trip point.

    I havent shown the Power switch (that actually switches the heater), but it could be a PFET like in the cutoff circuit.
     
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  14. phototron

    Thread Starter New Member

    Nov 22, 2013
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    #12, thank you very much for that good explanation. I understand it much more thoroughly now.

    MikeML, thank you for the new circuit! I will look at this project with fresh eyes tomorrow, but I expect to be able to add a PFET switch to the circuit after having seen the example you gave me in the voltage cut-off circuit.
     
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  15. phototron

    Thread Starter New Member

    Nov 22, 2013
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    Ok, I'm drawing the circuit as I think it should go, while trying to find schematic drawing software for my Mac. I know there is software for Windows but I don't have that installed. I'm still fuzzy on something: If the load is 2 watts, wouldn't the components in the switching circuit need to be able to handle that power of 2 watts?

    In the example circuits where the PFET switches the load, is the voltage available to the load reduced from battery voltage after the current passes through the transistor? I want to ensure I order resistors for the right voltage.

    Edit: I found that for now the program with the shortest learning curve to draw circuits is an Android app called Schematic. Here is my first attempt to unify the two circuits Mike ML has provided. I added a PFET at the output of the final TL431. I didn't include the actual load resistors yet as I am not fully sure how many I will use or what their values will be, depending on the final voltage they see.

    I also see that there are duplicate labels for the resistors and other components, such as two R3s. I can clean this up - I assume each must have a unique identifier.
     
    Last edited: Dec 1, 2013
  16. #12

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    No. If the load is 2 watts, the load has to handle 2 watts. A switching transistor, slammed on as hard as it will go, almost disappears in the P=IE because there is only 2 or 3 tenths of a volt across the transistor. P = I (.3V)
     
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  17. phototron

    Thread Starter New Member

    Nov 22, 2013
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    Thanks for simplifying it for me. I keep thinking of it like the capacity of a wire, but clearly that isn't how it works.

    If the transistor has some voltage across it, such as .3V, does that mean the output voltage will be reduced by that amount? For example, 4.8 - .3 = 4.5V?

    Here is a link to the app I used to make the schematic above.
     
  18. Alec_t

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    Sep 17, 2013
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    Aren't R1 and R3 a tad high in value? IIRC the 431 doesn't regulate properly with a collector current < ~1mA.
    Correct.
     
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