Project: 12F675 based Temperature Controller Project

Discussion in 'The Completed Projects Collection' started by THE_RB, Nov 14, 2009.

  1. THE_RB

    Thread Starter AAC Fanatic!

    Feb 11, 2008
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    Here is a complete temperature controller for a small paint curing oven, using LM335 sensor and a relay to drive the "heater" which is some light bulbs. It is based on a cheap PIC 12F675 microcontroller and also uses a 16x2 LCD display.

    All the C source code is provided, it is in MikroC for PIC.

    A rotary encoder knob sets the temperature, and a button resets the 999 hour count-up timer 000:00:00. Display and control resolution is 0.5'C steps and range is the full range of the LM335; -40'C to +100'C (although this oven itself will only see about 30'C to 70'C).

    WARNING! The project uses mains voltages!
    This project was built by someone with many years experience working with
    mains voltage appliances. It uses a mains-rated relay to provide complete
    isolation between the control circuit and the mains voltages. HOWEVER it is
    important to state that you should NEVER build mains operated equipment
    unless you are completely competent to do so, and have checked with your
    local electricicty laws. Please do not attempt anything unsafe!


    [​IMG]

    It's nothing too fancy to look at inside, as it was put together fairly quickly, but it is a good performance temperature controller for the simple job of a drying oven.

    The full project is here;
    http://www.romanblack.com/shift1/sh1_p2.htm

    The PIC 12F675 based controller is built on the cheap little open-source Shift1-LCD board I designed recently which lets you make simple projects with a 8pin PIC and a text LCD;
    http://www.romanblack.com/shift1/sh1_projects.htm

    [​IMG]

    The second link also has some other open-source C projects using the same PIC hardware;
    * Tacho RPM meter
    * Clock
    * Solar battery charge controller
    * RC Servo tester with uS displayed
     
    Last edited: Nov 15, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Great, RB!
    I hope a Moderator moves this over to "The Projects Collection" forum; it's a keeper!
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    It sure is. So moved...
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Is this project safe? Building all sorts of heating applications using mains power is potentially hazardous. I do not think the hobbyist should be encourage to do this kind of application. I have to admit the design is nice and well done. But i still think it is unsafe. As it has NO thermal fuse as one example. I will urge the moderator to review it once more
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    I see a fuse, transformer, and relay to put power out to the lamps. All terminals in the case. Good layout.

    Do you have a specific problem with the project?
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    To be honest yes. Building all sorts of heating/oven applications is potentially hazardous, especially then using mains as power. Even if it is low temperature. I have worked as safety consultant/inspector and every home-brew of this kind give me goosebumps. I have seen to many burnt-out ruins caused by say homemade clothing dryer arrangement.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    t06afre,

    Your concerns are certainly appreciated, and safety is certainly a big issue around here.

    Perhaps THE_RB could have given some additional warnings in his project about the potential dangers of working with mains power, and a disclaimer about "use of the project information/material is at your own risk".

    However, it looks to me as RB has covered all of the potential safety issues; the line power is fused, transformer is used for the LVPS, the HV relay wiring is insulated with silicone (I would've used heat shrink tubing myself; no risk of corrosion from the ascetic acid that silicone emits), and the project is fully enclosed.

    If someone followed his project guidelines to the letter, they'd have a pretty safe end product.
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    I meant what I said, and I said what I meant. Nobody can control what people do at home. We can only give guidelines. This device is not a doom machine. But as I said it potentially hazardous. I would say more hazardous than some railgun postings closed in this forum. A heater device like this would never have passed a safety approval for commercial use.
    If I can give any guidelines so do not use any device like this unattended. Like starting it and so go to bed. Also have thermal fuse inside the "oven"
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    The thermal fuse is an excellent idea; it should be incorporated in the wiring to the lamps.

    Yes, it's impossible to control what people do at home. That's why it's important to make a specific list of parts to use in the line power section, with dire warnings against substitution of alternate parts.
     
  10. THE_RB

    Thread Starter AAC Fanatic!

    Feb 11, 2008
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    You have raised some good safety issues t06afre, and I would like to address them.

    1. Warnings have been added to the top of the thread here and to the project page itself.

    2. I was remiss in not covering the mains safety issues when I posted the thread. As someone who has been a qualified electrician for 25 years and worked on all kinds of mains equipment during many years as an electronic goods repairer I often take mains wiring for granted. The safety inside this device exceeds that inside MOST modern mains electronics goods, especially since it has covers over all the mains connections now it is finsihed (but not shown in photos).

    3. If this device had used a triac for mains control then it would probably not have been published (even though this forum is full of triac threads). The mains-rated relay was used as a safe simple way to control the mains heater, I actually prefer this system based on my many years experience as opposed to triacs and optocouplers which I do have stocks of and use occasionally.

    4. This is not the first low temperature oven I have built. There is another one that has been going almost constantly for 10+ years, yes even without supervision!!

    5. The "oven" uses 240v light bulbs in commercial sockets. These are rated as safe for continuous use. They do not cause fires when properly mounted, UNLIKE just about every other type of electical element.

    6. The light bulbs are chosen based on power that will JUST get the oven to max temperature. This design factor means that even if the controller or relay fails and the bulbs remain ON, the oven will simply sit near its normal safe upper temperature.

    7. This was never intended to be an example of how to build commercial or pro level mains powered equipment. Believe me I would do that rather different and it would have greater component costs, take more time, and look a whole lot nicer. This was an example of a quick easy low-voltage project, that just happens to have a safe mains-rated relay at the other end to switch on a couple of 240v light bulbs.

    8. It would be a fair assumption that people on the forum who would tackle a project like this are the same people who can comfortably wire up a mains transformer in (say) a home made DC power supply. There is nothing unreasonable or unsafe about the project apart from it uses a mains transformer and relay, like many many other projects on this forum.

    9. The silicone I use is non-acetic and has no nasty fumes and does not corrode metal like acetic cure silicone. It is strong enough to hold 500 pound fishtanks together and it's safe enough that many electrical goods manufacturers use it to secure high voltage parts and wiring to increase their appliances safety.

    I apologise for the safety issues caused by the original post, and I hope these issues have now been rectified.
     
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