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Low Temperature Thermostat for Marine Heat Pump
Unlike conditions in the northern states, boats in the mid-Atlantic do not
have to be hauled from the water for the winter - the waters here typically do
not freeze. Temperatures, however, do drop occasionally below 0°F, causing us to
have to either winterize the boats, or provide a source of heat in order to
prevent freezing. A common source of heat is a small (1000w/1500w)
electric heater. These always have the danger of starting a fire from
tipping over, or due to some flammable material being to near and catching
fire. 2003-2004 wass the first winter that
has been in the water that I have not lived aboard her - so I
was faced
with how to let her handle the winter. I wanted to use the 16k BTU heat
pump, but the thermostat for the unit will not allow you to set the temperature
below 60°F. Experimenting with the heat pump, I found that if it is on
when power is removed from the unit, it will be on when power is
reapplied. Taking advantage of this, I created a circuit based on a Dallas
Semiconductor DS1620 Temperature Sensor/Thermostat.
Operation
The DS1620 can act in one of several modes. With a microprocessor
attached it is capable of giving current temperature with a resolution of
0.5°C. A second mode is a stand alone function. In this mode, a low
set point and a high set point are programmed into onboard EEROM. When the
temperature falls below the low set point, a low threshold line becomes
active and stays active until the temperature exceeds the low set point. Similarly, when the temperature is above the high set point, the high threshold
line becomes active and stays active until the temperature falls below the high
set point. A common output line combines the low and high threshold
lines. This line goes high when the temperature exceeds the high set
point, and remains high until the temperature drops below the low set
point. In my case I programmed the low set point for 37°F and the high
set point for 40°F. The common output of the DS1620 goes to an inverter
which provides a signal which goes high when the temperature drops below the
lower set point and then goes low when the temperature exceeds the high set
point. The output of the inverter drives a IRLD014 HexFet which in
turn drives a 30A solid state relay to turn and off the heat pump.
The circuit was bread boarded and mounted in the engine room, except the
DS1620 which is on a multi-conductor cable and mounted in the main cabin.
There is an LED to indicate when power is being supplied to the solid state
relay. A toggle switch, also mounted on the box, provides constant power
to the solid state relay, allowing for normal operation of the heat pump.
Programming
I was in a bit of a hurry when I built this project and did not want to take
the time to figure out how to program the DS1620, or to have to write the
software to program the DS1620. After a little searching on the Internet,
I found an article not only explaining how to program the DS1620, but also
provided the circuit needed and the complied software. I have included all
parts in the link to the left. The "programmer" is just a couple
of capacitors, resistors and the parallel port of a Win98/95/3.11 or DOS
computer. There are no instructions for the software, but it was not
difficult to figure out.
Results
While the boat came out of the water in February 2004 for
bottom work, the system did not run all winter. But while the boat was in
the water, using the controller, the
temperature (according to a recording thermometer) had not dropped below 37.5°F.
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