Warning: This construction article uses dihydrogen monoxide. Make =
sure you=20
have researched and understood the dangers associated with this =
substance before=20
proceeding.
When your home=E2=80=99s deep freezer, full of food, experiences an =
AC power failure=20
while you are out for an extended time, its contents can thaw out. If =
the AC=20
power is then restored before you return home, the contents can =
re-freeze and=20
you may never know that your food is spoiled. This has given rise over =
the years=20
to a number of freezer-alarm circuits and methods to detect thaw and=20
re-freeze.
In this post, I=E2=80=99ll show you how to build a freezer alarm =
using a power=20
operational amplifier (op amp). As an added benefit, it has zero power=20
consumption =E2=80=93 so you=E2=80=99ll have no worries about losing =
power or dead batteries.=20
Figure 1 depicts the complete schematic of the freezer alarm.
Figure 1: Complete schematic of the freezer =
alarm; the OPA2541 actually has two of these circuits
TI has an excellent line of linear power op amps capable of =
delivering=20
currents up to 10A, or that work on voltages up to 100V, with a wide =
range of=20
features. For this application, what we need in particular is the metal =
case=20
TO-3 hermetic OPA2541.
The first crucial step is selecting the housing. For this example, a =
cleaned,=20
empty can of pet food made an ideal housing.
1. Fill the housing approximately two-thirds full with di-hydrogen =
monoxide=20
(Figure 2).
![3D"]()
Figure 2: To construct the circuit board, fill =
the housing=20
approximately two-thirds full of dihydrogen monoxide
2. Place the housing in the freezer until the dihydrogen monoxide has =
fully=20
solidified.
3. Set the power op amp on top of the solidified dihydrogen monoxide =
(Figure=20
3).
![3D"]()
Figure 3: Simply set the power op amp on top of =
the=20
hardened dihydrogen monoxide to make the freezer alarm
4. Place the alarm in the freezer.
Set the alarm in a convenient place that=E2=80=99s visible every time =
you open the=20
freezer. As long as you can see the power op amp, the food is safe to =
eat=20
(Figure 4). If the power op amp is not on top and has become frozen =
into=20
the dihydrogen monoxide (Figure 5), you have had a power loss and =
re-freeze and=20
should discard the food in your freezer.
![3D"]()
Figure 4: Typical placement of freezer alarm =
in freezer,=20
and what you will see as long as everything is OK
=
Figure 5: If you see this, your freezer has lost =
and=20
regained power, thawing and then re-freezing its contents; you should =
discard=20
the food inside. Even when you see that amplifier partly submerged, it =
would be=20
a good practice to consider discarding the food.
Power can be lost for a host of reasons ranging from technical =
problems to=20
natural disasters. Here we have shown the "power" of TI op amps. When =
you are=20
truly looking for a high power linear solution, check out the line of TI =
power=20
op amps.
Does this solve your freezer problem? Post your feedback in the =
comments=20
below.
Happy April Fools' Day!
For more from Jerry and his colleagues:
Learn how to design with precision op amps and data converters twice =
a week=20
on the Precision Hub blog.
Read about analog signal chain tips and technologies on Analog Wire.
See op amps in action in reference designs in the =
TI Designs=20
library.
Search TI=E2=80=99s entire op amp portfolio, most of which are much newer =
than the OPA2541. To =
limit your=20
search to power op amps, use the set the output current column of the =
parametric=20
table to a minimum current of >200mA.