Driving small brushless DC "cooling fan"

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I'm planning a project where I will be driving a small brushless DC "cooling fan".

[This is so important that I should put the "HELP!!!" and URGENT!!! in the title, as this is for air-flow control for my outdoor cooker (UDS -- Ugly Drum Smoker). I built a 16-gallon.]

I (in all probability) need very little "push" on the air flow. I plan to adapt the business end of the fan through couplings and into one of the four ~3/4"/19mm air holes at the base of the smoker that allows combustion air to enter. (A normal cook at 220-230 degrees F has 1.5 holes open.)

I've found the fans as low as US$1.08 each on eBay from China, shipping included. Anyway, absolute cost isn't critical.

I've been looking at 25mm and 40mm units, two wire, 5V and 12V. The rating is like 100mA give-or-take.

Q1: I should be able to vary the drive voltage to control speed, right?

So, if I use AVR8 PWM, what filter do I use between the AVR and the fan to get a rough approximation of DC voltage? Or if I use a fast PWM (tell me what "fast" is --1kHz? 10kHz?) can it just be done directly?

Q2: As the 5V fan is rated at e.g. 100mA but I only want to drive it at say 1/10 speed, what will be the power draw?

Q3: I was thinking of just using a pair of AVR pins from the same port with current-limiting resistors (which then serve as part of the RC filter?), and driving direct. Bad idea?

Q4: Or do I use a MOSFET? If I do, then do I want fast PWM or slow PWM?

As I anticipate that very little extra push will be needed from the fan, I was thinking of doing very coarse bang-bang control. Like "on for 1 second out of 20" for lowest setting; 2 seconds of 20; etc. Good or bad idea?

I might do this part first for feasibility. The plan is to integrate a thermistor or PT100 probe for temperature setpoint and closed-loop control.

Awful lot of work for a UDS, eh?

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Just use the fan out of an old pc and a lab power supply. It'll read out the amps and everything.

Imagecraft compiler user

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I'm not a motor expert.

That said, I don't think you want to convert the PWM to DC (and ripple). If you convert to DC then you need a linear driver. That would be old school, a big old 2n3055 power transistor on a big heat sink. Note, however, that that design would work fine.

You don't want to directly drive the motor with the uC's I/O pins, unless you are going to parallel a full port. Doable. I would put a small load leveling series resistor on each port pin. Watch the max current per port, per V+, per Gnd, etc. limits in the footnotes of the electrical specs for the uC chosen. Note that the full drive current has to pass through the filter's resistor ( s ) if you do this in an analog drive manner. You really don't want to do this.

You can use PWM to drive a low side NFet. I think this is the easiest approach.

I think you might want a "smart" motor controller, even for a small fan. The fan won't work well at low PWM duty cycles, and the motor might stall and not run. I suspect you might want to start the motor at 100 % to give the motor some current to get it rotating, then "quickly" back the drive signal down, when you want a low level drive.

The faster the PWM freq the more times / sec the NFet has to switch through the linear phase, during which time it generates heat. Run it too slowly however, and you can hear the circuitry whine. Like Goldie Locks, the PWM frequency in the middle was just right.

You did not mention if the fan has an RPM output signal you will be tracking, or if you plan on adding one with an LED and an photodiode. You then know if the motor is spinning, and how fast.

I think it will take very little extra air flow to significantly increase the fuel burn rate and the heat being generated.

JC

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PC fans generally are PWM controlled at about 25kHz. Some of them include the FET switch, and can be controlled with an open collector I/O. Look for fans with a 4 pin PWM connector.

- S

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just use a npn transistor on a 12V computer fan, and turn it on off with ? 1/5 sec.(no real PWM needed), perhaps you start it that way, but run it direct from a 20mA AVR IO, (12V fan can normally run on 5V but not always start).
if you yse a pt1000 you can use the ADC direct (not so good with a pt100).

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Quote:

You did not mention if the fan has an RPM output signal you will be tracking,

Not if I go with the $1 fans. ;)
http://www.ebay.com/itm/16108736...
Quote:

you yse a pt1000 you can use the ADC direct (not so good with a pt100).

Indeed, I totally understand. Some UDS builders spend many hours and sandblast and add real pipe fittings and globe valves for air regulation and ...

I decided on this first one that it was going to be minimalist. After much networking, I picked up the drum for the cost of a drink. Four holes in the bottom for combustion air---not very costly. The lid is the type that has "ears all around, with a bunghole in the middle. Used as-is for lid and exhaust.

One new "Smokey Joe" grilling replacement grate from Home Depot. That was less than $10. As my food goes on it I splurged there.

OK, back to the PT1000. I've done several RTD apps and understand what you are referring to. Now, I wanted temperature feedback. First thought was a SS thermistor probe. With a decent cable, over $10. On eBay I found lots of PT100 at China prices; one type has a strain relief and armored cable. So I'll probably go with that and spend a bit more effort on the electronics.
http://www.ebay.com/itm/13055524...
http://www.ebay.com/itm/31092291...

Remember that the end result ain't brain surgery. ;)

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Above I described how I built a UDS "on the cheap". those readers who saw my recent post here about GCLD may be aware that I've got several TFT displays coming for exploration. So now I should be able to plot on the GLCD the temperature and airflow over time, and ... No effort too great when it comes to AVR apps.

BTW, we have here a surplus of 6V SLA batteries that are pull-outs during one of our controller builds. So I know what my power source is going to be. Next thread will probably be about picking an SLA charger design. ;)

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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You're aware of the Circuit Cellar project from May 2003 by John Moyer?

Thank Atmel for hosting a copy of the project.

http://www.atmel.com/Images/controller_3_04.pdf

Stan

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Quote:

You're aware of the Circuit Cellar project from May 2003 by John Moyer?

Thank Atmel for hosting a copy of the project.

http://www.atmel.com/Images/cont...

Stan


No, I wasn't aware of that. About what I'm planning to do. Notes:

1) Indeed, the "ceramic cooker" is better insulated and more thermal mass than my bare metal drum.

2) I outlined my miserly approach to my UDS build. Maybe $50 and a few hours? Popular ceramic cookers include the Big Green Egg line from about $500-$1000 to the Komodo at several thousand dollars.

3) It sure would have been nice if the Circuit Cellar article had the correct schematic, and not one totally unrelated. Maybe in the reprint it got messed up?

4) Indeed, my [exploratory] choice of the PT100 is somewhat arbitrary. The author used thermocouple9s). We've done production controller apps with thermistors at higher temperatures, RTD, and thermocouples so none of them are really scary. IME thermistors are a bit tricky at higher temperatures. I wanted a probe with a probe a few inches long, to reach toward the middle a bit for a more usable reading. If I extend the design to a second probe to put into the food itself, I want stainless for cleaning and food safety.

5) Digital temperature monitors such as the popular Maverick ET-732/ET-733/ET-73 have a common complaint on probe failure.
5a) A "nice" Maverick model is about $50 and about $100 for wireless. Ignoring my design/dev time, I figure I can get my temperature monitor -- and controller -- for that amount. Links above are for a $5 PT100 probe and $1 fan. Would it be easier to buy one? Certainly. But don't all the newbies post that they want to write there own character LCD library from scratch because "they will learn so much more"? :twisted:

6a) I have a wealth of already-built AVR8 boards in dozens of designs around the office/shop that are considered "surplus". Many will have the subsystems I desire for this app. Some have wireless modules. SLA batteries I mentioned earlier. Enclosures are around as well. In the end, even with closed-loop control, it is a simple low-current output circuit for the fan, and a few temperature inputs. pretty much everything else is "gravy".

7) Back to the article: Maybe because of the nature of the well-insulated ceramic cooker but that is an awfully flat temperature curve for on-off control, don't you think? In my unit I don't expect an immediate (or even within one minute) change to the new stable temperature.

8) Before corporate right-sizing, my outfit >>made<< temperature controllers as well as re-marketed some models, e.g. http://www.ebay.com/itm/HANYOUNG... All I have to do is dig one out of the junk boxes, attach a J- or k-thermocouple probe, and hook my DC fan to the output. No fun at all. ;)

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Quote:

3) It sure would have been nice if the Circuit Cellar article had the correct schematic, and not one totally unrelated. Maybe in the reprint it got messed up?

Indeed, Atmel didn't do itself any favours. I found a copy of the article online with the correct schematic:
http://faculty.petra.ac.id/resma...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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3) It sure would have been nice if the Circuit Cellar article had the correct schematic, and not one totally unrelated. Maybe in the reprint it got messed up?

Oops, I should have caught that. Was trying to post the schematic here when my DSL internet connection died.

For control of a larger fire, two individually controlled fans, a larger and smaller maybe, would have better control if necessary.

Years ago, I made a smoker with two 55gal drums stacked, fire in the bottom, three shelves in the top. Not insulated, took a lot of oak to smoke five chickens. This was not as efficient as it's model from my uncle's arrangement, where several, ganged, full-height large gym lockers had 3/8" rod run through to shelve trays. Oak fires at the bottom, doors were opened periodically to rotate trays up. Enough chicken for over a hundred. The chicken was great, neither system was used again.

Stan

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Quote:

neither system was used again.


lol

The "upright drum smoker" or "ugly drum smoker" is a lot simpler arrangement and build than any type of offset smoker (according to my reading).

Indeed, the construction took little time:
-- Four 3/4 holes near the bottom for combustion air
-- Exhaust vent needed; my lid with "ears" had the bung-hole for oil pump in the middle, so "done"; no construction needed
-- A way to support the cooking grate
-- A suitable charcoal basket. I bent expanded metal into a circle, added expanded metal bottom, wired them together with fence wire ;) , more fence wire for the handle (the most time-consuming part)

I've now done about half-a-dozen cooks. For target temperature in the chamber of 220-230 F, leave open about 1.5 of the four holes. I use 1" diameter magnets to control the air flow. So putting one tiny fan into one of the holes should (I speculate) give me plenty of air with only an occasional puff from the fan. You can fuss with it every few minutes but it is best to walk away.

I put my charcoal mixed with fruitwood chunks in the coal basket. There is an empty tin can in the middle, and a unrolled tin-can barrier from the middle to the side. A gap is left next to the metal barrier, and about a dozen well-lit coals are placed there. The fire burns itself around the circle. About 8 hours of burn for one load. That isn't much charcoal, really -- 12" diameter basket, about 2 to 3 layers deep.

"Set it and forget it."

[UDS is most often a 55-gallon drum. "Mini" UDS is often a 30-gallon drum. I made a "micro" with a 16-gallon drum. I have an empty nest so the capacity is fine for now. I might do a 55-gallon at some point.

Interestingly, there are three sizes of the popular Weber charcoal grills. The lid and cooking grate for the 22.5" fit a 55-gallon drum; the 18.5" fit a 30-gallon drum; and the 13" Smokey Joe fit the 16-gallon drum. Which came first, the Weber or the drum?]

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Smoking is bad for you unless you are smoking ham in which case it's bad for the pig!

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:
more thermal mass

I've got a grill out back, and the wife cooks on it. I don't know much about this topic at all, but:

My grill has lava rocks lining the grill. I replace these every other year or so. I think their purpose is to increase the grill's thermal mass. Easy, cheap.

Question: Do you just blow some air into the bottom of the grill, or are you running a pipe with holes in it the length of the grill, and distributing the air? I would think that would give a more uniform fire, more uniform combustion (usage) of the fuel, and more uniform heating. It would be analogous to the pipe my gas grill has for dispersing the gas and flame.

Lastly, are you attaching the fan directly to the metal frame of the grill? Will it tolerate the heat?

I had envisioned a pipe leading from the grill, to the fan, perhaps with a ceramic pipe insert, (thermal insulator).

But hey, I can make any simple project more complex!

JC

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You're cooking your wife? :shock:

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:

Do you just blow some air into the bottom of the grill, or are you running a pipe with holes in it the length of the grill, and distributing the air?

Well, the first I guess. Four holes in the bottom of the barrel to allow combustion air to enter. As I mentioned, the fire burns in a circle so only really burning on the frontier.

Quote:

Lastly, are you attaching the fan directly to the metal frame of the grill? Will it tolerate the heat?


While I am indeed planing to use some conduit to separate, remember that the intake area below the fire isn't as fiery hot.

Like this one http://www.bbq-brethren.com/foru... swamprb posting, little black one next to the red 55-gallon. I eschewed the paint job.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Here is another idea you can experiment with.

Stick a 1" wide by X inches long metal bar through a small slot in the wall of the container, so that the bar sticks inward to the center of the container, just above the fire.

A short tab sticks outside the container.

Mount it on a couple of ceramic or quartz or whatever standoff / thermal insulators.

Now aim an infrared thermal sensor at the part sticking out to measure the temperature. Given the high thermal conductivity of the metal, the tab sticking out will be just under the temp of the part mounted hanging over the fire.

The infrared thermal sensors are cheap these days.

You can buy the entire hand held infrared thermometer for $25 at Lowe's or Home Depot last I looked.

Totally avoids the temp sensor and low offset op-amp, etc.

JC

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Spark Fun sells the MLX90614 Infrared Thermometer sensor for $20, of the commercial hand held thermometer for $28. Not where I'd buy these these days, but it gives you an upper limit to the cost, and a starting point to work from.

Digikey sells a bunch of the sensors, $12 - $31 USD.

JC

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Cool....

I am building a sort of similar device. But for me it is to get additional airflow through my caravan fridge. It has been replaced by a table top model( the previous owner did that to save money) Now I use 2 8cm fans to create an airflow over the thermal exchange unit (is that called the heat-exchanger ?). I have looked at a large number of schematics to for the same motors. It seems to be best to have a large FET in the negative connection and a large CAP/ELCO over the motor with flyback diode. Then if you PWM the FET you should be able to control the resulting DC voltage on the motor hence controlling the speed.
The problem here seems to be how are you going to detect if the motor is running at all? If you set the voltage to low the motor will stall. Your control system will not see this. and as you need a lot more voltage to get the motor running again you would need to turn it on briefly max voltage. In the end I just bought 2 fans with PWM and Tacho here the difference was only 1 or 2 euro so the extra investment was worth it for me. This also ensured a simpler design. no need for switching FETs. All I now have is a number of resistors to convert the tacho signal to the right voltage and the PWM that goes directly to the FAN. INthe past I have once biuld a suplpy with an LM317 and a transistor that adjusted the output voltage through changing the feedback resistors value. BUt then you need to ensure that the transistor is really DC fed, so you need at least a double RC filter on the base of the transistor to keep the supply stable.

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Quote:

The problem here seems to be how are you going to detect if the motor is running at all? If you set the voltage to low the motor will stall.

I'm planning to start with a "duty cycle" approach -- Divide up 20 or 30 seconds into one second intervals. Turn on the fan for n seconds of duty during the period of 20/30 seconds. I'm guessing one or two seconds per 20 will be about right. But we'll see.

This sets aside for the moment the question you raised about too low a voltage.

We have an app with a 12V fan. In continuous operation, it is run off 5x AA, or about 6-7V. It happily runs down to less than 3V, which is 1/4 rated. I don't remember testing at very low voltages for "stall" voltage.

Thanks for the circuit suggestion.

Re the infrared idea: Interesting. But if I'm browsing thermistor and thermocouple and RTD probes at $5 or less, and we already have the parts and knowledge and probably circuit boards, ...

Probably the simplest would be to find the right thermistor probe, which has a "close enough" to log curve R vs. T. Couple with an inexpensive eBay thermistor controller that compares against a pot. Use an appropriate log pot and Bob's your uncle, right?

How many ways can we skin this cat? ;)

In the end, I plan to first rig up the fan. Then connect to an AVR board with the 20 or 30 second duty cycle, maybe divided into 100 parts. Have it turn on the fan for that many parts of the duty cycle. Monitor the temperature, and see if this approach keeps a fairly steady temperature. So while the air flow is now computer-controlled the "setpoint" is manual--just as I now manually adjust the open area of the intake holes.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I'm the cautious type, but I've heard galvanized steel gives off toxic vapors. I have no idea, but might be worth checking if your drum is galvanized. Suppose I'd also consider what was stored in the drum and might leave a poisonous residue....

Edit:

Missed this, but it's worth reiterating.

Quote:
I want stainless for cleaning and food safety.

C: i = "told you so";