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Toaster Oven dimmer ?

Discussion in 'Electronic Design' started by Sylvain Munaut, Jul 29, 2005.

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  1. Hello,

    To try to approximate the temperature profile of a reflow oven, I'd like
    to make a computer controlled dimmer for a toaster oven.

    Does any one here as a schema / base where I can start from for the
    Hipower part (digital and pc interface is ok). I found some dimmer
    schema but they where all for light and a power of 500W max. Here, the
    heating element is more like 2500W. Is it just a matter of putting a
    bigger triac, that can handle that load ?


    Thanks,


    Sylvain
     
  2. Dave

    Dave Guest

    Sounds interesting
    Use a SSR (Solid State Relay). These come with opto isolated, logic level
    inputs which can be safely driven by any micro processor.

    Dave
     
  3. this was in circuit cellar
    http://www.circuitcellar.com/library/print/0704/Lacoste_168/


    martin
     
  4. Joerg

    Joerg Guest

    Hello Martin,
    And the author can sometimes be found on this newsgroup as well as on
    fr.sci.electronique.

    To be honest I didn't even know that there were toaster ovens with
    quartz top heaters. Guess I am barbequeing too much (ribs yesterday).

    Regards, Joerg
     
  5. Yes, thanks.
    I've seen similar article. But I was wondering if using a a dimmer to
    for example be able to set the heaters to 50% of it's nominal power
    would be better/worse than just turning it on/off bruptly.

    Is it really necessary to have top quartz heaters ? I just did 3 stores
    and none of them had quartz heater. They all had classical resistors
    (either in S shape, or several straight line ).

    I'm more a merguez type of guy ;)



    Sylvain
     
  6. Hi Joerg

    I've only just bought my first blender, ever, I dont even have a
    microwave. It is not advisable to light a BBQ here, serious drought
    problems/fires

    I'll stick with Tapas in the local bars, less washing up


    martin
     
  7. Ben Bradley

    Ben Bradley Guest

    There's a difference in heater control units and light dimmers.
    Light dimmers use an SCR or triac that turns on at different places
    during each half wave to get varying power and varying light.
    (Electronic) Heater controls turn on and off at zero crossings, and
    thus don't put a noise spike on the power line as dimmers generally
    do.
    I looked at the referenced Circuit Cellar reflow oven, it uses a
    mechanical relay to control the heating element! That probably
    produces spikes, but probably just one (or a few, depending on how
    bouncy the contacts are) every couple of seconds or so, or however
    fast it switches the relay.
    Someone else mentioned a solid-state relay, and that sounds like
    the solution, ISTR that some (or perhaps all nowadays?) only switch
    off and on during zero crossing, and that's what you want to keep from
    dumping spikes on the power line.
     
  8. Joerg

    Joerg Guest

    Hello Sylvain,
    The reaction time of a heater element is so slow that I'd guess it
    doesn't matter. Except that a dimmer for such power levels would be
    quite expensive. Small triacs are cheap because evey dimmer in the
    hardwares store contains one. Not so for the big ones.
    Email Robert Lacoste about it. There must be a reason why he ordered one
    via the web for $150 where he probably could have gotten a regular oven
    for $30 or so at Carrefour.
    AFAIK those have to be cooked in oil at the end. They say the same about
    bratwurst which should be pre-cooked in beer. But we didn't find much of
    a taste difference versus just slow cooking them on charcoal.

    Regards, Joerg
     
  9. Joerg

    Joerg Guest

    Hello Martin,
    Same here but we do not use any fossil fuel products. The charcoal is
    started in a "starter chinmney". Just a couple of crumpled newspaper
    pages in the cavity underneath is enough, you just have to wait 1/2
    hour. The typical American barbeque grill is a "Weber". It is a
    completely enclosed round thing with adjustable vents on top and bottom.
    Quite safe if you are careful.
    Tapas are more healthy, too. You aren't presented with a chock full
    plate of food that in the end is consumed much too fast. Do they still
    place the saucers on top of the beer or wine glasses?

    So, are you permanently lving in Spain?

    Regards, Joerg
     
  10. dlharmon

    dlharmon Guest

    I have done this with some success. I used a Sunbeam oven that cost $20
    and had the quartz elements. The oven is somewhat slow heating (less
    than 1 degree C per second and slows down the hotter it gets). I am
    considering moving the bottom element to the top and/or purchasing a
    second oven for more elments. I just used a relay to control it. That
    is all it should take. I used a MAX6675 thermocouple to SPI converter
    to measure temperature. The board I ran was a complete success
    including the 1mm pitch BGA.
    I simply used thermostatic control, and it was nearly perfect. Note
    that the pink line is the goal temp, and it is covered up by the fuzzy
    blue line (measured temp).


    There are temp profile from the oven linked at:

    http://dlharmon.com/smd.html

    Darrell Harmon
     
  11. Yes, I've seen that. Actually it's when I saw you successfully soldered
    BGA that I decided to try it by myself ;)

    I've just bought a oven, it's not quart element, just 2 heating resitors
    on the top and 2 heating resistors on the bottom. 1500W total power. The
    inside looked exactly like
    http://www.sparkfun.com/tutorial/ReflowToaster/reflow-toaster.htm

    To do a quick heat speed test, I placed a thermocouple inside. However,
    it was near the back panel, not really centered (it's a multimeter probe
    so i inserted it trhu a screw hole). And at posteriori the real inside
    temp must have been quite hotter than that.

    It heats at about 1-1.5 °C at the start and at 0.2 °C/s when above
    200°C. However these measure might be flawed see what follows.


    As a quick test, I just took a PCB where the silk screen was failed and
    where I did some solder test. I took some pads where there was a solder
    "bubble" on the pad and applied flux, then some 0805 and 1206
    components. Then I turned on the oven till I read 130 °C, maintain that
    for 2 min, then heat up till 220°C and maintain that for 30 s. However
    the temperature in there must have been higher as the pcb itself kindof
    melted ;) (the different layers separting themselves, bubble appearing
    inside) Guess I have to get a better reading than that. The solders
    looks ok though, even if the components and the pcb are near burnt ;)


    Sylvain
     
  12. dlharmon

    dlharmon Guest

    I would say that you were way above 220 degrees C on the board that
    seperated.

    I have noticed that most of the heating takes place by infrared. If
    the thermocoule was not in contact with the board, there could have
    been large temperature differences. I saw a 50 C difference between a
    piece of copperclad (very reflective of IR) and a board with green
    soldermask in testing. I placed my thermocouple directly on top of the
    board near the BGA.

    Your heating speeds seem reasonable. Mine is about 2C/second at room
    temperature and 0.5C/second at 200 C. Xilinx recommends ramping up to
    the peak temperature at 2-3 degrees C per second if I remember
    correctly. I believe I would be in that range with the second element
    since the decrease in speed is probably due to the poor insulation of
    the oven.

    Darrell Harmon
     
  13. Joerg

    Joerg Guest

    Hello Darrell,
    It could also be because of the large volume of the oven chamber versus
    limited heater power. Just like it takes twice as long to heat a room of
    twice the volume with a space heater of a given power, almost regardless
    how good the house is insulated.

    Unless you are talking about a different beast usually they design these
    ovens so that one or two regular size chickens will fit. I don't know if
    that would be safe: What if the oven were somehow modified to maybe a
    quarter of its usual height?

    Regards, Joerg
     
  14. dlharmon

    dlharmon Guest

    Before I bought the toaster oven, I had thoughts of making a very small
    reflow oven using 4 500W halogen bulbs.

    I would try to raise the floor, but the heating element is a single
    300mm long quartz tube, and I am afraid that I would not get even
    heating. I don't think any chickens would fit in this toaster oven. It
    was the smallest one the store had and I chose it for that reason. The
    label says 1350 watts, and I assume that would be with both elements
    on. I have been using just the top element (broil setting). The oven
    has no insulation (just 2 layers of thin sheet metal). The outside gets
    really hot.

    Assuming I have 675 watts of power going in whenever the element is on,
    I would estimate that I am losing about 500 watts of heat when the oven
    is at 200 degrees C based on the difference in ramp rate. So I
    effectively have about 200 watts going to heat the oven and its
    contents. If I double the power to 1350W, I will still be losing about
    500W, but 850W will be going to heat the oven which should result in a
    ramp rate of around 3 degrees C per second. Maybe I will test that
    theory out tommorrow if I have time.

    Darrell Harmon
     
  15. Jeff

    Jeff Guest

    Quartz supposedly gives off more infrared heat, vs. convection heat. This
    would allow for quicker thermal responses.
     
  16. Unless the process is very tolerant of wide swings in temperature, you
    can't control a quartz heater effectively with zero-voltage switched
    time proportioning because of the fast response-- you have to use
    something like phase control (which is normally considered
    undesirable). </run-on sentence>


    Best regards,
    Spehro Pefhany
     
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