This project arose from a sudden desire to stop inhaling solder smoke. I just got back from about three weeks of constant travel around Africa with a bad head cold, cough and sore throat. After being away, I was itching to solder up something, but didn’t want to push myself any closer to bronchitis. The solution: a fan to waft away the solder smoke.
I had a bunch of waffle fans removed from computers, but some of them proved very wimpy. One in particular was super-quiet and drew about 200 mA, but was barely able to move air. Since more is always better, I dug into the junque box and pulled out a bunch of fans that I have removed from a commercial server. They were small, but powerful. Alone, each drew about a half amp and when plugged in a 12V battery (or tapped into the bench’s 12V supply), would vibrate themselves around the table. If one fan is good… four is even better.
I created a wood frame with some channels made from angle aluminum. Since the metal impeller blades are like little razors, I stapled metal mesh over both sides. After assembling the whole thing, it occurred to me that all the fans should face the same direction. Luckily, the wood glue hadn’t set by that point, and I was able to disassemble part of the frame, flip around a couple fans, and screw the whole thing back together.
With four fans running full speed, this thing is loud and produces almost enough thrust to take off. So, the interesting part of this project is not the fan itself, but the fan controller. Any sort of rheostat or linear device that would ramp up or down the drive voltage would be very inefficient and produce a lot of waste heat, so I went with a variable pulse width modulator. Rather than reinvent the wheel, I used a design described on the PC Silencio website. Two sections of the quad op amp create a triangle wave, another serves as buffer for a voltage divider, and the final section serves as a comparator to slice off the triangle wave at the level set by the voltage divider. The threshold determines the duty cycle of the generated square wave, which drives the motor at full voltage.
I didn’t have a LM324 in DIP14 layout, so I substituted a TLC271 op amp, which worked fine. For the n-channel MOSFET driving the motors, I went with an STP16NF06 (my old standby, used in previous projects such as the TEAPOT and Texas Topper Amplifier). Regardless of setting, the mosfet remains cool since it’s essentially either on or off and not hanging out in the linear range at all.
In practice, running the fans at near the lowest setting works fine for pulling away smoke from routine soldering. Since the question will inevitably arise — I should mention that in order to solder together the fan, I used a breadboarded version of the PWM circuit to run the fans so I could solder them together — thus getting around this bootstrapping issue.