Cheap and Easy Thermostat Mod

I want to build a controller that can be used for Ales, Lagers and to cold crash. To do this on a tight budget I looked into modifying a standard home thermostat to work at lower temps.

What I came up with was a VERY easy and cheap mod. All you have to do is solder a resister across the wires for the thermistor. This will put the new resistor in parallel with the thermistor. This will in essence fool the thermostat into thinking it is getting a higher temp reading from the thermistor than the actual temp.

Note: A digital thermostat will show this bad info and another temp source will be needed to check the real temp inside the chamber. I plan to print out the temp conversion list below and tape it onto my cheap thermostat. I will also hook up an inside temp source to verify what is going on inside. Maybe stick a meat thermometer through the wall or something.

Now for the How-To part:
First thing you need to do is to find out which thermistor you have. Unsolder one side of your thermistor and get a couple of reading with an ohm meter. There is a big difference between the 5K, 10K & 15K thermistors, but you need to get at least two readings in order to be sure it is a NTC type thermistor. Then refer to the list below for your thermistor.

I have crunched the numbers for 3 possible thermistors used in thermostats.
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If you have what is called a 5k Thermistor
Temp = Resistance
70* = ~5.9K
60* = ~7.6K
32* = ~16.3K
Adding 25.3K resistance in parallel changes it this way:
Thermostat setting = real temp
80* = ~72*
75* = ~65*
70* = ~60*
65* = ~50*
60* = ~47*
55* = ~40*
50* = ~32*
-------------------------------------------------------------
If you have what is called a 10k Thermistor
Temp = Resistance
70* = ~11.5K
60* = ~15K
32* = ~32.6K
Adding 50.3K resistance in parallel changes it this way:
Thermostat Setting = real temp
80* = ~71*
75* = ~64*
70* = ~58*
65* = ~50*
60* = ~45*
55* = ~39*
50* = ~32*
-------------------------------------------------------------
If you have what is called a 15k Thermistor
Temp = Resistance
70* = ~17.9K
60* = ~22.5K
32* = ~49.1K
Adding 69K resistance in parallel changes it this way:
Thermostat Setting = real temp
80* = ~69*
75* = ~64*
70* = ~57*
65* = ~50*
60* = ~45*
55* = ~37*
50* = ~28*
-------------------------------------------------------------
Note: They only make a select number of resistors sizes.
For 25.3k get 22k and 3.3K and wire in series then parallel with Thermistor
For 50.3K get 47K and 3.3K and wire in series then parallel with Thermistor
For 69K get 47K and 22K and wire in series then parallel with Thermistor
Like this : Lead1---R1---R2---lead2
(By making a longer series of resistors I could get closer to the ideal temp range of 32-70*s but it gets expensive, bulky and ugly looking. And who wants that?)

I hope that is not too confusing. Basically you convert a 50 to 80* thermostat into a ~32 to ~70* one for $2 and labor.
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Added note: If your readings go down between 70* and 60* or 32* then you have a PTC type thermistor. For this just add a resister inline with the thermistor. I would suggest adding a resister close to the difference between 60* and 70*. In theory this will bump the 80* setting down to ~70* real temp and the 50* setting down to ~40* real temp. Giving you a ~40* to ~70* thermostat. Testing will give you exact figures.

 

That makes good sense to me. Good job providing all the resister and temp translation info!

I had thought of using a home thermostat but went the route of the ebay aquarium temp controller instead. It's about $25 shipped (I couldn't find a cheaper home thermostat at Lowes). Add a two outlet power receptacle and you have a cheap johnson controller knockoff that will control one heat and one cooling device.

http://www.homebrewtalk.com/f51/ebay-aquarium-temp-controller-build-163849/index44.html

Txinga

 

Thanks. I posted this Cheap Mod mainly for those who are already using a home thermostat.

Lowes has battery operated heat only ones for $10 and a NO/NC relay from Radio Shack is $4. Hook it to the relay's NC (Normal Closed) side and it works as an AC only thermostat. Add an outlet box for a few bucks and I think I can do it all for under $20, since I have phone chargers and several old power cords I can chop down. And I like the DIY idea.

Hunter Mechanical Heat Only Non-Programmable Thermostat Item #: 155771 | Model #: 40120

 

Here is the relay link. It is a 125VDC/10A SPDT or ”Single-Pole, Double-Throw type. Here is a Wiki page explaining what an SPDT means. And it is $4.99, so I was off a little. Sorry.

 

Edit: Need to redo this. Will repost after further testing.