DIY Temp Controller Questions

I've decided to attempt this build but I've got a few questions.
First, why is it that that thermistor is moved? Could you not mount the thermostat inside the freezer? Seems like extra work.
Second, I know the OP used a solid state relay but would a mechanical relay work the same? Something like this? I ask because I can source this part locally for a lot less than an equivalent solid state relay. Any help would be appreciated. My electrical knowledge is limited at best, but I do have my brother-in-law who is a Mechanical Engineer and has more than a few hours of EE under his belt to keep me from killing myself.

neumann said:

I've decided to attempt this build but I've got a few questions.
First, why is it that that thermistor is moved? Could you not mount the thermostat inside the freezer? Seems like extra work.
Second, I know the OP used a solid state relay but would a mechanical relay work the same? Something like this? I ask because I can source this part locally for a lot less than an equivalent solid state relay. Any help would be appreciated. My electrical knowledge is limited at best, but I do have my brother-in-law who is a Mechanical Engineer and has more than a few hours of EE under his belt to keep me from killing myself.

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If you put the thermostat in the freezer it will be susceptible to condensation. i.e. electric bits getting wet, and those thermostats aren't designed to get wet.

I don't see why you couldn't use a mechanical relay. You also get the satisfying 'click' when it engages or disengages.

diatonic said:

If you put the thermostat in the freezer it will be susceptible to condensation. i.e. electric bits getting wet, and those thermostats aren't designed to get wet....

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It depends on how cold you plan on making it get. I doubt the thermostat will even let it get cold enough to cause condensation problems. Depends on enviroment too...but diatonic is right, be careful of condensation.

that relay looks fine.

if you go through the thread, i'm pretty sure you'll find someone who used a different relay and it worked fine. and you COULD mount the whole thing in the freezer, but I like to have my thermistor right up against a carboy with some padded insulation holding it in place. I also like to see what the temp is without having to open up the freezer or what have you. plus, like the others have said, this keeps all the wiring and everything out of harms way. it's not difficult to desolder and resolder the thermistor. it was my first time doing it and it took about 10 minutes.

I don't see why you couldn't use a mechanical relay. You also get the satisfying 'click' when it engages or disengages.

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You can use a relay, it all depends how often the relay has to cycle and load current.

Just some info.

Electro mechanical relays have two ratings.
Mechanical service life & electrical at rated load.

Here is an example 782 series Magnacraft relay.
10,000,000. Mechanical cycles
200,000. Electrical cycles at rated resistive load

16A rated relay used with a 14 A load won’t last to long in a temp control application.

If you use the same relay to drive a SSR which requires a few mA, the service life gets close to the mechanical cycles, big improvement.
Solid states relays do not have these limitations.

Cheers,
ClaudiusB

I used a mechanical relay on my temp controller, works just fine. It is switching a fairly light load (40 Watt light bulb). 1 reason not to put the thermostat in the cooler or whatever would be battery life. My thermostat was failing miserably during the winter when I had my ferm fridge outside essentially, so I had my roommate build a 3V voltage regulation circuit and hardwired the thermostat power into the control box.

OK, so relocating the thermistor is a good idea and not hard. No problem. Can some one explain to me what the AC adapter is for? I can't get around my head what it's powering because the thermostat uses batteries. Is it just to avoid having to change batteries? I know the batteries in a thermostat last nearly forever.

The AC Adapter flips the relay.

A relay is an electronic switch. it will have (at least) 4 terminals on it:

1. Your 110V input
2. Your 110V output (these two are normally not connected)
3. 12 vdc input
4. 12 vdc output (when voltage is applied to the dc posts, the switch on the AC side flips and connects the switch)

Example:
So the thermostat is set to "HEAT" mode. You set it to 65F. It's 66F in your box that the thermistor is measuring. The Thermostat is "Off" meaning it's not sending voltage out on the fan control. Then temp drops to 64F. The thermostat flips its own relay inside and sends 12V DC out the Fan connection (Labeled "G" usually") from the RC/RH labeled inputs. The Fan connection on the thermostat is connected to the 12VDC input on the relay. 12 VDC goes into the input and comes out of the output, back to the negative lead of the power brick. Because there's 12 VDC on the relay, it throws the switch on the 110V side, supplying voltage to the other half of the split AC outlet. Then your space heater turns on until the Thermostat reads 66F again. The thermostat shuts off its relay, removing 12VDC from the relay in the box, which opens the switch for the 110V AC leg of the split outlet, removing voltage from your space heater, thus turning it off.

The thermostat (in my setup) still runs off two AA batteries.

I'm also using a mechanical relay from Grainger on one of my controllers. It's lasted 3 months so far. When it dies I'll probably replace with an SSR.

Sorry for the long post, if it's not helpful, I can explain it further and better with pictures.