Mar 12, 2011

RGB LED Cube

I originally intended to have my projects and tutorials be on separate tabs in this blog but it turns out that blogger does not allow that to happen so instead I will have to post them on my main page. 

This first project is a really goofy sort of artsy project I did several months back when I started to seriously get into home projects. The circuit design could not be more simple and the look itself is based on similar projects from Instructables. The schematic is below. 

Figure 1. Schematic for RGB LED Cube

I used a few 4002 diodes to represent the LEDS in the schematic, which is not really accurate because they have turn on voltages around 0.7V where as the LEDS get as high as 4V but for purposes of illustration I think they work just fine. The entire circuit runs off a 9V battery and uses 3 resistors to balance the current through the three branches. The design is horribly inefficient (around 16% by power) but there are few considerations I wanted to highlight. Also, the circuit uses an SPST switch not pictured in the schematic.

First, the resistors are carbon-film 1/8W resistors. I was able to shrink the size using 330ohm resistors 1/8W resistors to drive the LEDS at 15-17mA without overloading their tolerances. The resistors can tolerate up to 125mW of power dissipation and the most any individual branch should achieve at one time is 112mW. Granted, that is pushing the limits of the part but the small size is the design's only real redeeming feature. 

Secondly, the LEDs I used are RGB LEDs that change color automatically thanks to a tiny IC built into the component. With each new color a new turn on voltage is regulated for the LED and that made it tricky to find out the worst case power dissipation of the entire system. Normally, RGB LEDs are driven by controlling the ratio of the three colors our eyes have cones for: red, green, and blue. At the time, I thought doing a full RGB display was too ambitious for a first project so I got the LEDs that did the work for me. If you would like to get the same kind, see my bill of materials (BOM) at the end of this post.

The final product (pictured below) uses a baseball display case to hold the circuit and the accent crystals. I needed something that would scatter the light from the LEDs and some of the projects I had seen online used clear marbles so I figured it was the same basic concept. 

Figure 2. The finished RGB LED Cube

I have also added a video of the cube operating in a dark room. You can actually stare at one for these for quite a long time without getting bored. Overall, it was a quick, fun project to get started with even if the design is lacking.

video 

Bill of Materials
ITEM QTY UNIT PRICE ($) PRICE ($)      LOCATION
RGB LEDS 3 0.16 0.48 eBay
Baseball Display Cube 1 2.99 2.99 The Container Store
1/8 W 330 ohm resistor 3 0.027 0.081 Radio Shack
Clear Luster Accent Gems 1 1 1 Dollar Tree
SPST Heavy Duty Slide Switch 1 1.57 1.57 Radio Shack
9V battery snap connector 1 0.42 0.42 Radio Shack
1.5" Round PC Board 1 1.05 1.05 Radio Shack





TOTAL PROJECT COST

$7.59

APEC: Day 5 Final Thoughts

This is my last night in Fort Worth so I thought I would do a recap of my time here in this entry. First, some awards:

Best freebie: Microchip’s retractable headphones

 Figure 1. Retractable headphones from Microchip

They are not exactly high fidelity devices but they are not bad for a quick and dirty on-the-go solution.


Most interesting presentation: Capacitive Power Transfer for Contactless Charging

Mitchell Kline of UC Berkeley is working on his PhD and chose to do a project on capacitive transfer as a means of wireless power as opposed to the more widespread inductive methods (see “Power Mat” from Duracell). This was a presentation I attended entirely because a friend of mine was going on the last day of the conference. I had little interest in the topic but the presenter was really good at pulling the audience into the research. He had a video showing how an iPhone could be powered through capacitive charging which was nice because no other session I had been to the entire week really showed the products in action. Most of the people presented the theory and experimental results without true proof.

What impressed me most about Mitchell’s project was the control scheme he used to ensure the highest possible efficiency in the power train. It was fairly complex in the theory, but what was incredible was that on the final PCB he managed to implement the controller using AND gates and ICs they had lying around rather than ordering specialized parts. During the presentation he mentioned it in passing, but it was definitely an aspect of his design that blew me away.


Best presenter: Dr. Dushan Boroyevich of the CPES group at Virginia Tech

Fine, call me biased for making this pick. I will admit I had a few close choices for this category and the fact that Dr. Boroyevich basically launched my interested in power electronics put him over the top. However, thanks to the folks over at IEEE.tv, you can watch it and judge for yourself here. Unfortunately, they don’t have the slides associated with his talk which is a shame because they really supplemented his speech and helped convey his ideas. I also picked him as my favorite because he was by far the funniest speaker I heard all week. Dr. Boroyevich is the head of the IEEE Power Electronics Society and at one point he said he was going to give some advice on reading IEEE papers (about 6 minutes into the video). Below is a summary.
  
   
1.)    “It is well-known”
             Translation: I didn’t spent any time to find a reference.

2.)    “It can be easily shown”
             Translation: It’s too complex to be explained in less than 10 pages.

3.)    “Correct within an order of magnitude”
             Translation: It’s wrong.

4.)    “Preliminary tests were inconclusive”
             Translation: It didn’t work.

5.)    “Typical results are shown”
             Translation: Either the best results are shown or the only results are shown.


On content alone I liked what he had to say because it was very much related to my field of interest (renewable integration to the power grid, digital controls, smart power electronics). See 28:30 in the video for a reinforcement of his IEEE writing standards.


Most Disappointing Presentation: AC vs. DC Distribution in the US

I mentioned this in one of my earlier posts, but I wanted to bring it up again because I was so disheartened with how it went. I don’t feel like there was any sort of discussion going on about the benefits of AC or DC. The entire session ended up being about DC standards and arc flash problems with DC grids (I will explain arc flash later). At one point, a student from the University of Illinois tried to get the panel to discuss the benefits and burdens of each and I applaud her for attempting to get something relevant out of the train wreck. Unfortunately, they were more interested in answering a completely unrelated question and rambling on about nothing for 10 minutes. Even with all the presentations about micro DC grids during the week, no one ever spelled out exactly how the grids would work or about how the infrastructure of the US would have to change and that was what I wanted from this talk.  


Final Thoughts

These last few days have been humbling but not overwhelming. To my surprise, the papers presented were, for the most part, completely within my realm of understanding. My time away from school working on my own projects has given me a greater appreciation for the engineering profession and the nature of the engineer. I have completely changed the way I approach a design problem now and I think that is the reason I have been able to relate to the more complex concepts so much better these days. Overall, I would say this was an enlightening conference.

Mar 10, 2011

APEC: Day 4

Tomorrow is the last day in APEC and my last night in Fort Worth. I will do one more entry on the days’ events tomorrow night and recap the week.

Today was another day of technical presentations and exhibitions. I actually ended up seeing some manufacturer representatives I knew from my job and had a chat with them. I am hoping to catch the head of the PSMA tomorrow for a chat on regulatory standards for power supplies. It may not be exciting to discuss standardization but it is my job to do so.

From the technical presentation side, each one lasted about 30 minutes and there were roughly six hours of presentations so I ended up attending around 10 just today. There were too many to name or discuss in detail but they all covered my main interests within power electronics: renewable energy integration, digital control of powers supplies, high efficiency DC-DC converters, and commercial lighting applications. For my current project, I was particularly interested in the LED driver circuitry lectures. There are definitely some interesting papers I need to read specifically related to current balancing in LED strings and maximum power point tracking the photovoltaic systems. Eventually, I want to build my own solar panel and use a tracking algorithm to obtain the most power possible depending on the orientation of the sun (or rather Earth’s orientation around the sun).

I have appreciated the mix of the presentations from this week. Some of the topics of the technical discussions have been purely theoretical while others have actually implemented their solutions. I personally like the approach because it gives me ideas to consider implementing in my next project while also giving me reference material in case I run into problems.

I have also noticed that no one has tried to pass off their research as the perfect solution. What I love about electronics design is that no matter what design you come up with you are going to have to make tradeoffs between one or more (usually more) elements to get the best product possible. In every presentation I have attended thus far, the researcher made it clear the drawbacks to their proposed solution. It is nearly impossible to improve one figure of merit without sacrificing another in electronics design. The question for the engineer becomes about how important those trade-offs are to the end use application, and I appreciated that the audience was made aware of the potential pitfalls of the research.

Mar 9, 2011

APEC: Day 3

Today started off really well. There were over 50 papers presented before 12pm so it was impossible to see everything. The papers are much more specialized than the presentations given by industry leaders over the last few days. I attended two out of four presented by the CPES group at Virginia Tech, including one by a friend I worked on a DC-DC converter design with about a year ago. His research focuses on how to measure core loss in inductive elements (transformers and inductors). Right now, manufacturers present their core loss measurements using sinusoidal waveforms to test the loss. It is not really accurate to take their measurements as fact when designing power electronic converters because most of the time the waveforms passing through the inductive elements are not sinusoidal. The presentation outlined two different methods that could potentially improve the way we measure core loss with the advantages and disadvantages of each. His paper was published in the IEEE archives last December.

The afternoon was mostly about the exhibits. I walked around and checked out the displays but, not looking for orders of 1000+ in my next design, I felt out of place. I did talk with some representatives from Microchip about their digital power development platforms. As I gain more experience with PICs, I may look into their dsPIC series to design some digital control loops for power supplies.

The day ended with a rap session about AC vs. DC distribution. I was very disappointed by the discussion. It ended up being a bunch of old guys talking about codes and standards rather than the advantages of each system or how to implement DC power. They titled the discussion Westinghouse vs. Edison but did not really discuss the history of this debate and how things may be different today. That was the discussion I expected to hear going in but was sorely disappointed. These guys also seem to be masters of deflecting because they rarely directly answered people’s questions and always brought the topics back to their own research/company work. There was another session on sustainability that I caught the end of and even the last 10 minutes were far more interesting than the power distribution panel talk.

Overall, today was mediocre at best. It started off strong and trailed off at the end. Sadly, I expect more of the same until the conference is over. The only thing I am really looking forward to at this point is a CPES initiative presentation on the very last day of the conference. On the plus side, at least I am not sitting at my desk all day at work.

Mar 7, 2011

APEC: Day 2

Day 2 of APEC was far more technical and involved then Day 1. In the morning session, I attended “The Dark Side of Flyback Converters” presented by Christophe Basso. Basso is a French power electronics engineer who has written at least one book. When I was doing my senior design project, I used his book on simulating switch-mode power supplies in PSPICE to generate my transient and frequency response curves. As luck would have it, I was designing a flyback converter supply for the Alienware MX11 gaming laptop which, at that time, had not yet come on the market. Flyback converters are the most common DC-DC topology on the commercial market. About 85% of the power supplies from consumer products use flyback converters because they use the fewest components and are therefore the cheapest solution.

Basso’s talk covered many of the subjects I had read about in his book so I was already familiar with most of it. His discussion of control loops took the basic concepts and went into the fine design and mathematic details. Overall, it was a very enjoyable talk with some dry moments.

After the morning session and lunch, the opening plenary session took place from 1:30 – 5:30 pm. The very first speaker was Slobadon Cuk, a legend in the power electronics community. Dr. Cuk was a student of Dr. Middlebrook, the “alpha” of power electronics. He unfortunately passed away this past year so there was a moment of silence to honor his memory before Cuk began his talk. Cuk himself received his PhD from Caltech and created his own converter topology, known as the Cuk converter. My power electronics professor in college, Dr. Khai Ngo, was once a student of Cuk’s at Caltech so his reach is pretty widespread.

Another notable presenter was Dr. Dushan Boroyevich, president of the IEEE Power Electronics Society and research professor at Virginia Tech’s Center for Power Electronics Systems (CPES). He discussed the implementation of “microgrids” in the United States. The idea is very intriguing because it means a complete revolution of the current power grid. This is actually a subject I will explore in depth in a future blog post, but basically each building in the United States could essentially be its own grid using high and low voltage DC rails. In the current grid design, high voltage AC gets transmitted across power lines to step-down transformers and is fed into the sockets in your house. For most consumer appliances and products, the AC is converted to DC to power the devices and power is lost in the translation. If you scale up these losses for nearly every device in your home, the losses add up. The CPES proposal is to set 1 high voltage and 1 low voltage bus through homes and building such that consumer products can be directly run off DC voltage or converted with DC-DC converters, which typically have higher efficiencies than AC-DC because the voltage does not need a rectification stage. Look for more details on this in future blogs.

I ended my day at the exhibition ballroom. Several companies dealing in power electronics showed up to hock their products to all the representatives. There wasn’t much for me to see since I am only a hobbyist engineer rather than a practicing engineer at the moment. One a sad note, the availability and quality of free stuff was lacking. I will make more trips back in the remaining days of the conference.

Tomorrow, some graduate student friends of mine in the CPES program at Virginia Tech are presenting their research papers so I am looking forward to hearing what they have to say.

APEC: Day 1

So I was lounging around my hotel room and I thought I would do a blog entry per day while I am attending the Applied Power Electronics Conference (APEC). These entries will be more for myself then to convey any concepts or discuss any technology in particular. Each day of the conference is a little different so I am hoping to get a little variety in the entries.

Today, things started at 9:30 am with registration. I took a few pictures of the APEC schwag they gave me in my welcome bag. Most of the packet contained the presentations given over the first two days in three giant books.

 Figure 1. The APEC 2011 Conference proceedings
 
They also threw in a nice looking flash drive with the APEC logo. Apparently, however, it seems its merely a prop because I can’t get my computer to recognize the drive.

 Figure 2. Bogus APEC drive

The first seminar I went to today was called “Introduction to Microcontrollers” with speaker Robert White. An MIT graduate, White now works for Embedded Power Labs designing digital control loops for power supplies. The lecture was about 2.5 hours long and worth every second. I was very surprised to find out how much I already knew about microcontrollers compared to the senior design engineers attending the presentation. White covered most of the topics I have spent the last year or so reading about including: RISC vs CISC, clock speed vs instruction speed, addressing, architectures, and digital controls. For me, this lecture was more about filling the in gaps then learning anything revolutionary. I think I got more out of this lecture though than I would have going into one where I was less familiar with the subject matter mostly because I have practical experience with microcontrollers.

The second lecture I attended was called “LED Lighting: Trends, Standard, Optics, and Power Electronics Drivers”. This was actually a last minute change based on the material I was reading in the books. For the last three months, I had been planning to attend “Using Digital Signal Controllers to Implement Switch Mode Power Supplies”. I wanted to stick with the idea of going to lectures where I have practical experience so I thought LEDs would be appropriate. Ultimately, the lecture was a wakeup to how much I don’t know about LEDs. Designing with these simple devices is far more complicated for even general purpose lighting then I ever imagined.

Lighting makes up about 20% of the national energy consumption in the United States so any efficiency improvement that can be made to the designs could potentially save billions (yes billions) of dollars a year. As of right now, a 7W LED bulb can produce the same amount of light, measured in lumens, as a 40W incandescent bulb with 20% efficiency. The incandescent efficiency is around 8%. The difficulty with the implementation of LED bulbs is the infrastructure we have spent the last 100 years developing. The United States is set to handle Edison sockets in nearly every commercial lamp. However, including power electronic driver circuits and necessary heat sinks into an Edison socket form factor is challenging.


In his lecture, Dr. Brad Lehman from Northeastern University discussed the ways engineers are overcoming these challenges. In addition, he covered tons about optics and discussed his personal research into biological effects of high frequency LED pulses. Overall, it was a bit more dense than the microcontroller lecture but for more eye-opening. When I do a blog on LEDs, I may need more than one entry now.

I am off to a good start during this conference. I actually managed to give out a business card too so that was pretty cool. We will see what tomorrow has to offer. Until then, yvan eht nioj!