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Beef Jerky Machine

Discussion in 'Electronic Basics' started by NJM, Mar 12, 2007.

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  1. NJM

    NJM Guest

    Hello all,

    I am wanting to build a copy of a dehydrator that an instructor at the
    local Vo-Tech uses. It is nothing more than an old fridge or freezer that
    is powered by a 150 watt light bulb. Very simple heh? It makes wonderful
    beef jerky. His set up was big enough to do 3 or 4 lbs at a time in 8.5
    to 9 hours.

    To my question. I am very new at electronics design and construction,
    however, I have a simple design for the temperature control circuit that I
    would like you all to critiqe.

    12vdc_________________
    | |
    100ohm /
    |_________________| npn trans.
    | |
    1K Thermistor \>________________dc controled 120vac relay
    |
    Ground

    I hope this is inteligible. I drew a schematic, but I can't post it
    here. I can post it on the internet if you can't decifer this.


    I would like to know if this circuit is a legitimate idea?
    Can it be made simpler?
    Do you think I will have trouble with the transistor only partially coming
    on, making the relay chatter, oscillate, or what ever the proper term is?



    Thanks for the advice,

    Ned
     
  2. Why not choose the right size of light bulb? It's not that critical.
     
  3. Lord Garth

    Lord Garth Guest

    You should use a solid state relay or an electro-mechanical relay to
    operate your load. The typical NPN circuit will have the emitter grounded
    and the load in its collector circuit along with a inverse parallel diode if you
    use a mechanical relay.

    To prevent chatter, you should build in hysteresis http://en.wikipedia.org/wiki/Hysteresis

    The high and low set points, or turn on / turn off if you prefer, should not be at the same
    point. http://www.maxim-ic.com/appnotes.cfm/an_pk/481 obviously, replace the fan
    with your relay.
     
  4. NJM

    NJM Guest

    I thought about that but I was worried that it would not give me enough
    control. Plus, there would be a bit of trial and error until the right
    bulb was picked, then, it would only work for that particular set up. I
    wanted to be able to vary the conditions so that if I wanted to make beef
    jerky one day, turkey jerky the next, and dry herbs after that.

    Of course, I have never done any of these things myself, so you may be
    right. I may be overcomplicating the process.

    Thanks.
     
  5. Most driers I have seen have a fixed heat setting.

    How about this - buy a slow cooker with a thermostat and use that as a
    source of heat? Or an electric frypan?

    I bought a convection oven which can also do drying although I've never
    bothered.
     
  6. Meat Plow

    Meat Plow Guest

    I had a drier that had one setting for everything. What varied was the
    time you left stuff in to dry. My favorite was deer jerky.
     
  7. Chris

    Chris Guest

    Hi, Ned. You may be biting off a bit more than you can chew with this
    project. ;-)

    Seriously, though, you may be overthinking this a bit. An easy
    solution which would get you there would be to put a triac lamp dimmer
    in series with your bulb (which should be somewhat oversized for your
    enclosure). These are available at any hardware store. Change the
    dial setting, and every 10 minutes go in and read the thermometer in
    the dehydrator and tweak again. In an hour, you'll have zeroed in on
    just the right dimmer setting for your chosen bulb to maintain the
    temp for your dehydrator.

    An even easier way assumes you'll be running your dehydrator in stll
    air which maintains a relatively constant temp (i.e. inside). Start
    out with the oversized bulb, then just judiciously punch a few small
    vent holes at a time until the stabilized temp is what you want. This
    does have the disadvantage that another bulb might mean you have to
    plug some of the holes. You could also place a moveable baffle in
    front of the vent to make adjustments.

    There are a number of ways to achieve very good temp control, but they
    might be a little difficult for an electronics newbie. Sometimes the
    easiest solution, even if it gets you only 90% of what you want, is
    the best one.

    Cheers
    Chris
     
  8. Never had that. I like the pork stuff.
     
  9. default

    default Guest

    I take it this is a thermostat? something you might find in a
    hardware store?

    One fallacy with your design is it uses a thermistor and transistor -
    it probably won't work the way you've drawn it.

    First the transistor's gain and switching point of the circuit will be
    dependant on temperature - takes less current into the base as the
    transistor's ambient temperature is higher (either the ambient or self
    heating from switching the load)

    Secondly, the 100 ohm resistor in series with the 1K thermistor will
    cause some "self-heating" of the thermistor. Generally speaking, you
    want to minimize the thermistor current so it doesn't heat itself.

    It could be made to work, but no designer worth his salt would do it
    that way.

    A very simple thermostat using one IC comparator (like an op amp) and
    a thermistor in a bridge circuit compared to a stable and adjustable
    voltage can control temperature within 1/100 th of a degree - only
    three active parts plus a relay. Lots of them on the web.

    Search: circuit, schematic, thermostat, comparator thermistor

    Then you only have to wade through 20K hits - might want to leave out
    the word patent in the search.

    Or just go into a hardware store and buy a thermostatic switch.
     
  10. Rich Grise

    Rich Grise Guest

    Your ASCIImatic is fine. :) However, (see below)
    This circuit probably won't do what you're looking for. Someone else
    suggested a comparator, with the thermistor in a bridge, and the
    comparator either drives the relay directly, or a switching transistor.

    It would also be a good idea to introduce some hysteresis, so you get
    "snap-action" - you do this by feeding back a tiny portion of the output
    back to the non-inverting input, probably a resistor of some megohms.
    This keeps the comparator/relay from "chattering" right at the threshold
    point.

    It won't be any simpler, but it could be built on a fairly small (say,
    1" x 2") piece of perfboard.

    Hope This Helps!
    Rich
     
  11. jasen

    jasen Guest

    Yes, just buy a thermostat intended for a hot water service, mount it 1/3 the
    way up, wire it in series with to the lamp and be done with it!

    Bye.
    Jasen
     
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