Thermostat/Thermistor Question - SOF Chiller

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!

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.

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.

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

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.

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.

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...

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

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!

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.

Ok. I'll assume noise isn't the problem then, since the thermistor and power attach to opposite sides of the thermostat.

Do folks still suspect a low voltage/amperage issue as in the initial responses? It seems like the power supply is big enough to handle the fan if its rated 12VD/400mA and the fan only draws 160mAh??

Also, BrewBeemer, thanks for the response. That sounds really complicated. Does your suggestion change at all knowing that the thermostat is battery powered?

Nope unless you wanted to run it without 24 VAC power completly as I believe this is required for the stat to pull in the interrnal relay for the compressor, furnace or what you want controlled off it.
I see your cube is big enough just DC vs AC for the stat.

I'm an idiot with electricity, so can you dumb that down for me (I have a feeling this is what I need to know, but I can't understand it).

The thermostat runs (at least detecting/displaying ambient temp) itself on 2 AA batts. When I plug in the power supply for the fan, the stat then needs a 24 VAC supply in order to switch the fan on and off and keep an accurate ambient reading?

It seems like so many people have done this exact mod (same model thermostat) with just a 12VDC wart (as I have it set up now). Why would AC be necessary here as it seems like others have not needed it? (I really have no idea about how the stat powers itself, etc. so that may be a dumb question). I feel like the thermostat is in good working order because I can get an accurate ambient reading when the power is disconnected. Is it possible that this thing is just a lemon if I need more than a 12VDC wart??

Thanks.

the batteries only provide backup power when ac power is down, just enough to maintain the clock and program memory and operate the temperature display. the batteries alone cannot power the switching circuit inside the thermostat. these are known as "power stealing" thermostats, since they "steal" operating power from their controlled appliances. so, in order for it to work in a non-hvac application, you have to fool it into thinking it's connected to a furnace or air conditioner....

the stat needs 15-24vac because most hvac systems have always used 15-24vac for their thermostat control loops... internally, the 15-24vac is probably rectified to dc and regulated to a lower dc voltage. if you try to feed it with dc, the internal rectifier's diodes will cause an additional voltage drop, 2-3 volts, due to the reverse bias of the diodes in the rectifier. i'm guessing that if you were to check the voltage at the power terminals in the thermostat with your 12vdc-400ma wall-wart plugged in the voltage will be below 12vdc.. a dc wall-wart with a higher voltage might help, but then you have a problem with 12vdc for your already-purchased fan..

if i were to build one of these i'd use a doorbell transformer for power, and have the stat control a 24vac relay which would switch the fan power. i would use a 120vac fan because it would eliminate the 12vdc wall-wart.. much simpler.

From the Manual

BATTERIES ONLY - This thermostat can run on batteries only using 2AAA
alkaline batteries. The batteries will last at least 1 year; replace the batteries
once a year or when the low battery icon comes on the display. If the batteries
are not replaced, the thermostat will shut off the HVAC and then stop working.

24VAC - This thermostat can run on the HVAC 24VAC (C wire) if available.
As shown in the wiring diagrams, the C wire is the other side of the 24VAC
heating transformer and can be found where the other thermostat wires
connect at the wall or at the furnace. Do not use the common or ground side
of the line voltage. If the C wire is used, the batteries are then for back-up in
case of power loss only and will last much longer. With the C wire connected,
the thermostat will continue to work if the batteries die or are removed.

Click to expand...

So you don't need 24V, batteries will last you about a year. If you want, you can use a 24 volt system with it, but why bother.

another issue... i reread http://www.wortomatic.com/articles/Remote-Sensing-DIY-Controller-for-Less-Than-$25 this article and it seems to apply to the model 8029B thermostat. you are using the model 6020, which could be completely different in it's power requirements....

I can return the fan to the shack, so that's not really a problem.

How would the wiring in your suggestion work? Run power from the wall to the doorbell transformer and then into the stat (which would get me 24vac to the stat, right?). Then from the stat to the fan (but wouldn't that only be sending 24vac out to the fan, so how would a 120vac get enough power to run in that example?). Also, what kind of wiring would I run to the doorbell transformer? Could I just use an old extension cord soldered to the transformer or do I need something else for that?

And thanks for continuing to help out and answer these REALLY basic questions.

Sparkle -- I read that too, but figured I was missing something.

marzsit -- The guy who did that page used the 8029B, but it stopped being produced shortly after. There were some comments (or another page based on that page) that used the 6020 with success, and it seems like all current versions of that mod are done with the 6020.

What I can't get through my head, is why have other folks not had issues just plugging in a 12v DC power supply. Could it be something else. Could I have this type of problem by not reattaching the thermistor properly or anything like that? Just trying to think of other possible causes...

it doesnt really matter what thermostat you use, they all boil down to being a temperature controlled switch. if you wire your power supply correctly, it should work just fine assuming your batteries are in decent shape.

So then that brings it all the way back to square 1. How can I get it to keep an accurate reading once it's plugged in??????

do you have the thermostat set to heat or cool or were you using the fan relay?

I have tried it on both auto cooling and "fan on." The drop in ambient temp readings are always the same (-3 once power source is plugged in, -10 once the fan kicks on). The change to ambient temp happens about 5-10sec after plugging in/fan going on.

In case it matters, I have the red wire from the power supply going to the RH terminal and the red wire for the fan going to the G terminal. Black wire from power source connected directly to fan's black wire.

Another way to go would be to use the thermostat to switch a relay I have 5 relays swithced with just the batteries in the thermostat that way the higher amperage circuit is seperated from the thermostat

jppostKW said:

Another way to go would be to use the thermostat to switch a relay I have 5 relays swithced with just the batteries in the thermostat that way the higher amperage circuit is seperated from the thermostat

Click to expand...

How many mA each relay draws times 5, this sounds like it'll drain those double A or tripple A battweries rather quickly. Why not use a cube to maintain the stat instead of batteries? Just throwing questions out.

BrewBeemer said:

How many mA each relay draws times 5, this sounds like it'll drain those double A or tripple A battweries rather quickly. Why not use a cube to maintain the stat instead of batteries? Just throwing questions out.

Click to expand...

The relays I'm using are 1 amp max with a 5 ma clamping current and a <.6ma holding current. Ihave been using them on the same set of batteries for 2 months now controlling a 12 volt mag drive glycol pump circulating coolant from a miny fridge thru a heater core and 4 5 inch 12v computer fans. My chamber is SOFC esque and it works better than expected

That's getting down there in the mA scale on coil current, the battery will about die by old age alone first. Hell my muffin fans draw 720mA each but make for strong yeast stir motors.

BrewBeemer said:

That's getting down there in the mA scale on coil current, the battery will about die by old age alone first. Hell my muffin fans draw 720mA each but make for strong yeast stir motors.

Click to expand...

My fans draw .39 amps each and the pump about 3/4 of an amp I run them off a deep cycle optima keep it on a trickle charger

jppostKW said:

My fans draw .39 amps each and the pump about 3/4 of an amp I run them off a deep cycle optima keep it on a trickle charger

Click to expand...

Sounds like your set, heck the charger alone can provide the power to run everything unless you live in a bad electrical area with many blackouts.

RedGlass said:

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).

Click to expand...

RedGlass, did you figure out how to get the RiteTemp 6020 to work? I have very little knowledge when it comes to electrical stuff but want to build a SOFC using this thermostat.

It seems I missed the boat as all the instructions for the thermostats are out dated (err, the thermostats are out dated).

Does anyone know of a current build (i.e. what thermostat is best to control a 12 V DC fan and how to wire it????) Thanks greatly as it would be nice to brew during the summer

RedGlass said:

In case it matters, I have the red wire from the power supply going to the RH terminal and the red wire for the fan going to the G terminal. Black wire from power source connected directly to fan's black wire.

Click to expand...

I’m a complete noob to wiring and electricity… but

According to the wiring diagram, the wire from the power supply goes to the G terminal, then from the RH terminal to the fan and then back to the power supply. Could this account for the voltage drop? (see "This is a general diagram on the hookup for the controller using a 12 – 24 volt fan and power supply" at the end of instructions)

I don’t know much about electricity and wiring but am pretty sure (direct current) electrons flow positive to negative, and the ‘work’ is in between. So, hot (red) to G, thermostat uses a switch to allow flow (close) to the RH terminal, electrons go to fan, does work, and returns to the power supply through negative (black) wire to complete the circuit.

Mista_Sparkle said:

it doesnt really matter what thermostat you use, they all boil down to being a temperature controlled switch. if you wire your power supply correctly, it should work just fine assuming your batteries are in decent shape.

Click to expand...

From what I understand, the batteries control the temp probe and internal switch (open/close between G terminal and RH terminal) while the regular 12VDC (AC to DC) power supply just runs the fan. The batteries don’t run the fan and the 12VDC supply doesn’t run the thermostat. If this is the case, then just a rewire should work fine and I assume the switch between G and RH terminals doesn’t care about AC or DC, you just can’t have too much voltage or amps going across that switch (like the load on a fuse)?????

Anyone, please correct me if I’m totally off base. I’m trying to figure this out before spending $$ on building this and maybe this might help others who “missed the boat” for older thermostats. Thanks,

I keep bumping this cause I wanted to know. And now I do... maybe someone else will too.

I just built a chamber using the 6020 ritetemp and it works great. I’m using battery power, left the default jumpers in place (reply if you want to know what they are cause I cant remember with all the hm cider right now), and wired it:

Power-wall-wort (hot) to G on Rite Temp, RH to hot fan, ground from fan back to 12 vdc wall-wort. And it works great! Don’t forget about the two minute delay between turning the fan on when changing temps on the thermostat.

Also, don’t forget to take out the temp controller and rewire that sucker to a longer cord. But it’s in the instructions… while they last. Also, good luck finding a 6020, I think that may burry this one… but then again, I assume all thermostats use the same crazy G, RH, etc.???

These things are actually on Amazon for $16!

I am gonna go ahead and order it, but I think I am a little confused about what all I need.

Do I just need the thermostat and a wire lead or do I need a relay and other confusing things?

This seemed like it would be pretty straightforward until I read this post.

If you are using it for a SOF then you need a thermostat (RiteTemp 6020), a 12VDC computer fan, a 120V AC to 12V DC wall plug, some spare electrical wire (24 or-so gauge), some shrink tubing (you can find that at O’Reilly, Checker, or any auto parts store – cheaper than Radio Shack- You will need a hot air source like a hot blow dryer to shrink the tubing), a pencil-point solder gun (like $10 at RS or wallyworld), and some solder wire (I used .032 dia.). -I tried posting links to the shrink tubing, solder gun, and wire solder but right now HBT only likes Amazon links!!!!

If you are using it for something else (to control a freezer) then you might need to look at other posts/threads.

There is no need for a relay or any other confusing things. For the 120V AC to 12V DC wall plug you could use a cell phone charger or better yet, if you have an old Linksys router use the wall plug for that… those are 12V DC. Some cell phone chargers pump out only around 6-9 V DC which will work but make the fan run slower. Use the same size wire of the power supply (or pc fan) as a guide for the rest of your wires.

Follow the instructions that are online or see my post above. As far as the OP’s problem, I suspect the wiring was wrong or it was a bad thermostat. As I posted above, it was super easy to do (I even soldered it with little to no experience). Use youtube for instructions on how to solder if you’re new at it.

Hope this helps! Let me know if you have any more questions

One thing to check is the output voltage on your power supply when it is not connected to anything, and then put a known load on it (say a 2K, 1/ watt resistor). Place the load across the output wires or terminals, and measure the voltage again. If it is not 12 V in either case you have what is called an "unregulated power supply." These are the most common kind, and each is designed for a specific load. If this is the case, you must fix it before we can do anything with your other readings, because voltage is not being held constant, so the addition of the thermostat and then the thermistor will change the voltage present, and hence the readings you are obtaining.

A simple fix for under a $2.00 is (first check that your open circuit voltage is about 14.5 VDC) to put a 1000uF/25V capacitor across the power leads, then connect the positive and ground leads to a 7812 power controller in a TO-220 case, then put a 10uF/25V capacitor (possibly tatalum) across the outputs (positive and ground) or the voltage regulator. Connect the grounds all at one position, by the way. Protect the regulator by placing an inexpensive silicon signal diode (e.g., 1N914, 1N4148, etc.) from the chip output to the chip input (pointing backwards, or so it seems when you draw it). Now you have 12 V to about +-3% regardless of the current in the system as you attach other components. If you are going to draw more than about 50 mA, use a heat sink on the TO-220 package (which will then allow up to 500 mA, much more than you would need). Some lower-cost sources for the partsare noted below.

Regards,
Tom Bellows
TSB Engineering Co.

www.bgmicro.com
www.allelectronics.com
www.mpja.com