Indicating Polarity on Diodes

Everyone knows which way current flows through a diode. Right? Of course they do. Diodes only permit current to flow in one direction.

Well, sort of.

In the case of your garden variety rectifier, barrier diode, or LED, that’s true. That line of thinking leads a lot of people to assume that you can indicate diode polarity by putting a plus sign “+” next to the anode.

Here’s why you can’t.

Zener and TVS diodes have a breakdown voltage. They are put in the circuit with their cathode on the positive side. In that configuration, they don’t conduct unless the voltage rises above their breakdown point. Zeners and TVSs are used for regulation, transient suppression, and things of that sort.

But wait! There’s more!

Regular diodes can be pointed backwards too. Anytime an inductive load is switched, like a solenoid or motor, you need a flyback diode to protect the switching logic. A MOSFET switching a solenoid on and off is a good case to look at.

When the MOSFET turns off, the current in the solenoid coil starts to drop. As it starts to drop, the magnetic field generated by the current flow starts to collapse. The collapsing magnetic field generates an opposite current, referred to as flyback, or back EMF.

To save your silicon switching device, you put a flyback diode across the coil, or motor, terminals, pointing backwards from normal current flow – with the cathode pointed toward +V. Doind so shorts the flyback current back into the coil, preventing damage to the MOSFET. These are typically Schottky diodes, but can be ordinary rectifier diodes.

A “+” plus sign alone, doesn’t tell anyone anything. For more information on what to do, read this post. Just for fun, read this post too.

Duane Benson
Diodes. Not just for breakfast anymore

Does Rising Nationalism Pose Threat to Electronics Supply Chain?

The amount of geopolitical discord around the world at present is stunning: Thailand, Vietnam, Korea and other major electronics manufacturing hubs are seeing a rise in nationalism and severe internal tension over how to address foreign pressure.

Thailand in May endured yet another military coup — its 19th since declaring independence from its monarchy in 1932. Some observers feel the military wants a permanent seat in the national parliament, a move that could hinder its democratic movement.

In Vietnam, citizens are outraged at what it feels is Chinese strong-arm tactics. Its Northern neighbor has provoked many Southeastern nations over the past few years, often by occupying seaborne territory that others had staked claims to in the past. (The Philippines have a similar complaint dating to 2012, when China evicted Philippine fishermen from their long-held fishing grounds.) Lately, Chinese oil rigs took up in Vietnamese waters, leading to riots at Fittec, Foxconn and elsewhere, where domestic workers took aim at their Chinese* employers.

Korea is losing business to Vietnam, aided in part by its own OEMs: Korea is now the largest investor there, pumping in nearly 23% of all outside investments in the first quarter this year. As Samsung relocates its cellphone manufacturing there, Vietnam is on track to produce 250 million handsets this year, versus 200 million in China and just 30 million in South Korea. As the linked article indicates, as of March 2014, Samsung Electronics subcontractors had invested an aggregate $2 billion in Vietnam. Meanwhile, while Samsung buys a reported 53% of its parts from Japan, South Koreans now view Japan as their second-leading military threat, next to North Korea, and resentment from World War II is rising once again.

Indonesia is suffering through a contested presidential election, one that involves an ex-general and the possibility of an overturned ballot result.

Japan, my friend Dr. Hayao Nakahara tells me, has essentially stopped investing in new manufacturing sites in China, with the only new developments minor capacity add-ons to existing plants. The two nations have been at odds over everything from possession of uninhabited islands in the East China Sea to a rehashing of wartime atrocities.

Southeast Asia is home to the bulk of the world’s electronics production, and holds the majority share of products built for the consumer, industrial/instrumentation, telecommunications, PC and peripherals end-markets (not to mention the vast majority of the raw materials and components supply). We’ve absorbed several of nature’s bullets of late — flooding in Thailand, the typhoon in Malaysia and of course the 2011 earthquake and subsequent tsunami in Japan. I am told that the media reports have exaggerated what’s happening on the ground in Southeast Asia, and that on a day-to-day basis little dissent is noticeable. That may be true, and to be sure, the self-inflicted disruptions have thus far been held to a minimum. Given the number of countries involved — unprecedented in recent times — and the enormity of what’s at stake, we can’t help but feel it will take some luck if the next supply-chain breakdown is only as bad than the last.

*Fittec is based in Hong Kong, Foxconn in Taiwan, but most employees and manufacturing for both companies are in China.

Design Tradeoffs

As many EMS firms are trying to grab a bigger piece of the design services market, one thing we’ve noticed when we tour their digs is how much more relaxed those designers appear. They are different in terms of setup – some sit in open cubicles, others have individual offices, and still others share a common but separate office, akin to a bullpen – but no matter the configuration, the occupants come across in control and unrushed.
Contrast that to the OEM designers we speak with, who almost uniformly come across as harried.
We’re not sure why this is. Perhaps those at EMS sites are more confident in their job security, knowing that more designs are being shipped their way each year, while their OEM counterparts feel under the gun, worried that their bosses, having already outsourced fabrication and (in many cases) assembly, might at any time let design go, too.
Even so, those designers who responded to our annual salary survey overwhelming were employed by OEMs. Does that suggest EMS designers are significantly fewer in number, harder to reach, or just less interested in filling out a survey? We don’t know.
What we do know, however, is that designers are as not easily compartmentalized as they once were. More have advanced degrees and increasing responsibility. They have become integral, even if more than one-third of respondents still worry about their jobs.
About three-fourths of those who responded were based in the US (probably because the survey was conducted only in English). Most of them have more than 20 years’ experience, suggesting that cost-cutting measures elsewhere aren’t decimating the field.
The average annual US household income was $63,000 in 2011. Given that, designers are doing well. Some 73% of respondents indicated their salary exceeds $60,000, with 17% revealing salaries topped $100,000. For comparison, the median income for a bachelor’s in engineering is $82,712. And most continue to get raises in line with or exceeding the average US raise of 2.8% last year. After the roller coaster of 2008-10, stability is welcome.
Keeping up with the Joneses is one thing. Keeping up with technology is something else. More than one-third of our respondents again said maintaining their technology fluency is their biggest challenge. That’s understandable – as consulting editor Jan Vardaman notes (pg. 20), advancements in everything from wiring materials to substrate systems are ahead. Moreover, an impending shift to copper pillar offers exciting possibilities for tighter silicon and package routing, but with those come the headaches of greater crosstalk and signal integrity issues. Technology, like life, is about tradeoffs.

One more note on the salary survey. Of those designers who recommend or approve products or services, only 78% get to weigh in on CAD tools. While we understand why some designers are out of the loop on this – many EMS companies buy tools as directed by an end-customer, user be damned – it’s still jarring in this day and age that those tasked with such a critical job don’t get a bigger say in how they perform it.

(I would be remiss if I failed to add that senior editor Chelsey Drysdale conducted the survey, compiled the data and wrote the report.)

Sharper Focus for New Year

Happy new year! Hope this one’s a sight better than the last.

What’s new at Circuits Assembly in 2010? For starters, we plan to double-down on our technology coverage. Poll after poll of our readers shows we are at our best when we focus on the tech side. Ergo, rather than pushing the same, watered-down press releases you can find anywhere, we are ramping our coverage of new (and improved) processes for assembling and soldering circuit boards, while trying to pin down the issues behind the same.

We also are adding more analysts to discuss the technology’s pros and cons, giving you better insight without having to hop on a plane to attend a pricey tech conference.

We are looking forward to 2010, which we will be a bounce-back year for electronics manufacturing. I don’t predict a return to double-digit growth, but I think gains in the high single-digits are very achievable.