DIN ROFIE wrote:
> I try to design a hot pouch that can heat up food to
> my specified temperature.

Now we're finally getting somewhere.

Where is the power coming from?  A regular wall outlet?  Have you decided
what kind of heater you will use?  If so, what are it's specs?  How much
power do you think it will take to maintain temperature?

Something that comes to mind is using a incandescent light bulb or two in a
thermally insulated box.  There was a girl's toy called a Easy Bake Oven
that used only a 40W lightbulb as the heating element if I remember
correctly.  It's surprising how hot 40W can make something if it's insulate=
d
well enough.

Incandescent bulbs last a lot longer if they are run even a little below
their design voltage.  This can be easily achieved by putting two higher
wattage bulbs in series, like two 100W bulbs.  They will probably only glow
orange, but will last a long time and still put out plenty of heat.

The control circuit can be as simple as switching a relay driving the bulbs
on or off every two seconds or so.  The temperature sensor could be just a
thermistor and a fixed resistor into a PIC A/D.  Every two seconds the PIC
decides whether the temperature is above or below the set point and turns
the relay on or off.  You may want a small fan in the box to keep the
temperature even and move the heat from the bulbs around.

A long time ago I did a darkroom water bath temperature contrller on a
similar principle.  I had a pot and a thermistor bridge into a comparator.
The pot adjusted the setpoint and the comparator output indicated whether
the temperature was above or below the setpoint.  A 6 bit counter divided
the 60Hz power line frequency to produce a clock about once every second.
This clocked a flip-flop that latched the comparator output for the next
second, which drove a relay that switched a immersion heater on or off.  I
was amazed at how steady such a simple control system kept the temperature.
Once it hit regulation, I couldn't see the temperature move on my
photographic thermometer.

I suggest switching the lightbulbs more slowly because it's the transitions
that will ultimately kill them.  Still, with two in series they will likely
last for the life of the unit.

> I am not attending any of lecture and class.I am just a waiter who
> interested to PIC

What is the level of your general electronics knowledge?


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