Resistor question

[meadowstonefarm]

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?

 

[Skogsgurra]

Absolutely. If it is an NTC thermistor with a monotone ohms vs temperature curve, then it will work.

Other sensors will probably need another approach, but it seems to be an NTC that you have.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[coconutalley]

Depends on the way the controller reads and interprets the temperature. Some have compensation to correct the non-linearity. Thermistor resistance are non-linear over temperature range and that is probably why the range is limited. I think it'd work but there might be small errors.

 

[meadowstonefarm]

This probably doesn't help, but the unit is a Toyo (japanese) DC inverter driven heat pump. I haven't taken it apart yet to see exactly how it works. I received it yesterday to find out the range is too limited. I looked at the wiring diagram last night and it seems to have a board with most functions integrated.

 

[itsmoked]

I would guess that you are either going to see a horrible waste of power or you will destroy the heatpump if it was designed for 15C higher temps.

Are you trying to pull heat out of 5C air? BRUHAHAHAHAHA!!

(excuse me)

or are you trying to create 5C cooling with it?


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Yes, what are you trying to do exactly? I was only answering the thermistor part of the question - not the thermodynamic aspects.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[IRstuff]

Note also, that it's very likely that trying to fool the controller might cause instability in the control loop.

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

 

[Skogsgurra]

Hey Keith!

"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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Hi Gunnar!

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

 

[ScottyUK]

Suggest a cross-post in forum403 might be appropriate. If you do start a parallel thread then leave a note here.


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Sometimes I only open my mouth to swap feet...

 

[meadowstonefarm]

Thank you for bringing up these points. The application is a cheese aging room that needs to run (normally) at 55F, but may need to run short spurts above or below that. I am not worried about the design of the system being capable of it, as the air temp off the coil is 40F in cooling, I just want it to run the fan longer to bring the room temp closer to the coil temp.

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.

 

[meadowstonefarm]

http://www.eng-tips.com/viewthread.cfm?qid=179402&page=1

thread continued here.