HeaterMeter 'Lite' project help?


 
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Dougie_Hess

New member
Hello everyone,
Frequent lurker but new member here. I signed up to get some input on a small hobby project I have started that might be considered a “HeaterMeter Lite”. I started this project as a way to learn about microcontrollers and electronics, and if I can make something useful then all the better.

Here’s the idea. The HeaterMeter does a great job with temperature control, remote temp monitoring, temp logging, etc… But I already have an ET732 for temp monitoring (as do many others) and all I really want is temp control. So I set out to design a blower system that picks up the ET732 radio signals to monitor temperature and PID controls the grill temperature from that.

I have successfully captured and decoded the ET732 radio signals using a cheapo 433mhz radio receiver and an Arduino Uno (or a Tiva-C Launchpad). Temperature packets are sent in 4X replicates every 12 seconds. From there I can write code to do some error checking and decoding to get the grill temperature (and meat temp, if I want).

I bought a 12v brushless blower fan off ebay (model HT07530). It is a similar package to the HeaterMeter recommended one, but not exactly the same. I am hung up on getting good PWM control of this fan at anything less than ~95% duty cycle and this is where my knowledge of electrical circuits is failing me. I have tried various circuits involving a low-pass filter on the 5v PWM input, a TIP120 transistor, a IRF510 MOSFET, and/or a capacitor across the blower terminals. Still the fan stalls at lower duty cycles. So first question, for those more knowledgeable in such things… Will the circuit in the HeaterMeter schematics using a BC337 and a FQU11P06 likely work for this fan, or should I just suck it up and get the same fan everyone else is using? Also, any alterative to the FQU11P06 that I could get at Radioshack?

Second question is about PID update frequency. If I can only get temperature updates every 12 seconds form the ET732 radio signals, is the interval going to be too long to effectively control the grill temp in a PID loop?

Finally, I built a 3D printer a while back and have seen some really great blower / servo damper mounting designs posted here. It seems worthwhile to integrate a servo damper into my design. In terms of operation, could I just use the damper to regulate air flow by letting the blower run at 100% then PID tuning the damper, or is that going to be too taxing on the blower motor?

Sorry for the long post… glad to finally be a member! Hope I can also get some insight into high altitude grilling (I’m at 7200 feet), but that is a topic for another thread.

Thanks!
Dougie
 
I had similar problems getting a linear response curve out of the blower. Here's a graph from 2010 using an IRF510
fanvoltages.png


As you can see, the capacitor across the fan makes all the difference, but that value depends on a lot of factors but mainly I think fan draw current. You just need to find something that works for you. The fan shouldn't make a difference in if it works or not. You can also see here even with the HeaterMeter v4 series the problem of linearity still exists (until TruVolt(tm) mode)


I don't think Radio Shack carries a P-MOSFET with similar specs or any power P-MOSFETS at all.

12 second input is fine. The temperature isn't changing that quickly. HeaterMeter folks tend to see 5-10 minute periods per cycle.

I don't know if you can just leave the blower running. Maybe? You could make it so that there at least are some small vents or something before the damper so at least a small amount of air is always passing through the blower. I don't think it matters though.
 
Great! Thanks for your input Bryan. I'll mess with the fan circuit a bit more and see if I can get a decent range of output using the IRF510. Willing to move to a P-MOSFET if I need. A static starting routine to spike the voltage prior to setting a lower duty cycle may help as the biggest issue is starting up the fan from a dead stop.

I've already tried a 22uf and 47uf capacitors, neither allow me to drop below ~90% duty cycle. Have you tried changing the PWM frequency on the Arduino to help smooth the voltage (ie. PWM Fast mode)?
 
What problem are you having, is it stalling out below that duty cycle or is it running too much? You can also try a different topology, as opposed to just putting the capacitor over across the fan leads, you can also put the positive capacitor lead on the negative fan lead and the negative capacitor lead to ground.

I played with other PWM frequencies and the default gave me the most control. That's where you start to get out of my depth as to why it doesn't work. Something about the fan RPM vs the PWM frequency I'm guessing. Phase-correct vs Fast PWM makes no difference because they're both pretty bad for motor control.
 
I'm running into both problems. Using a IRF510 with a 22uF cap I can run at ~80% duty cycle only if I come down to that value after starting at 100%. From a complete stop, I cannot start the fan at 80% power. Running anything less than 80% will stall the fan regardless. The range of output of the fan is pretty good between 80% and 100% PWM using the 22uF cap, but doesn't slow down much using a 47uF within those same ranges. What I need is to be able to run the fan at a much lower speed overall, as even at 80% I think it'll be too much airflow... maybe that just isn't possible with this blower.
 
Rebuilt my fan circuit today and got it working. Turns out I need glasses as I was using 2.2uF and 4.7uF caps instead of 22uF and 47uF. The 22uF cap along with the IRF510 MOSFET works much better. Not even close to linear response, but I can hit at least hit the lower speeds I need.

Thanks for your help Bryan. I'll update when I progress a bit further.
 
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