Air Conditioner Controller Mod

[zm50cc]

Hey everybody, I have a temperature controller in my place that I'd like to modifiy. It is a basic LightStat controller (it does not have the light sensing feature). The temperatures always seem to run higher than they actually are. I am able to deal with this but my roommate can't seem to do the translation. I was hoping there was just a simple coil to determine the temperature (which I was planning to bend). But, upon opening the case, I noticed they must have used reversed biased zerner diodes to find the temperature (as their reverse biased current varias presicely with the temperature). I believe this company miscorrectly calculates the temperatures within the controller or does not have have the temperature sensor placed appropriately within the housing.

Any ideas on how to modify this to be able to have the same effect as 'bending a coil'? Thanks alot for any insight.

The board:
**broken link removed**
A close up of the possible air sensor or controller:
**broken link removed**​

 

[Roff]

RT1 is your sensor. It's a thermistor in a DO7 package.

 

[zm50cc]

are the coils on the RT1 significant and is there any way it may be manipulated through bending or being replaced by another component or and identical compenent of a different magnitude?

 

[kchriste]

Mark the current position of VR2 with a felt pen. Then try adjusting it a little bit (No more than a 1/4 turn as we are not sure what it does) and see if that effects the temperature displayed. If not, turn it back to it's original position.

 

[Roff]

are the coils on the RT1 significant and is there any way it may be manipulated through bending or being replaced by another component or and identical compenent of a different magnitude?

I believe the coils are an attempt to thermally isolate the thermistor from the circuit board. I don't think bending will help.
Did you see the **broken link removed** on LightStat's web site? I didn't read them, so I don't know if they will help.

 

[stevez]

Without knowing what the controller actually does it's tough to say what would solve your problem. Some things to consider:

1. Does location of RT1 actually represent the temperature that is of concern? Room temperatures can vary widely as you move around the room, from floor to ceiling.

2. If the control turns something on or off then the temperature of the space will vary based on changing loads and the on/off of the air conditioning equipment. If the set point is what you actually want the room to be at - and it turns the air conditioner off at that point then on average, the room will never actually be at the setpoint for very long.

I'd consider some external type of control signal modifier before messing with the insides of something you don't understand. What isn't obvious - is this 'control' a thermostat that goes to a remote piece of equipment so that the outputs of the thermostat are contact closures? If this board actually contains other conditioning system controls, besides the temperature part, I'd avoid tinkering with it entirely. It's possible that VR2 allows for some adjustment and one would expect the factory manual to indicate that.

 

[zm50cc]

I appriciate all of your input. I think you guys have brought up some interesting points. This controller is for an air conditioning unit. It is wired to the HVAC unit and seems to also interface with the compressor outdoors.

I too would like to be able to condition an external signal to solve this problem, but I believe the thermostat is built-in to the device itself so it could not be conditioned from the outside directly. If any of you have any other ideas for interfacing a device to provide some sort of beneficial external control of this unit that would be great.

I have checked out the FAQ and installers/operaters manuals. They are very brief to say the least.

Although I have not studied this board in great detail, it seems somewhat obvious to me how it must work - it reminds me a lot of microprocessor class. But indeed, I do not know the internal wirings and workings of this board and would prefer not to mess with it.

Maybe the best solution is to find another thermostat - but it just seemed somewhat interesting to mess with this one. But, here in the south it would stink to mess it up and be without air for a few days!

The company who designed this has some great energy saving ideas - including limits on what you can set the temperature at (this one is set up for a range of only 10 degrees). Unfortunatly they are not so suitable for this particular installation.

I did try to see if I could adjust the position of VR2 - it seems very much stuck where it is. I did not want to try to pursuade it to move too much and was afriad there may be other unwanted effects to adjusting that. (You should check out Iskra's website - interesting devices.)

If I could have modified this board before it was built, I would have considered putting in a small pot so that the current through RT1 could be modified. Although, this may require a few more devices since the current through the RT1 may be very small (my guess is its a leakage current). It may not be worth the extra expense to the company, but since some of the diodes have tolerances in manufacturing that might make it less accurate, it would be awesome from the consumer side to have a 'qualified technician' be adjust it through a pot. I suspect it may need an a very small amp (op amp or FET) to raise the current to a point where it could be adjusted practically through a pot.

Also, to address the temperature variance within rooms. I have tested the temperature at the thermastat with a meter I have that is accurate to +/- .1 degree and also my Digital MultiMeter. The temperature meter and DMM both agree - and the thermostat is off by quite a bit from the other 2.

Sorry for this very long post - and I appriciate all your input!

 

[Roff]

How long have you had this thermostat? If the warranty is still in effect, I would either have them replace it or refund my money.

 

[zm50cc]

yeah, a refund may be in order. thanks again for all the input.

 

[Hero999]

I hope you havent invalidated the warranty by opening it up.

 

[zm50cc]

nah, there were no seals and absolutely no evidence of it being opened up. The housing litterally just poped on and off.

 

[philba]

maybe this is too low tech but how about getting one of those stick-on thermometers and slap it over the temperature display maybe then your room mate will stop freezing you out..

 

[zm50cc]

haha, great idea!

 

[1Steveo]

If RT1 is a thermistor, Than you can add a series resistor . I think that thermistors values go down when temps. go up.

 

[Nigel Goodwin]

If RT1 is a thermistor, Than you can add a series resistor . I think that thermistors values go down when temps. go up.

It depends entirely on the type of thermistor, you can get both negative and positive temperature coefficient ones.