Archive for the 'Power' Category
This is the second incarnation of my tps61040 based LED driver (here and here). As I wrote just a few posts ago, I’m trying out a new layout strategy to make my gizmos more breadboard friendly.
The 300 mil (thanks Dave) DIP16 package proves to be very small, so small I had trouble trimming it completely while depanelizing.
Another problem I ran into is a high voltage output cap. Seeing that this circuit generates upwards of 28 volts, the typical inexpensive ceramic or tantalum capacitors just don’t have the dielectric strength to work well. So, that leaves few options. Option one involves parallel smaller value high voltage caps. I ordered a bunch of 50v 1uF 0603 caps, so we’ll see how that goes. Second option is electrolytic. Sure I’ll incur some losses in the capacitor, dipping the efficiency a bit, but hey, it’s not a perfect world. I found some 10uf 4.3mm x 4mm caps that should do nicely. Third option is expensive ceramic … weighing in at $1 to $5 ea, these caps must be made of lunar rock. I have not ordered any of these, but I will look into harvesting some from dead / old electronics.
Notice the cute little inductor. That baby is 10uH, 1 amp, shielded and only 6mm square. Designed for high power applications, it has a generous saturation current, and rather low resistance. Even better, it’s only like 2mm tall, and to top it off is the cost; 59 cents each at quantity 10. In case you’re looking for an easy to use and flexible inductor, the digi-key catalog number is 587-1707-1-ND.
This time, in order to have a simple board layout, I chose to permanently enable the chip, so they’re be no dimming on this version. I’m not sure if the chip supports a hot load disconnect, I did manage to kill my earlier prototype somehow, one of the output leads broke off the pcb while I was holding it, in a dark room. After repairing the damage, I only get a very low output. Perhaps my capacitor or diode was fried.
Here are the breadboard compatible pins. The three pins are the output area, with the one inboard pin being the led sink, where the current sensing resistor is attached. This layout required two ground pins, and an external jumper to connect them. I’ll remedy that in the next iteration.
This is the little critter doing it’s thing. Do you like that battery brand? SHAZZAM – it just screams power. I bought a BUNCH of these at a traveling tool sale show, 99 cents for 16. They’re not half bad for light loads, this little switcher sucks ‘em dry in a mater of hours however!
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.
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.
MintLite – The Luxeon Powered Mint Tin Flashlight!
This idea has been rattling around in my head for more than a month now, and I finally have thought it out enough to do some doodling in Eagle. The basic idea is built around a six watt Luxeon K2. I plan to use a pair of 2.5aH lithium batteries to provide approximately eighteen watt-hours of power. The Luxeon will be controlled by a microcontroller, providing different brightness levels, as well as protecting the luxeon from excessive current when the batteries are fully charged. The microcontroller will also monitor the battery voltage; dimming the light as needed and eventually shutting down completely to prevent over-discharge. The light will contain it’s own battery charger, powered by USB using the MAX1811. The MAX1811 will charge a single lithium cell (or cells in parallel) at up to 500mA off a self-powered USB port. The 1811 allows charging from a bus-powered port as well, but for sake of simplicity, I will ignore that option.
The circuit keeps things fairly simple. Switch Q1 provides pwm control of the luxeon. Header SW1 will connect to some sort of switch, for turning the light on and off, and changing brightness. The MAX1811, IC2, takes 4.3 to 6.5 volts as input, and regulates it to 4.2 volts for charging the lithium cells. Charging status is indicated by LED2, which will light when the charger is in bulk charging mode (current mode).The microcontroller, IC1, is a PIC12F683. The 683 provides a lot of bells and whistles for such a small chip. I will be using analog input 0 to monitor the battery voltage. General purpose input 2 will monitor the charging status, perhaps to disable charging when the batteries are in bulk charging mode. General purpose input 4 will use an internal pull-up resistor to monitor the switch. General purpose output 5 is controlling a mosfet transistor responsible for PWM of the led.
The pcb layout is in it’s early stages, and designed mainly around parts I have on hand. I don’t think I’ll actually prototype this PCB, since it’s much too large, and the wrong shape. Whether it gets printed or not, it was fun to draw. There are two main things I want to change. First the FET (Q1) in the TO252 package is rated at something like sixty amps – way more than I need for this project. ON Semiconductor has some nice SOT23 fets rated at 4 amps that should fit the bill nicely, and save a lot of space. Secondly, I need to find a smt version of the usb connector, perhaps a mini usb instead.
Hopefully this weekend I’ll be able to breadboard this circuit and see how it all goes together. Stay tuned for “Part II”.
I’m already up past my bedtime, so just a few pictures for now. Write-up coming soon!
Specs: Input 2v to 6v DC … Output constant current 50mA up to 28v DC
Efficiency: Initial measurements, somewhere around 75%
Chip Texas Instruments TPS61040
size comparison, american quarter dollar piece
circuit detail – design is one-sided PCB with two through-hole jumper wires and the diode, everything else is smt
twin 1uF tantalum capacitors … the output capacitor I had originally selected was limited to 16v, so this was the best I could come up with on a Sunday. Note the top of the coil is missing – these things are fragile!