The "multimeter" is short for "universal meter," a crucial tool in electronic production. It can measure current, voltage, resistance, and some models even support measurements of transistor gain, frequency, capacitance, logic level, and decibel values. There are various types of multimeters, with the most common being analog (mechanical pointer) and digital multimeters, each offering unique advantages.
In this article, we'll explore the working principles and usage of mechanical pointer multimeters.
**Basic Principle of Mechanical Pointer Multimeter**
At its core, a mechanical multimeter uses a sensitive magnetic-electric DC ammeter (microampere meter) as the measuring head. When a small current flows through it, the needle moves to indicate the value. However, the meter itself cannot handle large currents. To measure larger values, resistors are either connected in parallel (shunt resistor) or in series (multiplier resistor) to extend the range for current, voltage, and resistance measurement.
**Measuring DC Current**
As shown in Figure 1a, to measure DC current, a shunt resistor is connected in parallel with the meter head. This allows most of the current to pass through the resistor, while only a small portion goes through the meter. Adjusting the resistance of the shunt resistor changes the measurement range.
**Measuring DC Voltage**
Figure 1b illustrates how to measure DC voltage using a multiplier resistor. The resistor is connected in series with the meter head to limit the current and allow for higher voltage readings. Changing the resistance of the multiplier resistor adjusts the voltage range.
**Measuring AC Voltage**
When measuring AC voltage, as shown in Figure 1c, the meter head—designed for DC—requires a rectifier circuit. A half-wave rectification circuit converts AC into DC, which the meter can then measure. The method for extending the AC range is similar to that of DC.
**Measuring Resistance**
For resistance measurement, as depicted in Figure 1d, a combination of resistors and a battery is used. The current passing through the resistor under test determines the measured value. Adjusting the shunt resistor changes the resistance range.
**Important Notes:**
- The ammeter in a pointer multimeter does not require a battery.
- The ohmmeter requires a battery to function.
**How to Measure Resistance:**
1. Short the two probes together to calibrate the zero point.
2. Adjust the "Ω" knob until the needle points to 0.
3. Touch the probes to both ends of the resistor.
4. Read the value on the ohm scale line and multiply by the selected range. For example, if the range is R×100 and the needle points to 80, the resistance is 8,000 Ω.
**How to Measure DC Voltage:**
1. Estimate the voltage first.
2. Set the switch to the appropriate V range.
3. Connect the red probe to the positive terminal and the black probe to the negative terminal.
4. Read the value on the DC scale line.
**How to Measure AC Voltage:**
The process is similar to DC voltage measurement, but there's no polarity. Simply connect the probes and read the value on the AC scale line.
**Note:** Always check the polarity when measuring DC voltage or current. Incorrect connections can damage the device.
**Basic Principle of Digital Multimeters**
A digital multimeter relies on an A/D converter to transform analog signals into digital data displayed on an LCD screen. It includes several conversion circuits:
- I/V (Current-to-Voltage)
- R/V (Resistance-to-Voltage)
- C/V (Capacitance-to-Voltage)
These circuits convert all measured signals into DC voltage before processing and displaying them.
**Measuring Resistance, Current, and Capacitance:**
- Resistance: Converted into a DC voltage signal for display.
- Current: Measured by detecting the voltage drop across an internal sampling resistor.
- Capacitance: Also converted into a DC voltage signal.
**Digital Multimeter Operation:**
Digital multimeters work by taking a sample every 0.3 seconds. While this makes them precise, the readings may vary slightly due to sampling. Unlike pointer meters, they do not offer real-time visual feedback.
**Key Differences Between Pointer and Digital Multimeters:**
1. **Pointer Multimeters:**
- Provide intuitive, visual readings based on needle movement.
- Simple internal structure, lower cost, and better overcurrent/overvoltage protection.
- Lower internal resistance (e.g., MF-10 has 100 kΩ/V sensitivity).
2. **Digital Multimeters:**
- Offer faster, more accurate readings with a digital display.
- Higher internal resistance (often 1 MΩ or more), reducing impact on the circuit.
- More advanced features like temperature, frequency, and capacitance measurement.
- Require batteries for operation due to their complex internal circuits.
In summary, both types of multimeters have their own strengths. Choosing between them depends on the specific needs of the task at hand. Whether you're working in a lab, field, or home workshop, understanding the principles and proper use of these tools is essential for accurate and safe measurements.
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