CRAC Class B (2025) — Questions 991–1000
U1066. Which of the following components can protect other components in a circuit from overcurrent surges?
以下哪种元件可以保护电路中的其他元件免受过电流的冲击？
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A 熔断器 (fuse) is a sacrificial overcurrent protector: it opens the circuit when current exceeds its rating, protecting downstream parts. A MOSFET, vacuum tube, or capacitor does not inherently protect against overcurrent, so B/C/D are wrong.
U1067. Among the components used in assembling amateur transceivers, those typically marked with a rated current are: (Choose all that apply.)
在组装业余收发信机所用的元器件中，标有额定电流的通常 是：
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A 熔断器 (fuse), a 开关 (switch), and a 继电器 (relay) all carry a rated current that must not be exceeded in normal use. A 压电 晶体 (piezoelectric crystal, e.g., for an oscillator) is not rated by carrying current the same way, so D is excluded.
U1068. In amateur projects, fuses are often glass- or ceramic-bodied fuse cartridges used to enhance circuit safety. Useful lessons for their use include: (Choose all that apply.)
业余无线电作品中的熔断器常为玻璃或陶瓷封装的保险丝管， 是用来增强电路安全性的。关于这类熔断器的使用，可供借鉴 的经验有：
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Sound fuse practice covers choosing correct voltage/current/fusing-speed ratings (A), using a holder for high-current cartridges so a burst can't hit nearby parts (B), and placing the holder accessibly for service (C). D's "RF filter on a fuse" is not a real or useful technique, so it is excluded.
U1069. A relay is a switch with electrical isolation, often used to control high-power or high-voltage circuits with low-power low-voltage signals. In amateur projects, relays also switch RF paths. Useful lessons for relay application include: (Choose all that apply.)
继电器是一种具有电气隔离能力的开关，常用来以小功率低压 电控制大功率电路或高压电路。在业余无线电作品中，继电器 也用来控制射频电路的通断或切换信号路径。关于继电器的应 用，可供借鉴的经验有：
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Good relay practice: size contacts for voltage/current (A); switch T/R only when no RF flows to avoid hot-switching damage (B); for signal switching, check the relay's small-signal performance like contact resistance and isolation (C). D is wrong — for HF/VHF you need HF/VHF-capable RF relays, not microwave relays (overkill and often unsuitable).
U1070. If a switching device is used to control whether a relay's armature coil is energized — for example, with the coil in the collector circuit of a transistor — which of the following may happen?
如果利用开关器件来控制继电器的衔铁线圈通电与否，例如是 将线圈接在三极管的集电极电路中，则可能发生的情况是：
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A relay coil is inductive; when the transistor switches off, the collapsing magnetic field induces a large back-EMF (V = L·di/dt) that appears across the collector, spiking well above the supply (A). The spike occurs at turn-off, not turn-on, and is a voltage spike, not a current surge, so B/C/D are wrong.
U1071. If a switching device controls whether a relay's armature coil is energized, a small capacitor or a reverse-biased diode is placed across the coil. Its purpose is:
如果利用开关器件来控制继电器的衔铁线圈通电与否，则线圈 的两端需要并联一个小容量电容器或是一个反向偏置的二极 管。其作用是：
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The diode (or snubber capacitor) across the coil provides a path for the inductive kick when the circuit opens, clamping the back-EMF and protecting the driver (A). It protects the coil- driver, not the contacts (B); it does not reduce operating current (C) nor directly reduce relay wear (D).
U1072. If a relay controls a DC load, we sometimes connect a small capacitor across the relay's contacts. Its purpose is:
若用继电器控制直流负载，我们有时需要在继电器的触点两端 并联一个小容量电容。其作用是：
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A capacitor across the contacts snubs the arc that forms when DC contacts break, reducing the RFI/EMI from the spark (A). The coil back-EMF suppression (B) is done across the coil, not the contacts; the cap does not reduce operating current (C) nor directly cut wear (D).
U1073. Electromagnetic induction is the phenomenon where a conductor placed in a changing magnetic field develops an induced electromotive force. Its application scenarios include: (Choose all that apply.)
电磁感应现象是指置于变化磁场中的导体产生感应电动势的现 象。应用场景包括：
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电磁感应 (electromagnetic induction) underlies the transformer (A, coupled windings), the generator (B, conductor moving in a field), the AC induction motor (C, rotating field inducing rotor currents), and radio communication (D, antennas/induction in RF). All four are valid applications.
U1074. The electromagnetic term "mutual inductance" (互感现象) means:
电磁学术语"互感现象"是指：
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互感 (mutual inductance) is the magnetic coupling between two circuits: a changing current in one induces an EMF in the other (A). The other options describe impedance interaction, electric-field superposition, and mixing — not mutual inductance.
U1075. Why is a transformer's iron core built from many insulated silicon-steel laminations of large cross-section and high permeability, rather than using a solid core like a relay armature?
为什么变压器中的铁芯要由截面积大，导磁率高的多片矽钢片 堆叠构成，而不像继电器中的衔铁那样使用整体铁芯？
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A solid iron core would let large eddy currents circulate, wasting power and heating. Laminations (矽钢片) insulated from one another break up those loops, cutting eddy-current loss (A). The other reasons are not the purpose.