# Designing a linear speed fan amp control for a smoker

Discussion in 'The Projects Forum' started by mr x, Dec 31, 2010.

1. ### mr x Thread Starter New Member

Dec 31, 2010
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Hello there, I'm so rusty at this stuff I squeak. Quick into, I graduated from a CET about 20 years ago. Back then, I could build circuits in my sleep. Since then, I've moved onto industrial instrumentation, and my circuit level electronics knowledge is long gone.

The project I need help with is a smoker temperature controller for my Primo XL. The simplest and most well known option is a small fan (around 10 cfm) to provide the intake draft, with feedback and control provided by a PID temperature controller.

Generally, the 12 Vdc fan is run in the on/off mode, but this presents a few issues. I would prefer a linear control, but that has problems as well. Is it possible to design an amplifier that will take the output of a PID (1-5, or 2.4-12 vdc), and allow me to set the amplifier output to the fan at an adjustable lower voltage (because it probably won't start at 2.4 volts) up to the maximum 12 vdc?

2. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
your talking about altering the P component of the PID loop, possibly to a degree of negating the benefits of full PID control. I would rather look at PWM for motor drive, which would allow you to maintain your loop control, if you thought the application could benefit.

3. ### mr x Thread Starter New Member

Dec 31, 2010
5
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The PID algorithm in the controller is a separate calculation, so the P will still do what it should do within that output calculation. I just need some sort of offset adjustment on an amplifier circuit, I guess, to account for the fan start-up voltage probably not being 0V dc. I would like to be a able to adjust that bottom voltage so that the 0% output from the controller (4mA, 1Vdc or 2.4vdc to the amplifier input) is 0Vdc out of the amp, and 1% becomes a voltage of 3-6 Vdc (or whatever the startup voltage for the fan is). And 100% is 12vDC.

Last edited: Dec 31, 2010
4. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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A smoker is going to operate at a fairly high temperature so you may need to stick to one of those large thermistors that isn't encapsulated or a simple thermocouple you can get from any heat/air wholesale parts supplier.

With a little creativity this IC might work for exactly what you're describing. Regardless it's a good read as far as a resource goes as you're thinking about it:

http://www.mouser.com/Search/Refine.aspx?Keyword=TC642

I'm using these in a design but I've pretty well got the US market cornered on them and they aren't building any more until late February next year and can't even guarantee that they'll have any extras, however if you can't use one of the others I'll be glad to sell you one for \$2.50 including proper packing and shipping provided you're in the US, if you aren't I can always look up postage to other countries.

Last I saw Farnell has 50, Mouser has 2 and I cleaned out Digikey.

Pretty simple circuit, voltage in = PWM out.

5. ### mr x Thread Starter New Member

Dec 31, 2010
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OK, I happened to dig up a controller that I can cycle at .1 sec. I think that should be quick enough to do the PWM. Not what I wanted, but it saves parts and money, which isn't growing on trees at the moment. But I'm still not sure it will work. I was looking at this idea (remove Arduino and insert PID):

But I'm having trouble with the controller output connection. It is a 120Vac model (Watlow 965) with a switched DC output. The V+ above will be 12Vdc from a transformer. If anybody has a moment, take a look at the top of page 10 for the output circuit design (sorry it's a .pdf and I can't C&P), Switched DC Output, and let me know what you think. Thanks again.

6. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
if you wanted to use the linked controller to control your heating element, all is good. As for controlling a blower, not good.

7. ### mr x Thread Starter New Member

Dec 31, 2010
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That system will work. It's just a temperature feedback loop. I may just break down and buy an SSR off ebay to bump up the load capacity. It's serious overkill, but I know that it works...

8. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
it will work, as I've used similar on air cooled plastic extruders (blowers). However the output wasn't modulated on the cool cycle but rather an overshoot.

9. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Use a far cheaper 12V computer fan many already have a temp control built in that relies on an external thermistor thus easily modified. Simple adapter can provide your 12V.

10. ### mr x Thread Starter New Member

Dec 31, 2010
5
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That won't work from the fans I have seen, as there is no way to adjust the temperature. And draft temperature control is pretty touchy stuff, particularly where control is most useful - at low temperatures (250f and under)..