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Gerald Steele

Mar 31, 2015

Precision Designs

OPA2541

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Related

Make a freezer alarm using a power op amp with zero power
consumption

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Warning: This construction article uses dihydrogen monoxide. Make sure you
have researched and understood the dangers associated with this substance before
proceeding.

When your home’s deep freezer, full of food, experiences an AC power failure
while you are out for an extended time, its contents can thaw out. If the AC
power is then restored before you return home, the contents can re-freeze and
you may never know that your food is spoiled. This has given rise over the years
to a number of freezer-alarm circuits and methods to detect thaw and
re-freeze.

In this post, I’ll show you how to build a freezer alarm using a power
operational amplifier (op amp). As an added benefit, it has zero power
consumption – so you’ll have no worries about losing power or dead batteries.
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
currents up to 10A, or that work on voltages up to 100V, with a wide range of
features. For this application, what we need in particular is the metal case
TO-3 hermetic OPA2541.

The first crucial step is selecting the housing. For this example, a cleaned,
empty can of pet food made an ideal housing.

1. Fill the housing approximately two-thirds full with di-hydrogen monoxide
(Figure 2).

Figure 2: To construct the circuit board, fill the housing
approximately two-thirds full of dihydrogen monoxide

2. Place the housing in the freezer until the dihydrogen monoxide has fully
solidified.

3. Set the power op amp on top of the solidified dihydrogen monoxide (Figure
3).

Figure 3: Simply set the power op amp on top of the
hardened dihydrogen monoxide to make the freezer alarm

4. Place the alarm in the freezer.

Set the alarm in a convenient place that’s visible every time you open the
freezer. As long as you can see the power op amp, the food is safe to eat
(Figure 4). If the power op amp is not on top and has become frozen into
the dihydrogen monoxide (Figure 5), you have had a power loss and re-freeze and
should discard the food in your freezer.

Figure 4: Typical placement of freezer alarm in freezer,
and what you will see as long as everything is OK

Figure 5: If you see this, your freezer has lost and
regained power, thawing and then re-freezing its contents; you should discard
the food inside. Even when you see that amplifier partly submerged, it would be
a good practice to consider discarding the food.

Power can be lost for a host of reasons ranging from technical problems to
natural disasters. Here we have shown the "power" of TI op amps. When you are
truly looking for a high power linear solution, check out the line of TI power
op amps.

Does this solve your freezer problem? Post your feedback in the comments
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
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
library.

Search TI’s entire op amp portfolio, most of which are much newer than the OPA2541. To limit your
search to power op amps, use the set the output current column of the parametric
table to a minimum current of >200mA.

Ian Williams

Excellent circuit, works 100% as advertised. I wonder if we can reduce
current consumption even more with a smaller package?

I also heard that
large quantities of dihydrogen monoxide can be toxic - any handling tips or
recommended PPE?

Apr 1, 2015

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John Caldwell

Hi Jerry,

Did you consider the local cooling effects on the alarm by placing it in
the vicinity of a large quantity of (delicious) ice cream? In the event of a
loss of freezer power, foods with less mass may thaw before your freezer alarm
is triggered due to these local cooling effects. If the freezer regains power
you may never know that half of your food is bad!

The solution to this problem is clear. A large number of freezer alarms
(all using OPA2541s in hermetically sealed packages of course) must be
dispersed around the freezer to guarantee total safety.

Apr 1, 2015

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Ken Dillinger

This circuit only works if the op-amp has infinite bandwidth.

Priced at $0.00.

-Ken

Apr 3, 2015

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