CRAC Class B (2025) — Questions 911–920

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.

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).

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).

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.

A basic multimeter directly measures the three fundamental electrical quantities — voltage, current, and resistance (电压、 电流和电阻) — using its DC/AC voltage, current, and resistance ranges. Signal strength/noise (B), the reactive part of impedance (C), and SWR/RF power (D) require specialized instruments such as a spectrum analyzer, an impedance bridge, or an SWR/RF power meter.

On the AC voltage range a multimeter reads the RMS (有效值) value of a sine wave. The peak (最大值) is larger by √2 (about 1.414) and the peak-to-peak (峰-峰值) by 2√2, so options B and C are too large; the full-cycle average (D) of a sine is zero.

A DC voltage range is not designed for AC; a pure sine wave has equal positive and negative areas, so its average (what the DC range integrates) is essentially zero. Only a true-RMS meter on the AC range reads the RMS value; the DC range does not give the peak (B), peak-to-peak (C), or RMS (D).

An ammeter must be placed in series so the full load current flows through it (串联). A voltmeter is connected in parallel. Options C and D are meaningless connection descriptions for current measurement.

Using the current (amps) range across a voltage source shorts the source through the meter's tiny shunt resistor, blowing the fuse or destroying the meter — the classic damage case (A). A large range on a small voltage (C) only loses resolution; B and D are not damaging.

Resistance measurement uses the meter's own internal current source, so the circuit under test must be de-energized; otherwise you get wrong readings or damage the meter. Option A is the required precondition; B is wrong (powered), and C/D are irrelevant.