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Low Temperature Thermostat
Digital Readout Windlass Zero Shift Focuser Low Temperature Thermostat

 

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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 0F, 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 60F.  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 0.5C.  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 37F and the high set point for 40F.  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 37.5F.


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This page was last modified: 01/22/14
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