Dixell Controls Probe Temperature Readings

Hi,

I am trying to understand how Dixell controls determine the temperature readings through an NTC/PTC probe. Please see the attached image for reference.

Attachment 10319

Based on my research and understanding, the way I believe it would work is that a voltage is passed through terminal 11, through an NTC/PTC probe, and based on the voltage drop on terminal 10 or 12 the resistance value is calculated and a temperature is determined from that. Is my understanding correct? If not can you give me a better description as I am just making an assumption.

The reason I am asking is that if a probe is connected to terminal 9 and 10, the evaporator gets a temperature reading, even though it is wrong, but I thought terminal 9 was a voltage free contact and therefore terminal 10 should get no reading?

If you anyone can give me any clarification I would really appreciate it.

Thanks in advance,

Terminal 9 must be a high impedance digital input. Terminal 9 must be able to detect the presence of voltage from terminal 11 (or not) by using extremely small consumed power, so small so as not to affect the temperature input.

You need to read the manual when you will see that the digit input is nothing to do with altering the temperatures, it is an alarm circuit only.

http://www.dixell.com/DETTA04Y/eng/

Select the XR06CX English download.

Terminal 9 has no connection between 10,11 and 12.terminal 11 is common analog line .terminal 9 is field option

Hi Guys, thanks to all of you for responses so far, but it seems I wasn't clear enough in my first post, sorry.

I understand how to properly wire the evaporator(between 10 and 11) and room probe(11 and 12). I also understand that terminal 9 has nothing to do with the temperature readings. My problem is if the evaporator probe is wired incorrectly to terminal 9 and 10, I expected there to be an error saying that evaporator probe is not working. However there is no error, there is a reading, but it's wrong. That is where I want to understand why.

From my understanding a thermistor needs a voltage passed through in order to measure impedance to get temperature. That is where I make an assumption that terminal 11, since it is shared is a terminal, sends a small amount of voltage through the thermistors, then terminal 12 and 10 measures the voltage drop in order to measure impedance. This is all an assumption as mentioned in the first post and I was looking for clarification.

Then I move onto the digital input, I read in the manual it was a voltage free contact. This leads me to believe there is no voltage passed through the thermistor and terminal 10 should get no reading and there should be an error, but that is not the case and I want to understand what is going on.

HVACRsaurus: If terminal 9 is a high impedance input, doesn't that mean that it doesn't send a voltage, therefore terminal 10 should receive no voltage and there should be an evaporator probe error?

Brian_UK: I did read the manual, that is where I got the wiring diagram from and the information about the digital input being a voltage free contact which is confusing me. But that's for linking the manual as it may help others answer my question.

moideen: I'm not too familiar with circuits so not sure if this answers my question sorry, I will try to look into it now.

Thanks to everyone for responses so far.

Hi JWray,

Is it just an incorrect temperature reading?

One way to test this is to immerse the temperature probe into an ice / water slurry, once stabilised the slurry will have a temperature of 0°C & the controller should indicate accordingly. Whilst I'm not intimately familiar with the Dixel, most controllers have an offset parameter for this purpose. If the sensor is out of calibration by more than a couple of degrees I'd probably replace it. Probably a good idea to even test the new one with ice / water slurry.

My experience with this kind of problem is that the probe usually has elevated resistance - caused by ingress of moisture & consequent corrosion. Sometimes the probe itself may be ok but I've found bad terminations on the interconnecting cable.

Good luck.