Fundamental of electrical automotive engineering - Understanding and Measurement use the Multimeter

Fundamental of electrical automotive engineering - Understaning and measurement use the multimeter. How to use the multimeters ?. There are 2 types of multimeters in displaying the measurement results are Digital Multimeter (DMM) and Analog Multimeter (AMM).

Digital Multimeter

Digital Multimeters (DMM), also called Digital Volt-Ohm Meters (DVOM), are a commonly used tool for measuring voltage, current, resistance and other electrical test functions. Digital meters have displays that indicate digital numerical values. DMM have very high internal resistance or impedance, in the range of 10 mega Ohms (10 million Ohms). This resistance means that the meter draws extremely small amounts of current and for all practical purposes does not affect the circuit being tested. DMM are safe to use when testing circuits with processors, control modules, and other electronic devices.

Analog Meter

Old-style analog meters have dials with needles that sweep instead of digital displays. Generally, analog meters should not be used for testing low voltage low current circuits. Most analog meters do not have very high internal resistance or impedance. When connected to a circuit, meters become another parallel branch in the circuit being tested. This is not a problem for general electrical testing, but when testing electronic circuits with low impedance meters, the increased current flow may damage circuits and components or adversely affect the reading obtained.

Features

DMM can perform many electrical test and measurement functions. A list of basic and advanced DMM functions is shown below. Basic DMM functions typically include measuring : DC voltage, AC voltage, resistance, continuity, capacitance, diode test, and current (amps/milliamps). Advanced DMM functions typically include measuring : Frequency, RPM, duty cycle, and pulse width.

Voltmeter Function

The volmeter function of the DMM is used to measure the difference in voltage or electrical pressure at two points in the circuit. A voltmeter reads available voltage such as battery voltage, generator output voltage, and voltage drops across components or conductors. Zero voltage (0 volts) at any point can indicate an open circuit or a short to ground. To measure voltage, the meter is connected in parallel.

DMM Voltmeter Functions :

• Measures the difference in voltage between two points.
• Read available battery or generator voltage.
• Measures the voltage drop across components or conductors.
• Can indicate an open circuit or a short to ground.
• To measure voltage, the meter is connected in parallel.

To measure voltage, meter and circuit polarity should be observed. The red probe is placed on the more positive test point in the circuit, that is, the test point closer to the positive (+) source. The black probe is placed on the more negative test point, that is, the point closer to ground or the negative (-) source. If the leads are reversed, a negative value will be displayed, but the absolute value will be accurate. Some meters auto-range and change scales automatically. On others, you must select the voltage scale or range. Always use the lowest possible scale for accuracy.

Unit Of Voltage

The basic unit for measuring voltage is the volt. Large and small amounts of voltage require other units. See the table below to see the common units, their symbols and mathematical multipliers. You will see these units and symbols used in service information and on test equipment displays such as DMM. The following may help you to remember the multipliers. ''Micro''=One millionth; ''Milli''=One thousandth; ''Kilo''=times one thousand ; and ''Mega''=times one million.

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