不尋常的結(jié)構(gòu)給予保險絲對瞬間浪涌電流的“記憶”能力。

　　在我事業(yè)中期，我成為一名器件工程師。不久之后，我到了新部門，面對一個一段時間內(nèi)都存在的問題。CRT顯示器中看似簡單的保險絲卻出現(xiàn)很高的故障率。我們已經(jīng)按照說明書徹底地測試了樣品。一個測試工程師以前曾設(shè)計了測試工裝，以便對一批保險絲測試所有的指標(biāo)條款。說明書要求保險絲在超過正常溫度、振動和擺動指標(biāo)的某個百分比后幾毫秒內(nèi)斷開。原先的工程師進行了很多測試工作，驗證了保險絲滿足所有的規(guī)范。

　　然而，在應(yīng)用中，高失效率仍在繼續(xù)。步，我測量了實際應(yīng)用中的電流，確保保險絲的選擇合適。我發(fā)現(xiàn)，除了小且簡短的上升電流外，標(biāo)稱值在保險絲的額定范圍之內(nèi)的，會快速上升到標(biāo)稱值。我懷疑是簡短的上電浪涌引起了問題。仔細(xì)檢查原先的測試結(jié)果和應(yīng)用測試后，我認(rèn)為這不能解釋高失效率的問題。絕望中，我送一些樣品到定點材料實驗室，讓那里的同事測試保險絲的橫截面直徑，并鑒別所使用的合金。很幸運地，實驗室將工作分配給了一位有能力的材料工程師，他把保險絲做了瞬間電流脈沖實驗后，送去做了額外的分析。幾天后，我拿到了漂亮的縮影照片顯示了意想不到的結(jié)構(gòu)技術(shù)。照片顯 
示保險絲由三種金屬組成，而不是使用某種低熔點金屬的單一合金。它有一個大圓形鎢內(nèi)核。穿過鎢是一個銅薄板；另一個銀薄板又覆蓋了銅薄板。更令人驚訝的是工程師送去的經(jīng)過瞬間過電流的保險絲。他發(fā)現(xiàn)通過電容充電到各種電壓，用保險絲短路放電，可以形成一個可控的浪涌電流。照片顯示若干次浪涌后，銀板達到熔點液化。更多浪涌后，銀板完全熔化，只留下帶薄銅板的鎢核。因為銀有很強的電傳導(dǎo)性，實質(zhì)上所有浪涌的電流初都完整的通過外部銀層流動。然后，另外的浪涌大多數(shù)通過薄銅板流動，因為銅具有比鎢更高的傳導(dǎo)性。那個層終熔化了?，F(xiàn)在，只剩下高阻的鎢核。隨著更多的浪涌，所有電流現(xiàn)在都不得不通過剩下的鎢核流動。當(dāng)更多浪涌發(fā)生時，鎢逐漸加熱到足夠變薄，斷開。

　　我們隨后意識到這種層構(gòu)造技術(shù)使保險絲具有了“記憶”瞬間過載電流浪涌累積的能力。上電的每個浪涌貢獻了小改變，終導(dǎo)致保險絲斷開。穩(wěn)態(tài)測試沒能顯示出這個特性。其三層構(gòu)造和金屬使用的結(jié)果，使保險絲具有記憶性。解決方法是改變?yōu)閭鹘y(tǒng)的單一元素低熔點合金的保險絲——也就是不具有記憶能力的保險絲。這個實現(xiàn)是許多這種發(fā)現(xiàn)的開始——可以通過溯源到基礎(chǔ)知識來診斷大多數(shù)問題。

　　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.