NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Support Forum » Accurate Temp Controlled Frig...

May 13, 2009
by tony
tony's Avatar

I am have been brewing beer for some time. I need to accurately control the temp of a mini frig to keep my kegs at the right temp. I have seen the Freezer set up you elude to on this site. Can you please list some of the parts you used to control your temp?


May 19, 2009
by mrobbins
(NerdKits Staff)

mrobbins's Avatar

Hi Tony,

We used the LM34 that we include with the kit, and we also used a relay to switch the compressor on and off. There was additionally a 2N7000 to drive the relay coil, like we show in the Servo Squirter That's really it!

One of the important concepts is hysteresis -- the idea of having a "dead zone" near your desired operating point where your controller simply refuses to change its current state. Because the temperature sensor data comes in with some noise, if you're trying to maintain 39 degrees in your fridge, you don't want it to keep switching the compressor on and off every time it's 0.1 degrees higher or lower! (If it did that, you would quickly burn out the compressor.)

So instead of picking one temperature to maintain, you basically pick two temperatures, maybe 38 and 40 degrees. The compressor turns on when the measured temperature is greater than 40 degrees. The compressor turns off when the measured temperature is below 38 degrees. But between 38 and 40, it will simply remember it's current on/off state, and won't change it. You have to adjust this hysteresis width (here 40-38 = 2 degrees) depending on the noise level, the "inertia" of the system (how slow it is for temperature to respond with time), and how often you want to be restarting your compressor.

Hope that makes sense.


May 19, 2009
by tony
tony's Avatar

Mike thanks for the help and the explanation. Sorry I've taken so long to come back but summer classes are gearing up...

One more question, when choosing a relay what should one consider?

Post a Reply

Please log in to post a reply.

Did you know that first-order systems have a exponentially decaying response to step inputs? Learn more...