Verifying my Zero Crossing detector & 230VAC switching circuit

Discussion in 'The Projects Forum' started by Zen-, May 25, 2014.

  1. Zen-

    Thread Starter New Member

    May 25, 2014
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    Hello,

    I'm trying to build an Incubator.

    The goal is to control the temperature inside the incubator threw Arduino.
    My heating elements are resistors that connected to 230VAC.
    The idle needed temperature is 37.5C stable!
    It can variate up and down something like 0.2C, for getting the best hatching results (better accuracy better hatching results).
    BTW i'm using the DS18B20, i know that it's not as accurate as needed but i got external thermometer with 0.1C accuracy that would help me to calibrate the temp.

    With that part i'm quite doing okay for now.

    The other part of the project is the tricky part for me, using parts that handling the high voltage.
    After searching and checking a lot of the information on the web i'm finally ended with this circuit: http://i.imgur.com/QsFVxFe.jpg
    (sorry for the hand written circuit).

    The thing is that those kind of circuits come in many different ways with slightly changes from one circuit to another, including the values of the resistors too.

    I would be glad if you will check my circuit, the components and the values of the resistors considering the right currents and voltages that are needed to make this circuit work.

    If it's okay i will purchase the components and continue with the project, if not i will make the needed changes of course.

    Waiting for your help.

    Thank you in advance.
     
  2. pwdixon

    Member

    Oct 11, 2012
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    Why do you need a zero crossing detector?
    If you want to switch at zero crossover why not use a zero crossing opto-triac?
    Do you really need that amount of power?
     
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  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Agreed, an electronic relay with built in opto would be the way to go.

    I've built a lot of low temp incubator/ovens and you will have difficulties getting it to less than 0.2'C variation.

    Your control system regulation can be better than 0.2'C BUT there will be different temperature areas within the space. Even with a fan within the space to circulate all the air and semi-insulated walls you may still get a 0.2'C difference between the space centre and the areas near the external walls.

    Re the sensor, the DS18B20 uses 1/16'C internal resolution. You can access that internal resolution my reading the internal registers. Still it would not be my snesor of choice for a precise incubator setpoint. I would use a thermistor that can be wired up to give much greater variance per degree C, ie much better sensitivity.
     
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  4. t06afre

    AAC Fanatic!

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    If you just have an on/off controller setup. A proper rated relay may be a better and simpler way to go
     
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  5. #12

    Expert

    Nov 30, 2010
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  6. Zen-

    Thread Starter New Member

    May 25, 2014
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    First of all i want to thank you for your help!

    Second, maybe i didn't explained the way that i would use the heating element.

    I want to cut the wave in different places in order to get different temp out of the resistors, so for that i need to know when the the wave crossing the zero point, eventually i want it to work as a dimmer.
    I'm not going to use it like an on/off switch because it will make a quite big variation.

    About the DS18B20 i know that the accuracy isn't as good as needed, i know about the option using a thermistor but i want to leave it for the next step.

    Waiting for your comments.

    Thanks.
     
    Last edited: May 26, 2014
  7. #12

    Expert

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    You can change the power applied by using a few whole cycles of the AC wave then not using the following x number of cycles. You don't have to chop the wave, it's electrically noisy to do that. Just do the duty cycle in the Arduino.

    Every container has a time constant for heating. If your container can change by 1 tenth of a C in 16 milliseconds, this method won't work. However, I suspect that the time constant for your container is longer than 16 milliseconds.
     
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  8. Zen-

    Thread Starter New Member

    May 25, 2014
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    At first i wanted to do what you are suggesting me but then i came across the ZC detector thing to make it work as a dimmer.

    I would be glad if you will explain me in shortly what is the difference between the dimmer thing and changing the amount of cycles in a period of time, how would it be different eventually with the resistors as the heating element.

    BTW, i'm going to use the PID library to control the heating element, i think that your suggestion is the one that should work with PID, correct me please if i am wrong.

    Thanks again for your help.
     
    Last edited: May 26, 2014
  9. #12

    Expert

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    Zero crossing and dimming are mutually contradictory. A dimmer works by delaying the start of current in each cycle. A triac can not be shut off until the current becomes zero. If you start the triac at the zero crossing point, you can only get whole cycles of the wave. So...either dim it or do the zero crossing method with whole cycles. Pick one.
     
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  10. Zen-

    Thread Starter New Member

    May 25, 2014
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    But in order to implement the dimming i need the ZC detector to know when the cycle starts, or i'm wrong?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I agree, with heating elements it is usually only necessary to burst fire rather than phase angle control.
    If you really need it, look up Fairchild app note AN3006 also zero cross detector.
    Max.
     
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  12. #12

    Expert

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    In the analog world, a simple resistor-capacitor circuit is used to let the voltage wave tell the triac when to start. Increase the resistance and it takes the capacitor longer to fill up, thus delaying the start of each power cycle. Google, "dimmer" to see how it's done.

    How you manage to make this difficult in digital electronics escapes me.

    AN3006 shows some very good ways to let the analog method be inserted in the digital implementation.
     
    Last edited: May 26, 2014
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  13. Zen-

    Thread Starter New Member

    May 25, 2014
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    Thanks for your help first of all.

    Of curse i want to make it the most simple as i can.
    If you telling me that it would work good just controlling the switching of the resistors (to control the temp of the resistors) by the duty cycle with the arduino without implementing the ZC detector it would be even better.

    In that case i would be using only that http://i.imgur.com/0T3JogQ.png
    and if so, is the circuit correct?

    Thanks.
     
  14. #12

    Expert

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    Yes. That will work.
     
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  15. Zen-

    Thread Starter New Member

    May 25, 2014
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    I appreciate your and the others help, thanks.
     
  16. t06afre

    AAC Fanatic!

    May 11, 2009
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    For safety reason you should go for a properly rated SSR (Solid State Relay). And mount it according the specs with proper heat sinks if needed. The datasheet will tell you this. A SSR will contain all the components shown in the drawing in post #13 If a setup like this is not done properly it will be a real fire hazard. And it is borderline what a hobbyist should do
     
  17. Zen-

    Thread Starter New Member

    May 25, 2014
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    I'm really confused, if using those components is dangerous why people are using them? as i understand they are commonly used.

    I found a SSR that should be okay with my needs http://www.sainsonic.com/zen/albums/ebay/nichegeek/20-018-200/05577d01.pdf

    what difference it will do regarding with fire hazard?

    I'm not trying to decry you, just want to understand more.

    Thanks.
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    These modules are low current and are tradionally used to plug in and populate/condition the 4 to 16 module I/O boards used with μp type I/O.
    I am not sure if you can get the Opto22 where you are but there are equivalent stand alone units that go to 25amps.
    You can get zero switching types.
    Max.
     
  19. #12

    Expert

    Nov 30, 2010
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    The components are not dangerous. Apparently somebody is assuming you aren't able to assemble them correctly.
     
  20. t06afre

    AAC Fanatic!

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    First of all it is big deference by playing with a 5 volt Arduino setup and construct something that is made to be connected to mains. Every unit that are made to be mains connected have to be approved and apply to a set of regulations. If this approval is missing your setup will not in any country be legal for mains connection. Let us take a bad scenario I am not saying it will ever happen to your hatchery. Your house get badly fire-damaged and the cause is your home made hatchery. Then your insurance company trace the cause of fire to your home made hatchery(and oh yes they will). They will say sorry but the fire was started by your negligence and we will not pay you a dime. You used non approved equipment have a nice day. I know you will probably go ahead with your project anyway. IF you use a proper SSR mounted correct. This device will at least have been subjected to some approval. And be a tad more safe than your bread board mains power controller. Perhaps more important is how you mount your heating elements. But is is also a lot of other pitfalls with your project. That you may not know about.
     
    Last edited: May 26, 2014
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