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Need for heat

Discussion in 'Electronic Basics' started by JeffK, Feb 4, 2006.

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  1. JeffK

    JeffK Guest

    Hi Everyone,
    I'm doing a experiment were I need to get a ¼" x 8" x 12"
    piece of aluminum to 150-200 degrees F. Outside temperature would be
    around 70 degree F. Whatever I use needs to run off of 6-12vdc. In the
    past I used a light bulb in a metal can (120vac), but it is too hard to
    regulate the temperature that way for me.
    I was thinking of using a Peltier type chip for a heat source. I find
    a lot of info about its cooling affect (or lack of) but not much about
    the heat effect. Could this be done effectively on battery power
    (6-12vac)?
    The other way I was thinking about is to use resistors. But I have no
    clue on the size of the resistors or how many I would need to get to
    the metal too that temperature. Anyone have a formula or past
    experience about this?
    Or is there something a lot easier that I'm not thinking of?
    Thanks Jeff
     
  2. Hi Everyone,
    I'm doing a experiment were I need to get a ¼" x 8" x 12"
    piece of aluminum to 150-200 degrees F. Outside temperature would be
    around 70 degree F. Whatever I use needs to run off of 6-12vdc. In the
    past I used a light bulb in a metal can (120vac), but it is too hard to
    regulate the temperature that way for me.
    I was thinking of using a Peltier type chip for a heat source. I find
    a lot of info about its cooling affect (or lack of) but not much about
    the heat effect. Could this be done effectively on battery power
    (6-12vac)?
    The other way I was thinking about is to use resistors. But I have no
    clue on the size of the resistors or how many I would need to get to
    the metal too that temperature. Anyone have a formula or past
    experience about this?
    Or is there something a lot easier that I'm not thinking of?
    Thanks Jeff

    Howbout a light dimmer with the globe setup?
     
  3. Rich Webb

    Rich Webb Guest

    For a ball-park, back of the envelope estimation, your plate should run
    around 2 to 3 C/W. You're going from about 20 C to 80 C, so you'll need
    to dissipate something on the order of 150 W to maintain that delta-T.
    Far too many unknown parameters. How rapidly does it need to come up to
    temperature? How closely should temperature be maintained? Any issues
    with water or critters getting into the works? Will it always be
    observed by an operator or does it need to run for long periods (days,
    weeks, more) unattended? Etc.

    A simple bang-bang thermostat controller could be put together with a
    bimetallic strip; or a thermistor, a comparator, and a few resistors; or
    go all out and drop in a small microcontroller for a PWM controller.
     
  4. JeffK

    JeffK Guest

    Hi Rheilly and Rich,
    If I stay with using a light, a dimmer would be the ticket. I don't
    know why I didn't think of that before, Great idea, Thanks.

    Rich. I would not need it to heat up right away. It could take as
    long as an hour or so to get to temperature. It will be out all night
    or longer unattened. I was thinking about using a Pic16F627a or the
    16F628a and a thermistor to control the temperature. Also, it would
    have a roof over it and a weather proof box for the electronics.

    So you think resistors could heat up the aluminum? If so how, what
    type of battery voltage am I looking at and how many mah (or amps)
    would I need? How long would it last?

    Thanks Jeff
     
  5. You can get metal-cased power resistors that are intended to be bolted
    to a heatsink - these would be most suitable for your application,
    because they are designed to make good thermal contact to the mounting
    surface (your aluminium plate). Rich calculated that you'd need 150
    watts - if you're powering this from a 12 volt battery, it would draw
    about 12.5 amps. A Group 24 (traditional car battery size) deep cycle
    battery is rated at about 60 AH, if I recall correctly - this would
    theoretically power the heater for under four hours. However, for
    best battery life, it is recommended that you don't discharge below
    50%, so you shouldn't run it for more than a couple of hours.
    (actually, the AH rating of a battery is based on a 20 hour discharge.
    Discharging in 4 hours will give you much less than the advertised
    AH.)

    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  6. Chris

    Chris Guest

    Hi, Jeff. If you're considering sticking with line voltage for power
    (a good idea -- 200 watts is something like leaving the car headlights
    on, your 12V battery won't last long), you also might want to consider
    just cannibalizing a drip coffeemaker hotplate. The heater element can
    be affixed to the aluminum without too much difficulty. The built-in
    bimetallic switch will take care of the temp control.

    If you're a little crafty in how you mount the heater, you can achieve
    pretty much the right temperature, and it will have the ability to
    compensate for changes in line voltage, or an occasional breeze cooling
    the surface of the aluminum. A lamp dimmer doesn't have this
    capability. Also, you'll have the capability to come up to temp
    quickly.

    On the other hand, the coffeepot hotplate temp controller will have
    some hysteresis, and the aluminum plate temp will go up and down a bit.
    However, from your original description, it doesn't seem that really
    precise control is much of an issue for your experiment, just that it
    be within the range.

    And yes, if you aren't familiar with using line voltage safely, this
    solution is not for you.

    Good luck
    Chris
     
  7. JeffK

    JeffK Guest

    Hi Peter,
    Thanks for the info. But I think Rich maybe wrong about the 150 watts
    needed. I only say that because when I was experimenting last year I
    was using a (110vac) 40 watt oven bulb in a 6" diameter metal duct
    pipe. It would reach 195 degrees F. in about 30 minutes or so.

    The only reason I was thinking of using a battery is so that it would
    make the unit portable, but unless I can figure out a way to get the
    amps down I don't it is going to work. Draging out a battery the size
    of a car battery and than having to charge it is to much work. I would
    be better off going with 110vac.

    If I used the 40 watt bulb again what type of switches would I need
    to use with the microchip? The chip is only good for 25ma dc. So what
    type of switch(s) does a person use to turn on and off 110vac?

    Thanks Jeff
     
  8. JeffK

    JeffK Guest

    Hi Chris,
    I can't believe all that great ideas I'm getting and how come I
    didn't think about them. Anyway the coffeepot heating element is a
    great idea. I happen to have a hot plate that I've been dragging around
    for 20 some odd years. I see now that I should be able to use that to
    get the heat I need.

    I have been using this to attract mosquitoes. It is a fun experiment
    to play around with. I'm very good at working with line current, it is
    the small electric circuits that I'm just begining to learn. Do you
    have any idea of what type of switches to use with the pic16f627 that
    only puts out 25ma. I'm thinking the switch would need a rating of 15
    amps @ 110vac.

    Thanks Jeff
     
  9. I would use a solid state relay that is drivable directly with the
    logic output. A mechanical relay is a bit cheaper, but takes more
    interface circuitry. At 15 amps load, the solid state relay will need
    a bit of a heat sink, since it wasts about 1.5 watts for each ampere
    it passes.
    Something like:
    http://rocky.digikey.com/WebLib/Crydom/Web Data/EZ Series.pdf
     
  10. Rich Webb

    Rich Webb Guest

    Well, there was a fair amount of hand-waving there. Some effects, also,
    for having the heat source enclosed in a duct as compared to being
    mounted on one side of a metal plate.
    You could use a relay, typically driven by a switching transistor that
    turns on (sinks to ground) to give a path for the coil current. Do some
    reading at Omron's web site for good info on the how-tos. For example:
    <http://oeiwcsnts1.omron.com/ocb_pdf...56AF80060A947/$File/M20Z54E11.pdf?OpenElement>

    You could also go with a solid state relay, or a diac/triac pair, or a
    MOC3011 or MOC3031 plus a triac, or ...
     
  11. Dale Farmer

    Dale Farmer Guest

    As I understand it, they are attracted by heat and CO2, so maybe a
    small propane catalytic heater would be more appropriate. That is what
    the mosquito magnet(tm) and it's imitators use.


    --Dale
     
  12. JeffK

    JeffK Guest

    Hi John,
    Thanks for the link. I'm thinking I need the EZ240A12 120vac @ 12
    amps. Do you hook up pin 1 and 2 like a simple switch, Black wire
    (power-in) Black wire (power-out)? I know there are other ways to wire
    a simple switch but is this the way EZ240A12 does it, meaning does it
    let the power go through Pin 1 to Pin 2 when Pin 3 goes over 3vdc? If
    so then all I have to do is have a input/output pin go from low to high
    to turn it on? Also do I have to protect the I/O with a resistor or
    diode? If it is this easy it is going to help me out with another
    project. I'm just finishing a air-hockey table for the kids. Actualy it
    has been in use for quite awhile but I am now finishing up on the
    controls. Scoring and being able to turn the fan on/off automaticaly. 2
    problems solved in one night, how lucky is that.

    Rich I hope I didn't make you mad it was not my intention. I know my
    description was n't the best, I guess I was hoping that I there might
    be a way to generate heat without burning up alot of amps. Like maybe
    charging up a capacitor and then have it go through a resistor so that
    I could still use a small battery power pack.

    Dale thery are attracted 4 different ways, heat, co2, moisture and
    smell. Actually I think there is a five and that is motion. My idea is
    to keep it simple and inexpensive, that way I can put more lures/traps
    up to get better results. Heat and co2 are the best attraction but I
    haven't gotten around to the co2 yet (except to run through a long
    tube, it works).

    Thanks again Jeff
     
  13. JeffK

    JeffK Guest

    John,
    After looking at the spec sheet I was wondering what rms stands for?
    It has Arms which I figure A stands for amps. mArms which is mill-amps
    and then Vrms v for volts. Is this correct? But what is rms?
    Thanks Jeff
     
  14. (snip)

    There are two pairs of terminals on these things. DC (3-32 volts)
    applied across one pair turns on the power path through the other
    pair. The two pairs of terminals (control and output) are isolated
    from each other. The DC control current turns on an internal LED,
    which triggers the circuits in the power switch. I can't find the
    data sheet for the variation you list (and Digikey has none in stock)
    so you need to look for a unit that has the 3-32 volt DC input and a
    120 or 240 volt AC output with a current rating high enough for your
    load. A little margin is a good idea on that last spec. I would also
    look for one that specifies zero voltage turn on, so it doesn't make
    any radio frequency interference.
     
  15. RMS stands for square Root of the Mean of the Square of the
    instantaneous value. This is a mathematical way to measure an AC
    current as a number that represents its heating effect on a resistor.
    In other words 10 amps AC RMS heats a heating element the same
    amount as 10 amps DC would. But 10 amps AC RMS hits peaks of plus and
    minus 14 amps each cycle and also is zero twice a cycle.

    Same goes for volts AC RMS or Vrms. As long as you are dealing with
    resistors and sine wave AC, Vrms and Arms is the normal way to measure
    volts and amps, and is what a multimeter will show.
     
  16. Rich Webb

    Rich Webb Guest

    Grrrrr...

    (Just kidding, of course.) Actually, for a back of the envelope swag I
    think I got pretty close!

    Chris's idea of using a coffee warmer sounds like the best so far for a
    one-off project.
     
  17. JeffK

    JeffK Guest

    Hi John and Rich,
    Thanks for all the info. I think I now have a better understanding on
    how to go about phase two of my project. The heating element out of a
    coffee unit sounds like the best way to go. I'll be ordering some
    relays this week and I'm also going to go around the thrift and
    recycling centers to see if I can get some coffee pots cheap.
    Thanks Again Jeff
     
  18. Rich Grise

    Rich Grise Guest


    Didn't we answer this same homework quesiton just last semester?

    Look up "thermal resistance" and stuff.

    Good Luck!
    Rich
     
  19. Rich Grise

    Rich Grise Guest

    If you get a hefty enough dimmer, you could use it on an ordinary toaster
    oven. That'd get you to temp in minutes - but with a real toaster oven, it
    might have a built-in thermostat that'd get you between 175-200 with a
    twist of the knob.

    Good Luck!
    Rich
     
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