Simple and Cheap DIY system for under $20


 
M

MarkN

New member
Hello everyone,

I'm a new member, and this topic interest me. I did a search found a link for a temperature control system someone did for their PC case. Wanted your thoughts as to whether this would work for a WSM?

Here's the link: http://www.heatsink-guide.com/content.php?content=control.shtml

It looks like a very simple design, and should cost no more than $20. Nothing fancy, you would have to calibrate your unit at a set temperature (say 240 degrees) and away you go. Instead of a NTC thermistor, use a PTC.

What do you think?
 
Looks cheap enough to build - well under $20. Why not just build it? Next time I'm at Radio Shack I'll pick up the parts and give it a try. If it doesn't work for a smoker I can use it in my PC.

Some thoughts:
The fans the Stoker and Guru uses are really "blowers" and work differently then PC fans. However, it's really just a DC motor in the end.

If you use a PC fan you'll need some way to dissipate the heat of the pit so it doesn't melt the fan. A stoker style WSM mount could work for this.

-rob
 
Yep, you would use a blower and stoker type mount instead of a PC fan. I recall seeing a post of this before.

I was doing more research and could not find an appropriate PTC type thermistor (on digikey). I looked for a 10K PTC type thermistor, but highest they went was 2K. Will that suffice? Also, they were rated at 10mA, which seems too low. The NTC type thermistor has more range.
 
The current rating of the therm is fine as it's just controlling base current. The problem I see is the therm's range. They top out at 257 F. Ideally you'd want a range starting at 200.

-rob
 
I found one that has a transition temperature at 110 celsius (230 F). Digikey: 235-1191-ND. This might work if you want to maintain a temp of around 230F. There is also another thermistor that has a 120C transition (digikey: 235-1190 ND).

Here it is in the digikey catalog: http://dkc3.digikey.com/PDF/T091/P1994.pdf
 
That's a PTC therm and you need a NTC. In the circuit as the temp goes up the therm's resistance goes down (NTC) and more current goes to the base therefore more current goes to the fan. A PTC won't work as it's resistance is proportional to the temp.

rob
 
NTC is what you want for the PC cooling fan, but in the WSM application, you want the reverse. As the temp goes up, you want the fan to stop. As smoker cools below 230F, then the resistance drops and your fan starts up. I think we want a PTC.
 
I see. You are right. In the PC application the fan controls cooling by venting hot air whereas were controlling heat by fanning the flame.

-rob
 
I just ordered the thermistor. Got one that transitions at 110C. I think I have everything else. I'll try it in a few days when the therm arrives. Should be a fun experiment.
 
I bought this off ebay. I found a couple on alliedelec.com that would work, but they charge $5 handling and $x.xx for shipping. So I didn't bother when i could get three for under $5 on ebay. The one on digi-key was not available.


The resistance jumps to 10K ohms at 130C. It starts to transition at 110C. So I'm hoping variable fan speeds from 110C to 130C. Fans starts to slow down at 110C and stops completely at 130C. I probably will adjust the potentiometer to stop the fan before 130C.

I'm also planning on using a PC fan instead of blower since air can flow more freely without the fan on. This would reduce the need for the fan running continuously.

For the fan mount, I was thinking I could use a stainless steel cup holder (I have an extra one from a poker table project). It has a 3.5" diameter.


http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageNam...IT&item=220235144308
 
Sweet! I just ordered mine too. I found all the other components as well. There are a couple of things to consider:

- The Guru and the Stoker blowers have a valve that looks like a flu of sorts. When it's blowing the air opens the valve otherwise it's closed.

- Thermistors have a lag in response time so that is going to add to the zig zag of the temp.

- The amps required to start a DC motor from a dead stop (breakover maybe) is higher then to keep it running once it's going. That's going to add to the zig zag as well.

- I think we are going to need to coat the legs with something if it's going to be in a moist environment. Any suggestions?

- Mounting the therm in the WSM - I need to find a quick connect that can fit through a 1/8 inch hole I have drilled in the WSM. I suppose I could just hand tie it and tape it. This is not a problem on the BGE so maybe I'll do it on there.


Once this is done I'll chart a smoke with the stoker and post the results. I think if it zig zags 10 degrees or less it's an option for others to consider. This is going to be cool if it works.

-rob
 
I don't have any suggestions for a coating for the therm leads. Hopefully it will work out okay. At least the sensor is coated.

For the fan, I assume that some air must be allowed to flow even while the fan is off (otherwise the flame would die). Hopefully the airflow permitted by the fan while off will not be too much. Of course this could be adjusted by partially closing the bottom vent that the fan is attached to.

For your stoker, does the blower run consistently?

I agree that the temp chart will be sawtooth. Results will vary depending on the therm and motor. It is conceivable that we could achieve a state where the fan runs continuously at variable speeds (which would eliminate the turn-up current). This depends on the range of the therm and would make the sawtooth smoother.

I'm going to officially name this project S.C.A.T.S for Simple Cheap Auto Temp System. Total cost for me is $1.38 (price of a thermistor + shipping) since I had everything else lying around :).
 
For the fan, I assume that some air must be allowed to flow even while the fan is off (otherwise the flame would die). Hopefully the airflow permitted by the fan while off will not be too much. Of course this could be adjusted by partially closing the bottom vent that the fan is attached to.

For your stoker, does the blower run consistently?
Click to expand...

All cookers have some air leakage granted the ceramics are pretty tight.

The stoker does a hard on/off but is has some predictive algorithms to deal with over/under cooking. When I do the chart it will not be controlling the blower - just cahrting cook temp.

-rob
 
Mark,

Did you build it yet? I just got my thermistors today. I'm going to put it on a prototype board and attach a guru fan to it then play around with either some boiling water or an oven. The one thing though is that I will need some type of insulation that is heat resistant - maybe teflon or silicon.

I might try it out this weekend on a rib burn if I figure this out.

-rob
 
Rob,

I have not built it yet. I'm under the weather and feel like crap. I have all the parts but probably won't get around to it until a couple more days of rest. I guess SCATS will have to wait.

-mark
 
Joe,

The project that we are modifying for BBQ use is based upon the PC Fan controller project listed here:

http://www.heatsink-guide.com/content.php?content=control.shtml

All we are doing is using a different thermistor that Mark found on eBay. I'm going to breadboard up the circuit tonight and play around with it with a Guru fan and 12 volt adapter and a heat gun. I'll take pics of the final project once I get that far.

You can find all the part but the thermistor at Radio Shack.

-rob
 
OK Here is what I did:

I built the circuit as shown in the PC Fan article using our PTC thermistor and an old Pentium CPU fan. I then Took the thermistor and taped it to the probe of a calibrated digital kitchen thermometer. I set the probe/thermistor about 6 inches away from a heat gun and applied 12v to the circuit.

The fan turned on instantly. I turned on the heat gun and slowly applied heat until the digital therm read 250. Then I adjusted the pot so the fan turned off thereby setting my pit temp to 250. Over the period of 30 mins I slowly decreased the temp of the heat gun to see at what temp the fan would turn on. As the temp was decreasing you could see the fan wanting to turn on. It would make slight movements but not turn on. It finally turned on at about 212 degrees.

Then I slowly increased the heat until the fan went off. That was about 246 degrees.


So it looks like we have a temp swing 40 degrees depending upon the characteristics of your fan. I chose a small fan thinking that the breakover current would be much less than a bigger fan like a Guru fan. It seems this breakover current is what's going to kill this project for use as a power drafter.

So how can we fix this? I think what is needed is a hard on/off of the fan rather then varying the current. This will get us over the breakover current issue. If we set up a voltage divider with the thermistor and another voltage divider with the pot and input those two voltages into a comparator then the output of the comparator can be used to turn off/on the transistor that's controlling the fan. I'll play around with this.

Another thing I will mention -- When I was doing this the heat gun shifted and the temp rose to about 300. That thermistor was toast. It became an closed circuit. Luckily I got 3 in the package. So in this application if your temps get too high the fan will stay on and feed the flames. Not good.

A comparator circuit won't add too much to the cost so I think we're meeting the requirements if it's added.

-rob
 
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