Monday, October 25, 2010

Falling in Love with Linux again

I've had an off again on again relationship with Linux over the years. I've played with quite a few different Linux distributions including: Slackware, Fedora, Knoppix, Debian, Damn Small Linux (on a very old Laptop), and settled on Ubuntu.

Maybe it is because I am a bit lazy and don't care to install everything via a command line and hack into config files to get my computer peripherals to work, that I like Ubuntu so much. Ubuntu, for the most part, is a very easy installation and adding additional programs (packages) is also very easy.

The open source concept of Linux is very romantic to me; it is a neat way to share knowledge and skills and not have to fork over a bunch of cash to try new development tools.

Some of my favorite Engineering programs that run on Ubuntu are:
- Element14/Cadsoft's EagleCAD Schematic & PCB Layout tool
- Qucs Circuit Simulator
- Linsmith RF Smith Charting tool
- Scilab (MATLAB like tool)
- Wine Windows software emulator, which will run LTSPICE very well.

I am not all business either, I love the free games available on Ubuntu as well. Super Tux, an old school 2D Mario World like game, is my favorite, the FreeDOOM first person shooter brings back a lot of memories of Collage LAN parties as well.

Sunday, October 24, 2010

Derating Tantalum Capacitors…. Why?

A few weeks ago I was at Arrowfest, an annual electronics training event put on by Arrow, and the Kemet Sales Engineer started talking about derating Tantalum Capacitors……

The thought of “derating” always makes me wonder “Why don’t the Tantalum Capacitor manufactuer’s just mark the proper usable voltage on to the parts?”,  that way the embedded designers wouldn’t have to rely on rules-of-thumb or safety-margin  guidelines.  A history lesson is required to understand why……

In the 1950’s the need for new Lower ESR Capacitors spawned Bell Labs to invent the solid-electrolyte Tantalum Capacitor, and drive Richard Millard of Sprague Electric Company to further patent the improved “reform-step” manufacturing process in 1955. These Tantalums offered < 2 Ohm ESR and provided much improved power supply filtering for higher-speed Digital Circuits over their wet-electrolyte predecessors.
The voltage rating of Tantalum Capacitors is a function of the geometry of the Tantalum powder used to create the porous conductive pellet that provides all the surface area for the charge to be stored. The finer the powder the larger the overall surface area in a given volume, but the finer the powder the lower the voltage can be across the dielectric. (Engineering is all about trade-offs) 

The voltage that is marked on Tantalum Capacitors is the DC rated voltage at 85 Degrees C, but I can think of very few applications were you are concerned about a capacitors “DC” voltage only… it is the “AC” ripple voltage spec that is important. The AC voltage amplitude and frequency of the application will determine ultimately the power dissipation “Heat” the capacitor will have to withstand. This increased power dissipation with AC voltages & surge currents is what prematurely causes the cap's MTBF to decrease. The MTBF can be increased by using a higher DC rated voltage capacitor in a given application; hence this is where the derating came from.

Kemet recommends 50% voltage derating factor for the MnO2 cathode system tantalums; but for the polymer-based cathode systems they recommend a 20% voltage derating factor.

Now in my mind Kemet should just mark 5V on a 10V MnO2 cap and let me not even have to think about the derating, but I guess 60 years of history is hard to erase.

“Characterization of Tantalum Polymer Capacitors” By Erik K. Reed
“Voltage Derating Rules for Solid Tantalum and Niobium Capacitors” By T. Zednicek and J.Gill