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Old 10-09-2020, 06:40   #1
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Isotherm Frig Thermostat Replacement w/Digital

When replacing the stock mechanical thermostat in an Isotherm unit with a BD35 compressor with a digital unit with a dry contact, what do you do with the "P" terminal on the control unit?
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Old 10-09-2020, 07:10   #2
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Re: Isotherm Frig Thermostat Replacement w/Digital

If you look at the info from Kollmann he says that you leave it unconnected. However mine had a jumper from C to P so I just left it. I am running a digital temp controller and I haven't had any issues with it. I'm using the same type controller on a DB35 and another on a DB50. The DB50 has the AB stainless box on it with the phone connector, so I just put a phone connector on my temp controller lead.
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Old 10-09-2020, 07:24   #3
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Re: Isotherm Frig Thermostat Replacement w/Digital

I finally found out that the "P" connection is for an optional low voltage cut out/in and these vary depending on the resistance between "C" and "P". Mine currently has a jumper, 0 0 resistance. I haven't found out what happens if you remove and it becomes an open circuit between the 2.

I will look at Kollman's page.
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Old 10-09-2020, 07:48   #4
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Re: Isotherm Frig Thermostat Replacement w/Digital

Use C and T...this isn't hard guys, its just a switch.
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Old 10-09-2020, 09:18   #5
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Re: Isotherm Frig Thermostat Replacement w/Digital

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Originally Posted by Coolerking View Post
Use C and T...this isn't hard guys, its just a switch.
the question isn't even about C and T
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Old 11-09-2020, 13:27   #6
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Re: Isotherm Frig Thermostat Replacement w/Digital

No wire on P terminal is generally the recommended low voltage battery protection in boats with 12 refrigeration. No resistor between P and C will cut off compressor 9,6 volts and restart will require 10.9 volts. If there are concerns about low battery voltage on morning engine starts after refrigerator ran all night, you may want to add a 24 ohm resistor between terminals C and P preventing compressor from running at battery voltages below 10.9 volts.
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Old 11-09-2020, 14:17   #7
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Re: Isotherm Frig Thermostat Replacement w/Digital

My systems (I have 2) have nothing on the P terminal.

I understand the shutdown feature at low voltage but to be fair, I get somewhat panicky if my voltage gets below 11.8v so to go down as far as 9.6v is, to me, having an effectively flat (read dead) battery. I never allow my system to cycle down that deep. Is this common?

I just installed a new battery monitor and have set a low voltage alarm at 12v. If it ever goes off in the middle of the night, I’ll start switching stuff off.
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Old 11-09-2020, 14:44   #8
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Re: Isotherm Frig Thermostat Replacement w/Digital

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Originally Posted by Richard Kollmann View Post
No wire on P terminal is generally the recommended low voltage battery protection in boats with 12 refrigeration. No resistor between P and C will cut off compressor 9,6 volts and restart will require 10.9 volts. If there are concerns about low battery voltage on morning engine starts after refrigerator ran all night, you may want to add a 24 ohm resistor between terminals C and P preventing compressor from running at battery voltages below 10.9 volts.
Ok thanks. That is the same as what the Danfoss manual says for 0 or 220 K-ohm resistor. I don't think there is a resistor in the line now, but I think I will put one some time as I replaced battery this year because I was gone a week and the solar had a problem and killed a battery.
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Old 12-09-2020, 12:37   #9
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Re: Isotherm Frig Thermostat Replacement w/Digital

Hello All,
Is there a source of information on how to install a digital thermostat for those of us who are less sophisticated in mods to fridge systems. I have a new CU-100, just installed and would like to add a digital T'stat but need step by step guidance. I also suspect that there are others on the forum wishing the same. Thanks


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Old 12-09-2020, 16:15   #10
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Re: Isotherm Frig Thermostat Replacement w/Digital

You just:
1- buy a unit
2 - provide power to it (I tapped of the power wire to the frig unit using a 1 amp fuse in line)
3- run a set of wires across the dry contact on the controller to the terminal board f the danfoss unit replacing the thermostat wires going to the C and P connections.

It is easy. If you are will are willing to try i would say you have the ability
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Old 12-09-2020, 17:21   #11
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Re: Isotherm Frig Thermostat Replacement w/Digital

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Originally Posted by jpstevens View Post
Hello All,
Is there a source of information on how to install a digital thermostat for those of us who are less sophisticated in mods to fridge systems. I have a new CU-100, just installed and would like to add a digital T'stat but need step by step guidance. I also suspect that there are others on the forum wishing the same. Thanks


Jim
Your CU100 has a 4 pin thermostat plug two of these wires are for thermostat switching compressor ON and OFF to control compressor, also in one of these wires is a resistor designed to preset compressor speed to match the K value of evaporator to compressor output. If you want the same performance after changing to another thermostat you need to add an equal value resistor to new thermostat's wiring or compressor will always run at slow speed. The other two wires in plug are the power wires for an electronic thermostat. See CU100 manual for correct color coded wires in 4 pin plug.
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Old 15-09-2020, 09:56   #12
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Re: Isotherm Frig Thermostat Replacement w/Digital

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Originally Posted by Richard Kollmann View Post
Your CU100 has a 4 pin thermostat plug two of these wires are for thermostat switching compressor ON and OFF to control compressor, also in one of these wires is a resistor designed to preset compressor speed to match the K value of evaporator to compressor output. If you want the same performance after changing to another thermostat you need to add an equal value resistor to new thermostat's wiring or compressor will always run at slow speed. The other two wires in plug are the power wires for an electronic thermostat. See CU100 manual for correct color coded wires in 4 pin plug.

Richard, will the digital controls for my old non-functional Glacier Bay system work in basically the same way so that I could use them with any compressor? I have been told otherwise. If not, they are for sale.
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