Measuring DC Voltage with a Multimeter (Task: Measure 9V Battery, 1.5V Battery)
(1) When measuring DC voltage, identify the positive and negative terminals (the red probe connects to the positive terminal “+”, and the black probe connects to the negative terminal “-”). Connect the multimeter in parallel to the circuit being tested. Before measuring the battery, check the rated voltage of the battery first. Select the DC voltage range: first the range, then the measurement. For a 9V dry battery, set it to the 10V range, where each large division represents 2V and each small division represents 0.2V, accurate to one decimal place. For a 1.5V battery, set it to the 2.5V range, where you read at the 250V range, where each large division represents 50V and each small division represents 5V (read at the 250V range and then divide by 100). Report the result to the examiner accurate to two decimal places.
(2) Measuring DC voltage has polarity. If you are unsure of the polarity, first touch the black probe, then lightly touch with the red probe to see the direction of the needle movement. If incorrect, switch the probes immediately. When measuring AC voltage, polarity does not need to be considered.
(3) Do not change the range while measuring to avoid arcing and damaging the contacts of the switch.
(4) The selected range for measuring voltage should be slightly higher than the voltage being measured.
Measuring Resistance with a Multimeter
When measuring resistance, there is no distinction between positive and negative terminals. The principle is that the internal battery of the meter supplies current to the resistor.
2. First, perform zero adjustment on the ohmmeter by shorting the probes; the needle should point to the zero position on the right side of the scale. If it cannot be adjusted to zero, it indicates that the battery voltage is insufficient and should be replaced.
3. When selecting the range, during exams, start with a higher range, but in practice, you can start with a middle range, such as 100 or 1K, for estimation.
4. Connect the probes to both ends of the resistor being measured; the needle will indicate a reading. If the value is too small or too large, if the needle is too far right, reduce the range; if it is too far left, increase the range, and select a more suitable range for re-measurement.
5. Each time you change the range, you must perform a zero adjustment. The needle indicating in the middle position has the least error; ensure to read the value only after the needle stabilizes. Measurement reading (value on the scale) x multiplier = measured value (2).
6. The circuit being measured must be disconnected from the power supply; do not measure resistance while powered, as this may damage the meter.
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