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Save Your Back by Using a High Desk March 14, 2008

Sitting actually puts a lot strain on your lower back. Over the years, working at a desk job can really injure your back so that you won’t be able to sit comfortably for extended periods of time anymore. After suffering such an injury myself, I found that the only way I could get back to work was to use a desk high enough to stand at while working. I also have a chair that is high enough so I can alternate sitting and standing. Even if you end up sitting most of the time as I do these days, it is still well worth it.

Most modular office furniture can be (more…)

Schematic and PCB Design Process March 7, 2008

I would like to know how a circuit designer in OrCAD plans the schematic and layout for the digital, analog, power, low speed high speed portion of the PCBs?

Wow, this is a big but good question! For anyone considering undertaking a schematic capture and PCB layout for the first time, it can seem like a daunting task. A modern PCB serves not just as a means to interconnect components, but also as a mechanical structure, heat conductor, noise shield, and even as a circuit element, and must go through a complex automated manufacturing process to boot.

Perhaps it is best to start with (more…)

Tips to Avoid Arcing December 19, 2006

I have gotten several requests for information about high voltage PCB design and how to avoid arcing.

For example, Ken writes:
“Could anyone explain surface arcing causing hi-pot test failure?”

I thought I would make a short post to serve as a place to discus these issues.

Current vs. Trace Thickness and Temperature vs. Copper Density December 15, 2006

Regarding the PCB Trace Width Calculator [1], Regit asks:

(a.) What is the relationship between current and trace thickness? And, (b.) what is the relationship between temperature and copper density?

Answers: (more…)

Reducing EMI in PCBs December 6, 2006

I got the following question by email:

When a data sheet says “minimize the trace area for lowest EMI”, do they mean thin trace? Does higher reactance minimize EMI?

It is a good question, so I though I would post something on this.

Minimizing EMI requires minimizing the electric and magnetic fields around the trace. If a trace has a lot of voltage that is changing fast, it will tend to emit a strong electric field. In such a case, making the trace have less surface area can help limit the emissions. Better yet, shielding the noisy trace with a trace that lets the energy return to where it came from is very effective - the shorter the return path, the better. In fact, if the noisy trace is completely surrounded by the shield, the electric field lines won’t get out and the noise will be contained. Once shielded, the area of the trace is not so critical in terms of noise, however more area also means more capacitance, which your circuit may not like. Therefore, yes, it is usually a good idea to minimize the area of an E-field emitting trace, but it must be able to carry the needed current of course.

Next is the magnetic field, which comes from a changing current. The higher the current and the faster it changes, the worse the EMI can be. The magnetic field can be somewhat canceled by routing the returning current as close as possible to the outgoing current. In other words, the loop area traveled by the current needs to be small. We are talking about AC current here, so it can be contained to a small area by effective use of decoupling capacitors. These should be placed near the source of the noise current.

Ground planes can help contain both electric and magnetic fields. They give the electric field lines a place to terminate, and those high frequency return currents will automatically flow near the outgoing currents due to an imaging effect where the ground plane will actually mirror the trace currents with currents flowing in the opposite direction.

Shortcomings of the IPC-2221 Current Capacity Guidelines October 31, 2006

Our friends at Flomerics have posted an interesting article [1] that describes some of the shortcomings of the IPC-2221 design rule for determining the current carrying capacity of PCB traces. The article concludes that the IPC rule is based only on a 1.6 mm thick, 2-sided FR4 board with a single trace on one side, solid copper on the back side, and 35 um of copper sheet thickness. Modern PCBs can typically have a lot more layers and more copper, and thus be able to take more current with less temperature rise in the traces due to better heat spreading and better heat shedding. The author then derives a new series of graphs showing the temperature rise vs. current and trace width for various PCB stack-ups. Also, it seems that internal layers can handle a lot more current than IPC-2221 would have us think.

[1] New Correlations Between Electrical Current and Temperature Rise in PCB Traces, Johannes Adam

Making AutoComplete Work with Cut and Paste April 12, 2006

In Internet Explorer, the AutoComplete feature for forms can be handy, but I have noticed that it doesn’t work if you cut and paste text into the form. On the other hand, if you manually type text into the form, it is remembered. I found that you can force AutoComplete to work with cut and paste by deleting the last character and retyping it. This is a great time saver – especially for things like tracking numbers.


Also note that you can delete a single item from the AutoComplete drop down by selecting it and pressing delete.