Archive for the 'CAD' Category
Testing SMD devices on a breadboard requires some sort of carrier. You can use the dead-bug method, affixing the smd to something, and using bits of wire to solder its tiny pins to larger ones that fit into a breadboard. Another method is using SMD converters, which is fine, but really limits what you can do with the chip, it’s not very portable, and it takes up a LOT of room for very little gain. So, I decided to try re-drawing some of my designs to fit in the footprint of a DIP style package, but be more or less self contained. These self contained modules will work on a breadboard, protoboard or where-ever.
Today’s theme is switchmode power supplies. To start, here is a ’single cell’ to +5v boost regulator, based on National LM2698. This circuit should accept as little as 2.2 volts and provide a solid five volt output. With 3.6 volts in, it should provide over one amp of current. Thanks to the large capacitors, this module resembles a 28 pin ‘wide’ dip, approximately 600 mil across.
This module is also a ’single cell’ to +5v boost regulator, based on the petite TPS61040 from Texas Instruments. The chip claims to support voltages as low as 0.9v, but I plan to use it with a single 1.5v AA. The amount of current it will provide is somewhere around 100mA. It can provide up to 500mA using a higher input voltage. This module resembles a 20 pin ‘narrow’ dip, or approximately 300 mil across.
Lastly, this is the smallest design yet. This module resembles a 16 pin ‘narrow’ dip. Also based on TI’s tps61040, this switcher is configured in constant current mode. My prototype design sources 50mA at 23 volts into a string of white LEDs, powered by two AA batteries.
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I’ve been working with a new design these past few days … I’m trying to build a portable power supply / charger for mobile usb devices. Inspired by Lady Ada’s Minty Boost, I set out to build something a bit more powerful. Perhaps I can call it the Minty XL? Alas, that is not the topic of this article. The charger is based on two “modules” which are discussed here. I felt it was a good idea to build these modules as separate units, so I can breadboard them and test out their design, before committing to have a PCB professionally fabricated for the actual charger.
The first module is a boost converter based on the National Semiconductor LM2698. This converter takes 3.6v from the batteries and boosts it to 5v. The converter should supply at least one amp and perhaps as much as 1.3 amps under ideal conditions. To test the design and layout, I’ve designed a small single sided PCB that will plug into a breadboard using a four pin header.
National supplies the LM2698 as a mini-so8 package, so it’ll be some challenging soldering to do with an iron! Although a 4 pin header was used, there are really only 3 connections. Vi will connect to the batteries or current limiting circuit simulating batteries. Vo is the boost output, which is also indicated by an LED. Two low esr tantalum capacitors provide input and output filtering and some smaller ceramic caps provide decoupling for the IC and a filter for loop compensation. Two resistors form a voltage divider, supplying 1.25v to the feedback circuit of the chip. The coil is a two amp 7mmx7mm shielded ferrite core inductor and the switching diode is just some schottky I picked out of the Digikey catalog.
What good is a powerful portable charger if its own batteries wear down? The second module for my project is a battery charger based on Maxim’s MAX1811 Li+ Charger. The MAX1811 is designed to be a USB powered charger, which seems a fitting complement to a portable USB charger. In fact, if you visit a dimension were conventional physics don’t apply, the device may be able to recharge itself! Anyway, the MAX1811 based circuit is very simple – the chip does all the heavy lifting of monitoring the cell health, temperature and state of charge.
Two capacitors, neither strictly required provide filtering on the input and output of the chip. An on-board LED indicates the charging mode. When the LED is on, the charger is bulk charging the cell, up to 500mA. When the LED is off, the charger is either preconditioning the cell (for severely discharged cells), maintaining the cell, or off. I would have preferred a little more information, but hey, I like simple and this chip is that, the blocking diode is even built in!
Thankfully Maxim supplies the MAX1811 in an so8 package, so it should be fairly easy to solder. This small circuit board also plugs into a breadboard using a three pin header. V+ supplies the charger with roughly four to six volts. B+ is the charger output to the battery, and both the battery and supply share a common ground.
Hopefully this weekend I’ll be able to fabricate these circuit-boards and will toss up a few pictures of the finished product.
Popularity: 31% [?]
The huge size of my previous pcb layout kept bugging me … it was about 1.3×1.2″ and would have consumed more than half of the mint tin space! So, I spent few hours coming up with something a little more compact.
This second layout is more compact, but still a bit on the large size; measuring 1.2×0.9″. I won’t be able to make it much smaller, without going double sided, and thats just not something that’s easy to do at home.
I’m not sure what happened to the weekend, but it’s almost over, and I haven’t even dusted off a breadboard yet. Oh well!
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As sort of a follow-up to my Inket Photolithography article, I have some more pictures of the process, from start to finish.
Using paintshop pro, I take individual PCB layouts and combine them into a composite panel, which is the exact size of the PCB I’m going to expose. Often I’ll repeat the same design a few times, in case there’s some glitch in one of them, or the board is damaged during the depanelizing process. I like to start with a black background for the panel, to mask out any unused sections of copper… this helps cut etching times and helps save the etchant life.
The inkjet transparency comes with a sheet of white paper attached, to protect the transparency and give the printer something extra to grab onto with the feed rolls. It takes about 30 min for the ink to fully dry.
This is my darkroom which also doubles as my house’s furance room. The exposure system is two cheap-o under cabinet lights, with “daylight” bulbs loaded in them. The timing is completely manual and is just an extension cord I unplug when time’s up.
After exposure, the PCB takes a bath in a sodium hydroxide solution to develop the resist layer. Within a few minutes resist that was softened from exposure to light is dissolved away. I rinse the board in hot water (per manuf. spec) to check for complete removal of the resist – sometimes a thin film remains and another dip in developer is required. After all is well, a rinse in cold water sets the resist and halts the developing process.
Here is the transparency used to expose the photosenstive printed circuit board. It is actually laying on top of the board which as already been etched.
The resist is a blue color when it’s “active” and turns green after it has cured, and is no longer photosensitive.
Protecting eyes, ears and lungs is a very important step.
My drilling station is a basic dremel mototool (single speed) mounted in a drill mini-press. I have two 20 watt halogen lamps to illuminate the panel, one is a flood/fill light, the other a tight focus spot right on the cutting head. Using expensive aluminum titanium oxide coated solid carbide drill bits (designed for drilling fiberglass and non ferrous metals like titanium), the dremel easily pierces the board, spinning at 35000 RPM.
Lots of holes!
Make sure your safety gear is still on!
A visit to the impromptu depanelizing saw aka a Skill Jigsaw turned upside down and secured in a bench vice. With the blade installed backward, the foot of the saw provides a nice table for resting the panel on. This arrangment will gladly take a finger or at least mess you up, so make sure you have no distractions and always know where your fingers are while the blade is moving. Sometimes you’ll end up with pieces that are pretty small, but it’s not worth the risk of serious injury trying to do them on the saw… scoring both sides of the pcb with a drywall knife should let them snap cleanly… also I’ve heard large sheer-type paper cutters work.
The end result, a pile of little PCBs.
Sometimes the saw doesn’t leave the cleanest edge or your line is a bit wavey – a visit to mr belt-sander will clean things up nicely.
editors note: some of the pictures for this article didn’t come out as well as I had hoped, or have yet to be taken, so the [image missing] tag is a place holder to remind me to reshoot.
Popularity: 58% [?]
I may be easily amused, but I’m not easily impressed.
Everything I have read about pcb prototyping using photolithography claims I needed to use a laser printer and some sort of transparent or translucent medium. Armed with this knowledge, I used acetate, or overhead projector transparencies. The film claims to be designed for laser printers and copiers, but it still distorts some when printed on. Anyway, the problem with my laser printer is toner pin-holes. For whatever reason, the output would have these little holes everywhere, and these little holes would swiss-cheese my traces and ground plane pours. This forced me to use thick traces, which would still get swiss cheesed, but generally retained enough composure to be electrically conductive. The pin hole problem seems to be getting worse … the last test I performed on my printer as an exposure test of varying width lines and different sized holes and pay layouts. In general, the performance was pretty bad and was not acceptable for more advanced designed I wanted to make, involving very small SMT components.
So, I tried printing the same exposure test, on paper, to my inkjet printer. The results were stunning. The lines were sharper, the holes clearer, the pads better defined. What a difference it makes going from a 600 dpi laser to a 4800(?) dpi inkjet. So, I felt it worth the risk, and decided to run my exposure test with the inkjet. Using some inkjet transparencies (they’re kinda coated with some type of fiber?), I printed my patterns. I exposed my board, and then developed it… the results were WOW! Of course, I messed up a few things with this first run, mostly I let the light cook for too long. So in the best un-scientific manner possible, I changed a bunch of variables at the same time, and tried another pass.
That is the result! … Please ignore the greasy thumbprint on the left side of the board – that was acquired after etching, and has no effect on anything aside from mild embarrassment on my part. My setup was fairly simple. I printed my design at best quality, monochrome mode onto a transparency. Next, in my darkroom that doubles as a furnace room, I laid down my PCB, emulsion up, then the transparency, then a sheet of 1/4″ plate glass. About 4″ above that, I have two 15 watt under cabinet lights, each loaded with a GE Daylight bulb. The lights are connected to an extension cord, so I can turn them on and off together. After making sure everything is lined up, I start my stopwatch and plug in the lights. After letting it cook for eight minutes, I turned off the lights and slid the pcb into a waiting bath of developer. The developer had been sitting for about a day, so it was a little slow. I left the board sit for about 2 min, before turning on the room lights. As the emulsion started to dissolve, I could see the results were good, real good! With the room lights on, I stirred the developer and lightly brushed the pcb using a foam brush (as recommended by the manuf.) My image grew sharper and sharper.
After I was sure all the emulsion that needed to be gone was gone, I rinsed the board and slid it into a waiting bath of ferric chloride etchant. A few min later, I pulled the pcb out, to make sure all the copper had turned pink. If the copper is not pink, it means some emulsion remains, and its time for another trip to the developer. Fortunately all the copper was pink, no problems here. I let the pcb soak for about 30 min while I had some lunch. After lunch, and without washing my hands i might add, I extracted the PCB from the etchant and rinsed it off. The results are excellent.
So, no longer will I shy away from tiny SMTs, since I can now lay down traces as thin as 8 mils without issue (determined by the earlier exposure test). Granted there were three flaws I had to fix on this board, I suspect they were caused by either dust on the transparency or those fibers that are embedded in the plastic. A quick touch-up with the sharpie solved them without a hitch.
Next stop, de-panelize with the PCB “suicide” saw.
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