CRAC Class A (2025) — Questions 621–630

U0620. A power source is a kind of power-supply device commonly used by amateur radio operators. Our understanding of a power source is: (Choose all that apply.)

电源是业余无线电爱好者常用的一种供电装置。我们对电源的理解是：

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All four statements correctly describe a power source: it converts energy into electricity (A); a battery is a chemical source (B); a DC supply converts AC/DC into regulated DC (C); and a transformer reshapes AC to form an AC source (D). Each is a valid characterization of a power source.

U0621. The direction of the electromotive force (EMF, 电动势) across the two terminals of a power source is:

电源两端电动势的方向为：

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The electromotive force (电动势, EMF) of a source is defined as acting from the negative terminal toward the positive terminal inside the source (A). This is opposite to the external terminal voltage direction (B/D are wrong), and it is an intrinsic property, not dependent on load (C is wrong).

U0622. The direction of the voltage across the two terminals of a power source is:

电源两端电压的方向为：

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The terminal voltage of a source drives current through the external circuit from the positive terminal to the negative terminal (A). This is the opposite direction to the internal EMF (so D is wrong), and is not load-dependent in direction (C wrong).

U0623. External power sources for supplying amateur radio equipment come in several types. Common ones include: (Choose all that apply.)

为业余无线电设备供电的外置电源具有多种类型。常见的有：

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Common external power sources for amateur equipment are switching power supplies, linear power supplies, and storage batteries (A, B, C). A standard cell (标准电池, D) is a precision voltage reference, not a practical power source for equipment.

U0624. Which of the following batteries can be recharged? (Choose all that apply.)

下列哪一种电池可以充电？

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Lithium-ion, sodium-ion, and lead-acid batteries are all secondary (rechargeable) cells (A, B, C). An alkaline dry cell (碱性干电池, D) is a primary (non-rechargeable) battery and should not be recharged.

U0625. How can a 12-volt lead-acid battery be charged when the mains power is out?

如何在电网停电的情况下给一个12伏的铅酸蓄电池充电？

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The safe method is to parallel the discharged 12 V battery with a running car's battery/alternator so the running engine's charging system tops it up (A). Adding acid (B), cooling it (C), or connecting directly to 220 V mains (D) are incorrect and dangerous.

U0626. When using a storage battery to power a radio station, how should the battery operating time be estimated?

使用蓄电池为电台供电时，应如何估算电池供电的时长？

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Operating time (hours) ≈ capacity (Ah) ÷ average current (A). Using the battery's nominal ampere-hour (安时) rating divided by the transceiver's average operating current gives the runtime (A). Watt-hours ÷ transmit-only power (B) ignores receive/standby current and is not the standard estimate; C/D are dimensionally wrong.

U0627. The effect of a power source's internal resistance on a circuit is:

电源的内阻对电路的影响是：

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Internal resistance causes a voltage drop (I·r) inside the source, so the terminal (output) voltage V = E − I·r falls as load current rises (A). The EMF (E) itself is unaffected (B wrong), and output power is not increased (C wrong).

U0628. Some transceivers have a fuse in series in the 13.8 V DC supply line, followed by a diode of very high rated current connected in anti-parallel for protection. What characteristic of the diode does this circuit exploit, and how does it work?

有些收发信机会在直流13.8V供电线路中串联一个熔断器，并在其后反向并联一个额定电流很大的二极管作保护之用。该电路利用了二极管的什么特性？是如何工作的？

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The reverse-parallel (anti-parallel) diode is a polarity- protection device. Normally it is reverse-biased and conducts nothing. If the 13.8 V supply is connected backwards, the diode becomes forward-biased and, with its very high current rating, effectively shorts the line, so the series fuse blows and disconnects the wrong-polarity voltage (A). Options B/C describe zener-like voltage clamping (not this circuit), and D describes thermal runaway (incorrect).

U0629. Some transceivers have a reset button marked with a number near the 13.8 V DC power socket. What is its function? (Choose all that apply.)

有些收发信机会在13.8V直流电源插座附近安装一个标有数字的复位按钮。其作用是什么？

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This reset button is the protective device that trips and disconnects the supply on overcurrent (A) and on reversed polarity (B); once the fault clears you press it to reset. It is not a temperature (C) or overvoltage (D) setpoint — those would be labeled differently. 测量与仪表 Measurement and Instruments