Resistor question
Resistor question
(OP)
Hello,
I'm an M.E., so electricity is greek to me....
I'm jsut looking for confirmationt hat my approach to this little problem is correct. I have a heat pump that uses a thermistor to monitor temperature. The problem is thatt he unit only operates in the range of 18-28 degrees C. I need to run down to 5 degrees C.
Basically I need to fool the control into thinking the resistance value it knows as 18C is actually 5C.
My idea was to take a few temperature/resistance measurements with a multimeter and calculate the Steinhardt-Hart equation. Knowing the temp vs. resistance curve I can offset it using a simple resistor, thereby fooling the control into thinking the resistance value it's reading is 15C higher than it really is.
Will this work?
I'm an M.E., so electricity is greek to me....
I'm jsut looking for confirmationt hat my approach to this little problem is correct. I have a heat pump that uses a thermistor to monitor temperature. The problem is thatt he unit only operates in the range of 18-28 degrees C. I need to run down to 5 degrees C.
Basically I need to fool the control into thinking the resistance value it knows as 18C is actually 5C.
My idea was to take a few temperature/resistance measurements with a multimeter and calculate the Steinhardt-Hart equation. Knowing the temp vs. resistance curve I can offset it using a simple resistor, thereby fooling the control into thinking the resistance value it's reading is 15C higher than it really is.
Will this work?





RE: Resistor question
Other sensors will probably need another approach, but it seems to be an NTC that you have.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistor question
RE: Resistor question
RE: Resistor question
Are you trying to pull heat out of 5C air? BRUHAHAHAHAHA!!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistor question
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistor question
This would be particularly the case with an NTC resistor, since you'd have to put something in parallel to bring down the resistance, which will change the apparent temperature coefficient.
TTFN
Eng-Tips Policies FAQ731-376
RE: Resistor question
"Are you trying to pull heat out of 5C air?"
We are pulling enough heat out of -10 C air to heat our house. It is not the temperature as such - it is the difference between air in and air out of the pump that produces heat. I think that our -10 C in leaves the pump at about -13 or -14 C.
Works quite well. And we have lowered heating costs from around 10 kUSD/year to 4 kUSD/year after we installed the heat pumps.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Resistor question
It can be done. But it is often a joke in that the only heat that ends up getting delivered is the waste heat of running the motor which gets carried (mostly) to the delivery side. This leads users to think, "Wow, look at all the heat we're getting from the cold outside."
Systems that are reversible are often the worst. They are necessarily designed to be refrigerators for the cooling mode. A cold source heat pump on the other hand really needs to be more of a freezer due to the required delta. So if someone is going to jury rig one to run in a non designed range they need to do it cautiously.
Glad yours are being effective for you. Do pay attention though as at some point of external cold you may think they are doing the job but they may not be. At some point an electric heater is better.
Are yours reversible or just heaters?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Resistor question
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Sometimes I only open my mouth to swap feet...
RE: Resistor question
The unit I bought was delivered yesterday (crushed), looks like Fedex drop kicked it off a roof. The unit is repackaged and will go back to the manufacturer today. Once I get the replacement I can better define the temperature control I am trying to modify for you guys.
RE: Resistor question
thread continued here.