Worklog - Re-designing the entrails (February 14 2010)
The thermistors are ready and connected to the Master Controller, so does the 2 DS1621 and the 4 fans as well. Only one thing is left: A long testing time. Before some days, i had another long-term test that did not really took so long to understand that the design of the PC box was completely wrong. So, i have to re-design it.
Engineering loves simplicity because it works!
This is the "old" design of the box. Completely inefficient with heavy thermal loads. A hell for the hard discs. The red arrow indicates the air flow.
The old design had very poor performance and caused heavy thermal loads
The first design was rather complicated. The air inlet and outlet were both on the right hand side of the box. I had 2 95mm plastic tubes inside the box. The two 120mm outlet fans were pulling air from inside these tubes. Two 92mm fans was pushing air in the box. In the middle, i had another 2 120mm fans to help the air flow. This makes a total of 6 fans with a very poor efficiency. From the very first test runs, the performance was very poor. With 22�C ambient, the HDD were cooked. The shell had more than 45�C.
The new design is much more somplier. The air flow is linear and the has less obstacles and more free volume. The inlet is now from the left hand side and the outlet from the right hand side. Two large holes, a about 125mm each will provide the fresh air in the box,and the hot air will be thrown away from another 2 125mm holes. I will run the system with only 4 fans, all 120mm, to check the performance. If i need more air flow, i have 2 120mm and 2 92mm fans to choose from. I believe that i will not need them though.
The inlet fans are placed vertical, and the outlet fans are placed horizontal. I do not know yet why i have done this... Maybe because i was too tired and too boring to make a construction like the inlet, as the outlet fans are simply placed over the holes... I will test this configuration a couple of days and then i will decide if i will put the outlet fans also vertical.
Using the ultra quick - ultra powerful lack leim 380, i glued 2 pieces of chipboard to make the cover for the side
A transparent piece of acrylic glass will be the cover
Between the 2 fans, a small piece of acrylic glass will seal the inlet from the box. The wooden cover can be seen right from the rightmost fan.
When the cover is placed, the fans will draw air only through the holes and push it in the box
Then, i closed the box and sealed it all around. If no overheat is detected inside, i will let it run for a week or so. From time to time, i will record the measurements from the sensors with different fan speeds and ambient temperatures:
I should have mention it in the article. I may add it later on. I add the cap because i could not drive with PWM this fan and get rpm feedback at low rpms. I have face the stall problem myself using other fans. But this fan has not any particular problem by reducing the voltage. After all, i will never drive them below 800rpms. And this circuit can go even lower w/o stalling the fan.
Actually, the problem with PWM was that (maybe you have face it also) from lower to higher rpms, the duty cycle was changed only a couple of steps %. I tried from very low frequencies up to very high frequencies, still the regulation was terrible. That's the reason for this cap.
I have not even had time to build your initial circuit - I\'m too busy supporting our original speed controller and designing a new temperature controller right now but will get to this soon. And when I do I\'ll send you the results.
I see that you have added a 470uF cap across your FET, converting your PWM back to a noisy switching power supply. Can you explain why you added the cap?
We tried a cap across our switching tansistor but found that the motor tended to stall at lower RPMs. But we did come up with a design using a smaller cap & second resistor in the power transisitor drive circuit that stretches the transistor on ramp and reduces the on-pulse kick from a loud growl to very soft pulses. This change would not work with your circuit because you are running at a much higher frequency than we are.
I realized after my last post that my logic behind sampling only when the fan is on flawed. With your higher frequency design, your RPM pulse train is slower than your drive frequency. To make this design work for you, you would need to convert the incoming RPM pulse into a shutter or gate then count the number of PWM pulses riding in on the pulse. And since the PWM frequency is a constant, you can translate the PWM pulse count into RPM.
Yes, i changes the 4049 with an 741 that i have in stock. The reason for the 741, is that i have a lot of them to foll around. I will post the circuit or course.
As for the second post, i suppose that this would work as well. This will dramatically reduce of course the signal reading interval(i read about once a second) because then it would be very hard to keep the rpm low.
Will you try this? If you do, please post the results.
I was sitting here looking at my O'Scope and eating my Dominos Pizza when the real solution just came to me!
Instead of trying to condition the short pulses that occur when the fan is off, you should restrict your speed measurements to only when the fan is on. I'm assuming you can trigger a internal loop on the rising edge of the PWM on pulse? Then you can count incoming tach pulses across a fixed time window and calculate the fan's RPM from the pulse count. You may end up slowing your frequency back down in order to provide a wide enough window to measure your fan speed at all settings. You can even use a tach output pull-up resistor that's tied to your controller's 5V rail, possibly eliminating any need for signal conditioning.
You don't state in the text but I assume you replace the circuit you built around the 4049N with your LM741 based trigger circuit? At least the components on your board supports this.
It's been a long time since I designed anything around a LM741 OP Amp and the design is getting old. Maybe you can design the entire analog side around one LM324 OP Amp or if you want to stay with two 8 pin packages, something more efficient like a TLE2021?
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