熔断丝彻底垮台了

详细内容

　　不寻常的结构给予保险丝对瞬间浪涌电流的“记忆”能力。

　　在我事业中期，我成为一名器件工程师。不久之后，我到了新部门，面对一个一段时间内都存在的问题。CRT显示器中看似简单的保险丝却出现很高的故障率。我们已经按照说明书彻底地测试了样品。一个测试工程师以前曾设计了测试工装，以便对一批保险丝测试所有的指标条款。说明书要求保险丝在超过正常温度、振动和摆动指标的某个百分比后几毫秒内断开。原先的工程师进行了很多测试工作，验证了保险丝满足所有的规范。

　　然而，在应用中，高失效率仍在继续。第一步，我测量了实际应用中的电流，确保保险丝的选择合适。我发现，除了小且简短的上升电流外，标称值在保险丝的额定范围之内的，会快速上升到标称值。我怀疑是简短的上电浪涌引起了问题。仔细检查原先的测试结果和应用测试后，我认为这不能解释高失效率的问题。绝望中，我送一些样品到定点材料实验室，让那里的同事测试保险丝的横截面直径，并鉴别所使用的合金。很幸运地，实验室将工作分配给了一位有能力的材料工程师，他把保险丝做了瞬间电流脉冲实验后，送去做了额外的分析。几天后，我拿到了漂亮的缩影照片显示了意想不到的结构技术。照片显
示保险丝由三种金属组成，而不是使用某种低熔点金属的单一合金。它有一个大圆形钨内核。穿过钨是一个铜薄板；另一个银薄板又覆盖了铜薄板。更令人惊讶的是工程师送去的经过瞬间过电流的保险丝。他发现通过电容充电到各种电压，用保险丝短路放电，可以形成一个可控的浪涌电流。照片显示若干次浪涌后，银板达到熔点液化。/iompany/detail12.html" target="_blank">ONSTrucTIon gives fuse a "memory" for brief current surges.

　　By Jim Sylivant, Engineering Consultant -- EDN, 1/17/2008

　　In midcareer, I became a ponent engineer. Soon after I arrived in my new department, I faced a problem that had been ongoing for some time. It seems that a simple fuse in a CRT display had been having high failure rates. My new department had thoroughly tested samples against its specificATIons. A previous test engineer had designed a test fixture so that he could test batches of this fuse for all spec items. The specification required the fuse to open at a certain percentage over its nominal rating within a certain number of milliseconds over a range of temperatures, as well as after shock and vibrations. The previous engineer had done a splendid job of testing to verify that the fuse met all specifications.

　However, in the application, high failure rates continued. My first step was to measure actual current in the application to ensure that we had chosen the proper fuse. I found that, besides a small, brief start-up current, the nominal value quickly settled to values well within the fuse's rating. I didn't suspect the brief start-up surge of causing a problem. After going over previous test results and in-application testing, I could find no explanation for the high failure rate. In desperation, I sent some samples to our on-site materials lab and asked the folks there to measure the cross-section diameter of the fuse element and identify the alloy used. Fortunately, the lab assigned the job to a very petent materials engineer who went the extra mile by analyzing the fuse after subjecting it to brief current pulses. In a few days, I got back beautiful microphotographs showing an unexpected construction technique. The photos showed that, instead of using a single alloy of some low-melting-point metal, the element consisted of three types of metal. It had a large, circular, tungsten inner core. Over the tungsten was a thin plating of copper; yet another thin layer of silver lay over the copper. Even more surprising were the photos the engineer took after subjecting the fuse to brief overcurrents. He found that, by charging a capacitor to various voltages and discharging it by short-circuiting it with the fuse, he could create a controlled amount of surge current. The photos showed that, after some surges, the silver layer reached its melting point, causing it to liquefy. After more surges, the silver pletely melted away, leaving only the tungsten core with its thin copper plating. Because silver has such high electrical conductivity, virtually all the current from the surges initially flowed entirely through the outer silver layer. Afterward, additional surges flowed mostly through the thin copper layer because copper has higher conductivity than tungsten. That layer eventually melted. Now, only the tungsten core with its high resistance remained. With more surges, all current now had to flow through the remaining tungsten core. As more surges ourred, the tungsten heated up enough to gradually grow thinner and finally disintegrate.

　　We then realized that this trilayer-construction technique gave the fuse the ability to “remember” the aumulation of brief overload-current surges. Each surge at power-on contributed to small changes that eventually caused the fuse to open. Steady-state testing had not revealed this characteristic.
As a result of its trilayer construction and the metals used, the fuse had memory. The solution was to change to a conventional fuse with a single element of low-melting-point alloy—that is, one that did not possess memory. This realization was the beginning of many such discoveries—that you can diagnose most problems by going back to an understanding of the basics.