Temperature control?

[Callsign_Vega]

I would try a 25 turn 50+ohm trim pot, as this would give you a good range of temperatures while still giving you a fine adjustment at the values your wanting.
Something like this: http://jaycar.co.nz/productView.asp?...T&SUBCATID=803

Using the rule for resistors in parallel;

1 / R = 1 / R1 + 1 / R2 + ......

where R = the resistance we want/the chiller setting (ie 8 Kohm @ 20c) and R1 = the resistance of the probe at running temp (ie 15 Kohm @ -17c) and R2 = the resistance of our trim pot, we can now find the minimum resistor size needed.

1 / 8 = 1 / 15 + 1 / R2
R2 = 1 / (1 / 8 - 1 / 15)
= 17.14 Kohm

if you want flexibility you want a larger resistor as the more resistance you add the closer you will get to the original value but the more sensitive the adjustments become as a much larger range has to be moved through in the same number of turns of the trimmer (ie 25 turns 0-XX ohms).

Some examples of probe resistances when the trimmer is set to max and the controllers trying to hold 20c (8 Kohm);

R1 = 1 / (1 / 8 - 1 / R2)

25 Kohm: R1 = 11.76 Kohm
50 Kohm: R1 = 9.52 Kohm
100 Kohm: R1 = 8.7 Kohm

so you can see that if you had a 100 Kohm trim pot set to max with the controller set to 20c your temp would be about 15c, where as with the same setting and a 50 Kohm trim pot the max temp you could adjust to is about 9c, and with the 25 Kohm about -4c.

Something some people like to do (I don't bother TBH) is use a resistor in series with their pot (which are both in parallel with the probe) to act as an adjustment safety eg put a 15 Kohm resistor in series with a 50 Kohm trim pot that way your resistance range becomes 15-65Kohm (17-9Kohm on probe with cont @ 20c) giving a temp range of about -24c - +12c while preventing anyone accordantly adjusting the pot to 0 ohm shorting the controller.

Replacing the thermistor is a viable option however you are limited to the range of temperatures settable by the controller also that range might be affected but the resistance/temperature gradient of the thermistors response.

Very helpful! I am off to Radio Shack.

You can easily be a "start delay timer" and add it to the main power loop though. That will prevent cycling by putting in a hard timer between off's to the next on.
In connection with a PID/thermocouple and an SSR or Contactor, you can have it all done for about $60.

Do you mean the main power from the outlet or the power coming from the capacitor to the compressor? I am not sure how that would work with a digital controller like my unit has. If I unplug it from the wall and plug it back in it stays off.

[n00b 0f l337]

Well then, time for some very basic rewiring.
Remove the ENTIRE circuitry of the current system. Unplug the run capacitor as well.
Rewire the compressor to run on a simple switch, and follow the standard diagram for rotary compressors for the run capacitor.
Then you can easily add the PID, contactor, and start delay timer.
The PID would be added post switch, the contactor and start delay in series to the power to the compressor.

[Callsign_Vega]

Well then, time for some very basic rewiring.
Remove the ENTIRE circuitry of the current system. Unplug the run capacitor as well.
Rewire the compressor to run on a simple switch, and follow the standard diagram for rotary compressors for the run capacitor.
Then you can easily add the PID, contactor, and start delay timer.
The PID would be added post switch, the contactor and start delay in series to the power to the compressor.

That seems like a lot of work. I will try the slight modification to the current controller as its all nice and tidy and has a remote.

[n00b 0f l337]

However, my method would give you redundancy and be built up and ready for dealing with the high loads.
I don't trust commercial crap

[Callsign_Vega]

I'ts working great so far, thanks for the help guys. In parallel I setup a 10k ohm permanent resistor and a 50k ohm potentiometer. I can now control the coolant temps with the stock unit controller.

[Red M]

Glad to hear its working out for you