I completed a fun little electronics project yesterday: a small solder fume extractor based on a PC fan. I've been calling it the Solder Snorter, in homage to Jon Thomasson's Solder Sniffer 9000 — this thing is more or less a redo of that idea.
The thing is really simple: an 80mm PC fan (runs off 12V DC), a lithium battery (a single protected 18650 cell), a battery charging IC (Microchip MCP73831), a boost converter to convert the nominal 3.7V from the battery to 12V (TI TLV61046A), the passives needed for those two, a charge indicator LED, a switch and a USB port for charging. Nothing to it! The goal was to learn a bit about batteries and battery charging, and a little about power conversion.
I took a sneaky peek at the schematic for Jon's Solder Sniffer, and I got some ideas about battery charging and power conversion from Chris Gammell's Contextual Electronics videos. Then I just dove in.
Selecting components for the battery charger and boost converter took a long time. There are a lot of choices, and I had very little idea what I was looking for to start with. I settled on the MCP73831 because it's the simplest of that range of Microchip chargers, and I'd seen Chris using a similar part in one of the Contextual Electronics videos. For the boost converter, I looked at a lot of datasheets, but settled on the TI TLV61046A because it's very simple to use, and even has a special default 12V DC output mode, which is exactly what I needed.
The schematic ended up being disappointingly simple, given the amount of time it took to get to that point. Most of it was a matter of copying the application circuits from the charger and boost converter datasheets.
I did make one mistake there though. The
STAT output from the
MCP73831 is low when charging and high when the attached battery is
charged. However, it also has a high-Z state, when there's no battery
connected. Sharing R1 between LEDs D1 and D2 is then a bad idea, since
STAT going high impedance connects the LEDs in series between the
positive power rail and ground with no current limiting resistor.
Fortunately I noticed that I'd done this before assembling the thing,
so I just didn't populate D2 and everything was fine.
Layout was quite interesting, because both the charger and the boost converter had layout recommendations, which was something new for me, and the recommendations were based on a lot of copper pours, which I'd not used much in KiCad before. It looks like this:
The board screws onto the mounting holes of the fan, which is what the little semicircular tabs are for. There's a Micro-USB connector for charging, a switch, the battery is mounted on the back with a couple of battery clips, and more or less everything else is following the recommendations in the datasheets. Making all the pours was fun, as was making the arrays of thermal stitching vias.
I made two mistakes in the layout:
I ended up without soldermask over all the stitching vias. Didn't matter for this, but it's something to remember in future.
I removed the thermal relief for all connections to the copper pours. My reasoning: the pours are there for thermal reasons, so why would you want thermal relief? It turns out that it's pretty hard to solder parts down when you have big unrelieved chunks of copper on the board like this. There's a balance between thermal properties and manufacturability, which I'd not thought about. Will do better next time!
Assembly was relatively straightforward, apart from the thermal relief problem. The most difficult part to solder was definitely the Micro-USB connector, which I did first. That worked quite well: tin all the SMD pads, hold the part in place with tweezers, blast it with the hot air pencil until reflow, then solder the through-hole pins for a solid mechanical connection. It worked more or less first time, and I ended up with a solid connection without getting lots of solder inside the connector.
Testing was simple: does it charge a battery? does it stop charging when the battery is full? does is make the fan spin when you switch it on? That all worked, which was a definite result.
It was in some ways a pretty trivial project, but I learnt some useful things. And I have a little fan that sucks streams of nasty solder fumes into its carbon filter, so I'm happy with that!