Thermostat/Thermistor Question - SOF Chiller

RedGlass · 05-31-2010, 03:46 AM

Question for anyone with a little electrical sense:

I built a "Son of Fermentation" Chiller this afternoon (using this plan). I wanted to mount the thermostat on the outside and still measure wort temperature so I extended the thermistor based on this plan.

I'm using a RiteTemp 6020 thermostat from home depot; a 3", 12 V (DC) fan from radio shack; and a old 12V (DC) power supply that was lying around the house. The build and the thermostat mod went fine (or so I think so far).

Here is my problem:

When the power supply is not plugged in, the thermostat (with extended thermistor) reads ambient temp accurately (based on known temp w/ calibrated thermometer). When I plug in the power supply, the thermostat immediately reads ambient a few degrees cooler (-4* or so), even though I have not touched the thermistor at all. As soon as I switch it to cool or fan on it drops another 10* or so, again not having touched the thermistor at all.

I have no idea how all this crap works, I just followed the directions in the plans above. Anyone have any clue why the thermostat would immediately start reading a cooler ambient, even though the actual temp has not changed? Let me know if I need to provide any more information about anything...

Thanks!

marzsit · 05-31-2010, 04:16 AM

hvac thermostats are designed to operate on 12-24VAC, the batteries provide memory and clock backup but don't actually power the thermostat. i'm guessing that your thermostat doesn't like running on 12VDC.

when the power supply is off, the thermostat runs on internal batteries and reads correctly. when you plug in the power supply, the internal batteries are disconnected and the thermostat then operates on external power. the low, DC voltage results in inaccurate reading, and gets worse when you switch on the fan due to increased voltage drop.

**BrewBeemer** · 05-31-2010, 04:58 AM

Oh he has VD!

I was thinking his cube power supply is too small and not enough mA output under the fan load.
Use a larger cube plus the plugged in cubes higher voltage than the specs state to the stat for the true temp readings.
I always pack rat those 750, 1,000 & 1,500 mA cubes I get off a street scrap collector friend. He'll get boxes full of electronic games with piles of cube power supplies that I dig thru and buy for a buck each in many voltages.

Mista_Sparkle · 05-31-2010, 05:13 AM

does sound like a voltage drop, however for a thermostat to cause a significant voltage drop, you most likely have a VERY small power supply. if you have a PC power supply laying around, i use those all the time(however they do use a lot more power at low loads, and take a little bit of wiring(more like a paperclip) to get to work)

yeah, just find a bigger wall wort

RedGlass · 05-31-2010, 07:21 AM

Well a new power supply seems easy enough.

Figuring out electronics is like greek to me, so let me dumb it down for a second with a few follow up questions.

What's the difference between AC and DC. I know the literal definitions about how the currents work...but I mean if I have a fan rated for 12VDC, how does a 12-24 VAC power source (which is what the thermostat apparently runs on) affect the fan? Will an AC power source short out a DC fan or something like that?

The fan specs are:
Rated Voltage: 12VDC
Voltage Range: 10.2-13.8VDC
Rated current: 0.16A (or 160mA) max
Rated power: 1.92W max @ rated voltage

What would the specs on a proper power source be to fit that fan and make the thermostat keep an accurate temp? In other words, I want it to work but I also want to make sure I don't set this thing on fire somehow.

Thanks again.

RedGlass · 05-31-2010, 02:21 PM

Oh, and the thermostat materials say it runs on 24VAC and should not get higher than 1.0 amps. Can this and the fan get along if I get the right power source?

Thanks again to anyone who knows, I'm completely dumb when it comes to anything electrical so I have no idea how all these values fit together.

Mista_Sparkle · 05-31-2010, 03:11 PM

is it a regular cheapie thermostat? if so the battery provides the power to run the electronics in most cases and the 24VAC 1A rating is on the relays that switch various parts of you hvac system on or off.
Also, if that is the case, your thermistor reading shouldnt change based on whether something is plugged in or not. There could be a bit of noise induced on the thermistor line or something throwing off your readings from the wall wort though.

You could have a low battery in the thermostat, or it could just be a ****ty thermostat...

**BrewBeemer** · 05-31-2010, 03:27 PM

Get a thermostat 24 volt AC transformer, same as a door bell if it's 24 volt vs 16 volt AC output, some come with muti voltage terminals. Then the stat will be happy. Look for a 24 volt AC fan another option with your new X-former. Another option same 24 VAC X-former add a bridge rectifier with a inline resistor to run your 12 VDC fan. Replace this resistor with a 12 volt zener diode and a resistor alone.
Use the chart below for your resistor value with using a zener diode as well the wattage of the resistor required.
This would only give you half wave DC, not good on the fan vs full DC.
Just punch in the numbers as well added a safety margin on the chart below. This would be a cheap way to go with just a 24 VAC transformer, zener diode plus a resistor for to purchase then you can use your existing fan.

Another way kind of goofy is still use a small 24 volt AC X-former so the stat's happy plus use your 12 VDC cube power supply you have to run just your fan but you'll need a 24 VAC relay off the stat's contacts to switch control the fan power when the stat wants cold. Those 24 VAC X-formers are rather cheap, Home Depot or Radio Shack item.

http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

RedGlass · 05-31-2010, 03:34 PM

The thermostat is a RiteTemp 6020 from home depot. It's a $25 thermostat -- I'm sure it's cheap, but I'm sure there has to be a way to make this work. Batteries are new.

I think you're right about the 24VAC thing, since that is apparently the power rating for another contact in case I wanted to wire it up to a house HVAC system. So maybe that's not the issue.

I looked at the power supply I was using and its 12VDC and 400mAh. The fan is only 160mA, so it seems like the power supply I was using should be big enough to power that fan. Maybe I'm missing something. What would "noise" on the thermistor mean? What could I do to eliminate that if others think that might be the problem??

Thanks!

Mista_Sparkle · 05-31-2010, 03:55 PM

noise would be induced voltage from the thermistor wires running near a high voltage components operating at some frequency. thermistors are generally less susceptible to noise than other measurement devices such as thermocouples. however, unless you wrapped your thermistor wires around the transformer or an AC cord, chances are your readings would be OK.