熔断丝彻底垮台了

　　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。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。更多浪涌后，银板完全熔化，只留下带薄铜板的钨核。因为银有很强的电传导性，实质上所有浪涌的电流初都完整的通过外部银层流动。然后，另外的浪涌大多数通过薄铜板流动，因为铜具有比钨更高的传导性。那个层终熔化了。现在，只剩下高阻的钨核。随着更多的浪涌，所有电流现在都不得不通过剩下的钨核流动。当更多浪涌发生时，钨逐渐加热到足够变薄，断开。

　　我们随后意识到这种层构造技术使保险丝具有了“记忆”瞬间过载电流浪涌累积的能力。上电的每个浪涌贡献了小改变，终导致保险丝断开。稳态测试没能显示出这个特性。其三层构造和金属使用的结果，使保险丝具有记忆性。解决方法是改变为传统的单一元素低熔点合金的保险丝——也就是不具有记忆能力的保险丝。这个实现是许多这种发现的开始——可以通过溯源到基础知识来诊断大多数问题。

　　英文原文：

　　Blown fuse has a meltdown

　　Tales From The Cube: Unusual construction gives fuse a "memory" for brief current surges.

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

　　In midcareer, I became a component 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 competent 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 completely 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 occurred, 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 accumulation 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.