Multimeter

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Usage

Multimeters are used to quantify as a value several electrical properties and conditions of the device in question.

To use a multimeter, you must first know what you want to measure, and the maximum value you might expect to occur while the measurement is being taken.

Here is a few examples of what each setting may be used for.

Voltage (DC) Voltage (AC) Resistance (ohms Ω) Current (milliAmps DC) Current (Amps DC) Current (Amps AC)
Batteries

DC Power Supplies

Solar Cells

DC Generators

In Circuit Testing

AC Power Supplies

Transformers

AC Generators

Signal Generators

Mains Equipment (1)

Resistors

Potentiometers

Cables And Wires

Light Globes

Semiconductors

Large (value) Capacitors

Small Load Testing

Testing for Current Flow

Heavy Duty Load Testing

Mains Load Testing (1, 2)

1. This may be illegal to do yourself in some countries and should only be done by those skilled in handling mains voltages and currents. See electrical safety

2. This may be alternatively done using a clamp meter

To use the multimeter, if it is analog, adjust the screw at the needle pivot to set its alignment exactly on the 0 marker. If it is digital skip this step.

Turn the dial to the correct Unit and Range you wish to measure. The numbers indicated on the dial specify the maximum value that can be read on that particular range without damaging the multimeter. A good choice is to chose a range that is above your expected measurement value. If unsure, start at the Maximum value and work downwards.

To chose the correct unit, here are some abbreviations and symbols that may be written on the dial:

V - Volts - Voltage

Ω - Ohms - Resistance

A - Amps - Current

DC - Direct Current

AC - Alternating Current

K - Killo - 1000

M - Mega - 1000000

m - milli - 0.001

μ - micro - 0.000001

See Units for details.

Next, carefully connect the multimeter probes directly in contact with the conductors of the device of which you want to measure across. (Note: For high voltage and current circuits you should connect the meter BEFORE powering the circuit, additionally it is wise NOT to adjust the multimeters range while the circuit is running.)

For DC Voltage and Current measurements, connect the red lead to the most positive side of the device or component, and the black lead to the other side. Some digital multimeters can make negative measurements, but on an analog multimeter incorrect polarity may cause wear on the needle against the leftmost position limiter.

Do not use the ohms setting to measure Voltage, or use the DC setting to measure AC or the AC setting to measure DC. This may damage the multimeter, especially in the first case.

Reading the meter:

digital: Multiply the reading displayed on the LCD screen by the unit type displayed on the dials setting. For example if the reading is 50 and the dial is on milli Volts DC then the voltage is 50/1000 Volts (50mV). if You are measuring Killo Ohms, then multiply 50 by 1000 (50Kohms, or 50KΩ)


analog: For example, if you are on the 250mV setting, and there isn't a scale with 250 as its far right value, you may use one that has 25. Simply multiply or divide what the needle is pointing to if needed to fit the scale, and then apply the unit, eg milli, killo etc.


Measuring voltage and resistance: Connect the multimeter directly across what you want to measure.

Measuring current: Place the meter in series with what you want to measure the current through. This may require that you cut the wire or track in order to insert the meter into the circuit. If you are measuring currents above 200mA or 500ma (depending on the model), the red lead should be unplugged from the multimeter and then reinserted into the socket labeled 10A

Digital multimeter

Cheap, accurate, many different functions. The input impedance is usually 1 MΩ or 10 MΩ if a voltage divider is used to measure high voltages or avoid loading the circuit too much.

Accuracy

The accuracy is given as a percentage and as a number of digits. If the accuracy is given as 0.5% ±2 digits and you measure 100.5 V the true voltage can be:

100.5 / 1.005 = 100.0 minus two digits = 99.8 V

to

100.5 * 1.005 = 101.0 plus two digits = 101.2 V

Resolution

The resolution used to be given in the number of digits but is now commonly given in counts. 3 digits would have a range from 0 to 999, that is a count of 1000. 3½ digits would range from 0 to 1999, a count of 2000. The fractional digit refers to the most significant digit and specifies that it can be either 0 or 1.

Analog multimeter

Simple, good at displaying changing voltages. Simple designs can reject more interference than most digital multimeters. The resolution on a large quality scale can be compareable to a 1000 count digital multimeter but requires more skill by the operator for repeatable high accuracy. The input impedance is usually quite low and voltage dependent on basic models, some of the more advanced models have built in amplifiers with high input resistance.

Alternating current

Normal multimeters are direct current devices that use rectifiers for measuring alternating current. That means the value will only be correct for a perfect sine wave. Some other waveforms can be measured by using a table of correction factors. To measure non-sine waveforms directly a true RMS meter is needed.


Errors and correction factors for a common multimeter (not true RMS)

Wave type Average value Effective value Crest factor Form factor Error Correction factor
Sine \frac{2}{\pi} \mbox{V}_\mbox{p} \frac{1}{\sqrt{2}} \mbox{V}_\mbox{p} \sqrt{2} \frac{\pi}{2\sqrt{2}} 0 % 1
Square Vp Vp 1 1 11 % 0.9
Triangle \frac{\mbox{V}_\mbox{p}}{2} \frac{\mbox{V}_\mbox{p}}{\sqrt{3}} \sqrt{3} \frac{2}{\sqrt{3}} -4 % 1.039
Pulse train DVp \sqrt{\mbox{D}} \mbox{V}_\mbox{p} \frac{1}{\sqrt{\mbox{D}}} \frac{1}{\sqrt{\mbox{D}}} 100 \times \left ( \frac{\mbox{D}\pi} {2\sqrt{2\mbox{D}}^{-1}}  \right ) \% \frac{2\sqrt{2\mbox{D}}}{\mbox{D}\pi}

D is the duty cycle (t/T)


Safety

Derating curve for oscilloscope probe at increasing frequency

Voltage ratings are usually specified for DC or a low frequency, high frequencies will usually make the insulation break down at much lower voltages.

It is common that a multimeter is not protected by a fuse on all combinations of ranges and units so when working on devices capable of high current great care has to be taken.

Digital meters are category rated based on their intended application, as set forth by the European Committee for Standardization EN61010 standard.

There are four categories:

  • Category I: used where current levels are low.
  • Category II: used on residential branch circuits.
  • Category III: used on permanently installed loads such as distribution panels, motors, and appliance outlets.
  • Category IV: used on locations where current levels are high, such as service entrances, main panels, and house meters.

If you don't know what rating your multimeter has it is not safe to use it where high currents are possible.

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