Multimeters (sometimes called VOM: Volt-Ohm-Milliampere Meter) are a combination of voltmeter, ammeter, resistance meter and other types of electronic meter. They are invaluable for troubleshooting home and automobile electrical and electronic problems.
While they may appear complicated, not all of their features need to be used. For home use, they earn their keep mainly by acting as voltmeters and continuity testers.
Prices for both analog and digital models range from less than $10, to a few hundred dollars for industrial or professional models. Popular brands include Fluke, Mastech, Cen-Tech, HP (Agilent), Amprobe, Actron, Wavetek, Beckman, Micronta, Triplett, Yokogawa, Sanwa and UEi.
Analog Multimeters
Old-style analog moving-needle multimeters have significant advantages and a few drawbacks.
Advantages
- Can measure fluctuations and transients. Analog meters can measure low frequency fluctuations, to a certain extent.
- No batteries required. Batteries are needed for the resistance meter but the voltmeter and ammeter will work without batteries.
- Good for quick readings. The analog needle reacts quickly, making it suitable for fast probing of multiple points. High, medium and low readings are easily seen with a glance at the needle.
- Good for tuning electronic circuits to a specified value (example: setting a trim or variable resistor) as the needle's speed and relative position provides useful feedback.
Disadvantages
- Limited accuracy. Accuracies range from plus or minus 1% to 4% of full scale (depending on the model and the type of measurement). A reading in the middle of the scale (half full scale) will have half the accuracy (2% to 8%).
- No autoranging. For more precise readings, the meter needs to be manually set so that the needle is almost at maximum (full scale deflection).
- Possible to read the wrong scale by mistake.
- Scale can be difficult to read.
- Delicate. The needle's mechanism is delicate and accuracy can be affected if the multimeter is dropped.
Useful features
- Many measurement ranges. The more ranges available, the better the chances that the meter can be used close to full scale deflection for high accuracy.
- 50 micro amps measurement range. More common is the less sensitive 200 micro amps range.
Autoranging Digital Multimeters
Digital multimeters are deservedly popular. However users need to be aware of their limitations.
Advantages
- Accurate. The best DMMs can have accuracies as high as plus or minus 0.1% but 1% to 2% is more common.
- Easy to use. Autoranging DMMs automatically switch the internal circuit to the correct measuring range.
- Wide range of features. Advanced DMMs can measure waveform frequency and duty cycle (edging into oscilloscope territory).
Disadvantages
- LCD display can be too dim to read easily. Some models have a backlit display but this will reduce battery life.
- Not suitable for measuring fluctuations and transients.
- No zero adjustment for resistance measurement, except on advanced models.
Useful features
- Automatic power-off.
- Back-lit display, with "always on" option.
- Analog bar graph for quick reading of fluctuations. This is a killer feature, available on professional models costing hundreds of dollars. It allows a DMM to have the advantages of both analog and digital meters.
- Selectable auto or manual ranging.
- Display of minimum, maximum and average readings.
- Measurement hold feature.
- Connection to PC for automatic periodic data-logging.
- Automatic AC/DC voltage detection and measurement switching.
- Non-contact AC voltage detector. For identifying live wires.
Other Multimeter Features and Measurements
The following features are useful for both analog and digital multimeters:
- True RMS AC voltage measurement.
- Continuity tester with beeping indicator.
- Diode tester with beeping indicator. Also used to test BJT transistors, two terminals at a time.
- Battery tester for 1.5 and 9 volt batteries. Measures voltage while loading the battery with a small current drain. This feature is more common on analog meters.
- BJT transistor gain (hFE) measurement.
- FET transistor gate leakage current measurement.
- Capacitance measurement.
- Temperature measurement with separate thermocouple probe.
- Table stand.
- On-board storage for test probes.
- Fused input circuit.
- Maximum 10 amperes measurement circuit. A separate test probe socket is needed to handle the high current. Cheaper meters use an unfused input circuit for the 10 amperes socket, even if the other socket is fused.
The Best Multimeter or Multitester
A basic, low-cost autoranging digital multimeter is easy to use: good for the non-technical DIY hobbyist. More advanced users can handle the complexity of an analog meter, giving them the flexibility of dealing with fluctuating readings.
Professional engineers, technicians and electricians will want to pay more for a digital multimeter with a bar graph display, allowing them to enjoy the advantages of both types of multimeters. Hybrid multimeters with dual digital and analog displays are a possible alternative.
The All About Circuits website has a good explanation of voltmeter circuit theory and design. For people who just need to check their batteries, a dedicated battery tester is easier to use than a multimeter.
Yuen Kit Mun - Kit Mun is a self-confessed information junkie, reading an average of a book a week over the past two decades. His growing Internet ...
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