Help with Small Project Please

[Jensonhead]

Hello all.

I'm trying to make a barbeque/smoker that is temperature regulated. I'm using what is called a "Texas Two-Stage" smoker, which is two offset drums joined together. The lower drum has the fire and the upper drum is where the meat gets smoked.

With a temperature sensor in the smoking area, I want to use a small squirrel cage fan to force air into the firebox at "maximum" when the temperature falls below 200 deg. F, and to be completely off when the temperature reaches 225 deg. F. "Maximum" has yet to be determined, but it will be "X CFM" (cubic feet per minute), something strong enough to kick the fire into life, but not so strong as to blow ash all over the food.

The squirrel cage fan was salvaged from an overhead bathroom "fart fan". I can provide the specs if necessary, but it's standard 120 VAC (US). At full voltage, it is probably 3 times the fan (CFM) I need, so I would expect it to operate at around 40 VAC max, and fall to zero as the temperature in the cooking area approaches 225 deg. F,

These are the basic parameters, as I have imagined them, but now I need to build a prototype and find out exactly what they are going to be.

For background, I have military training and experience in basic electronics, radio fundamentals and radio repair, but that was many years ago, and most of what I barely knew back then has been forgotten. I can solder, and can tell the difference between a diode, a capacitor, a transistor, a resistor, etc... but that is about the extent of my electronics knowledge. Maybe as things advance I might remember more, but that's a good starting point if anyone is generous enough with their time & expertise to help me.

FWIW, if there is anyone else with a dual-interest in building things and good smoked meat, I'll do all the beta testing so that at the end of the process, others can learn from my mistakes and possibly build my self-regulating smoker and do it right the 1st time. For those that do not know, smoking meat is a royal pain in the a**, as it requires you to stand there and manipulate the air intake and the exhaust in order to maintain the correct balance between them in order to achieve a temperature that is between 200 and 225 deg. F. Smoking meat is typically a 4 to 10 hour proposition, depending on what you are doing. So this is going to be a SERIOUS help if it works.

At this point, what I think I'm looking for is PARTS. What to use to sense the temperature, how to send it's signal to a thermostat (located away from the intense heat). I assume there will need to be some kind of adjustable potentiometer(s) in order to adjust the fan so that it's "performance curve" follows the desired temperature curve. I'd like it to spend most of it's time barely running, and only turning off when things get too hot, and only going to "max" when (for example) the wood is "changing gears" and needs some extra O2 in order to get going again.

But none of that is definite. I've got to get into real-life testing in order to know for sure what is and is not going to work. But I'd like to start-out with some good, workable parts and some help in configuring them.

Thanks in advance. Looking forward to moving this idea I've had into a practical stage.

 

[duke37]

A 120V AC motor may not like being fed with a low voltage. I would go for a 12V DC fan out of a car heater. This will have plenty of waft and will be easier to control with low voltage, safer electronics. A 12V fan out of an old computer power supply may be big enough.
The DC fans can be tested on a 12V battery before making any circuits.

You will need a temperature sensor, a thermistor will be very sensitive but I do not know whether it can stand the temperature.
A thermocouple (copper/constantan) can easily stand the temperature but will have a low output so will need some good low noise amplification.

It will be easiest to use on/off control but proportional control is possible i.e. the cooler the temperature, the bigger the blow.

 

[KrisBlueNZ]

This sounds like a really cool project.

For non-American readers like me, the temperature range of 200~225 deg. Fahrenheit is about 93~107 degrees Celsius.

I agree with Duke on the use of a DC fan. These are much easier to control.

Here's a PWM fan controller that should do it: http://www.ebay.com/itm/Reversible-...458?pt=LH_DefaultDomain_0&hash=item5aefd54a52

This is not a speed regulator; it doesn't get any feedback from the fan about what speed it is actually running at. It's an "open loop" controller. Some fans provide a running speed indication with a third wire, and you may be able to find controllers that use that signal to regulate the fan speed. That might be important to offset the effects of wind blowing, different amounts of wood on the fire and meat in the smoker, and deterioration of the fan.

I think you need to measure the temperature of the air inside the smoker, somewhere near where it enters, rather than the temperature of the metal or any object inside the smoker, otherwise the delay between changing the fan speed and getting the feedback will be too long. I think that was your intention anyway; just clarifying it.

In any case you will probably need to "damp" the fan control signal so it can't change too quickly, otherwise you could have problems with overshoot and "hunting".

As for measuring the temperature, I think Duke is right. (He has all sorts of experience!) For temperatures over 150 deg. C (302 degrees Fahrenheit), a thermocouple may be your only option. See Digikey's web site in the section Sensors, Transducers > Temperature Probes. The thermocouple probes are around USD 60.

There are some thermistor probes in that section but they are only specified up to 125 deg. C (257 deg. F), which is a bit close to your upper limit!

There are various semiconductor devices that are specified up to 150 deg. C (302 deg. F) that MIGHT be usable if you can figure out a way to mount them and get the necessary wires to them. I will ignore the digital devices, since I guess you're not into microcontrollers, so that leaves analogue devices with voltage, current, or PWM outputs, all of which are usable.

Digikey has a section Sensors, Transducers > Temperature Sensors, Transducers with various semiconductor temperature sensors from USD 0.30 up to USD 74! The cheap ones are mostly in SMT packages; some are in TO-92 (wire-leaded transistor style). Some of the more expensive ones are in full metal packages. For example the Analog Devices AD590JF (USD 19) is a current source with an output current of 1 uA per degree Kelvin. Have a look at the Applications section in the data sheet to see how you could use it.

I don't have much experience with process control unfortunately, so I can't say any more than that.

Good luck and please keep us posted on the progress of this project!

 

[duke37]

As Mrs Beaton would say "First catch your bison"

Thermocouples are used in gas boilers to detect the pilot flame, these are quite cheap. You could make your own from copper/constantan(resistance wire) but it would be a hassle to make the junction and provide the insulation. PTFE will go to 190degC.

I do not know the details of the gas boiler thermocouples but the output is likely to be about 4uV/degC so 100deg will give 0.4mV. An amplifier with a gain of 1000 will give 0.4V which will be plenty to go into a comparator.

A thermocouple needs a cold junction. If it is fed directly into an amplifier, then the amplifier is the cold junction, this could vary by 10degC but you are not after great accuracy. Cold junction compensation is possible.

 

[(*steve*)]

There are a stack of temperature controllers available very cheap from China.

I recently purchased one which is a PID controller and comes with a thermocouple on the end of a 5 ft cable shielded in metal braid. It is supposed to be good to 400C

Here is something similar.

It really only provides on/off control, so it may not be suitable.