This information covers the basic functions and operation of the digital multimeter. Although an analog multimeter and test light may be used by a service technician, the digital multimeter performs the more complex measurements on the newer electronic systems. Digital multimeters use the metric system, in order to make it easier to work with large numbers.
The digital multimeter is highly accurate. The digital multimeter is used to find the precise value of any type of voltage, current or resistance. The meter is powered by a 9 volt alkaline battery. The meter is sealed against dirt, dust, and moisture.
The meter has four main areas: the liquid-crystal-display, push buttons, rotary dial function switch, and inputs for the meter leads.
Liquid Crystal Display
The meter's liquid crystal display, or LCD, uses display segments and indicators. Digital readings are displayed on a 4000 count display with polarity (±) indication and automatic decimal point placement. When the meter is turned ON, all display segments and annunciators appear briefly during a self test. The digital display (1) updates four times per second. When frequency readings are taken. The display updates three times per second.
The analog display (2) is a 32 segment pointer that updates at 40 times per second. The display segments have a pointer that rolls across the segments indicating a measurement change. The display also uses indicators to abbreviate various display modes and meter functions.
The buttons on the meter are used to perform additional functions. The additional buttons will be covered later in the course as they apply to the type of measurement taken.
When the meter is first switched on and a measurement is made, the meter automatically selects a range. The meter will display the word "AUTO" in the upper left corner. Pressing the range button will put the meter in manual range mode. The range scale will be displayed in the lower right corner of the meter. With each additional press of the range button, the next increment will be displayed. Press and hold the range button in order to return to the auto range mode. The yellow button can be used to backlight the meter display.
Various meter functions are selected by turning the meter's rotary switch. Each time the rotary switch is moved from OFF to a function setting, all display segments and indicators turn on as part of a self test routine. Moving clockwise from the OFF switch, the first three positions on the rotary switch are used for measuring AC voltage, DC voltage, and DC millivolts. The top position is used for measuring resistance. The next position will allow the meter to check diodes. The last two positions are used for measuring AC and DC current in amperes, milliamperes, and microamperes.
Meter Lead Inputs
Depending on the measurement that you wish to make, the meter leads must be placed in the correct terminals. Notice the insides of the input terminals are color coded, red or black. The positive lead can go in any of the red inputs.
The "Com" or common terminal is used for most measurements. The black lead will always remain in the "Com" terminal.
The first input terminal that is on the left side of the meter is for measuring amps. This input is fused at 10 amps continuous (20A for 30 seconds).
The next position to the right is for measuring milliamperes or microamps. No more than 400 milliampere can be measured when the rotary switch is in this position. If you are unsure of a circuit's amperage, you may want to start out with the red meter lead in the 10 amp input jack (highest range).
The input terminal on the right side of the meter is for measuring voltage, resistance, and diode test.
Overload Display Indicator
While you make some measurements, you may see "OL" displayed. "OL" indicates that the value being measured is outside of the limits for the range selected. The following conditions can lead to an overload display:
- In autorange, a high resistance reading indicates an open circuit.
- In manual range, a high resistance reading indicates an open circuit or an incorrect scale that has been selected.
- In manual range, a voltage reading that exceeds the range selected.
- When you perform a diode check, voltage readings are greater that 3 volts or open test leads.
Input Terminal and Limits
The following chart shows the meter functions, the minimum display reading, the maximum display reading, and the maximum input for the 9U7330 Digital Multimeter.
|Function    ||Min Reading    ||Max Reading    ||Max Input    |
|AC Volts    ||0.01 mV    ||1000V    ||1000V    |
|DC Volts    ||0.0001V    ||1000V    ||1000V    |
|mVolts    ||0.01mV    ||400.0 mV    ||1000V    |
|Ohms    ||0.01Ohms    ||40.00 MOhms    ||1000V    |
|AC/DC Amps    ||1.0 mA    ||10.0 A (cont)    ||600V    |
|mA/µA    ||0.01 mA    ||400.0 mA    ||600V    |
|   ||0.1µA    ||4000 µA    ||600V    |
Measuring AC/DC Voltage
When you use the multimeter to make voltage measurements, remember that the voltmeter must always be connected in parallel with the load or with the circuit under test. The accuracy of the 9U7330 multimeter is approximately ± 0.01% in the five AC/DC voltage ranges with an input impedance of approximately 10 mOhms when connected in parallel.
To measure voltage, perform the following tasks:
- Make sure that the circuit is turned ON.
- Place the black meter lead in the "Com" input port on the meter and the red lead in the VOLT/OHM input port.
- Place the rotary switch in the desired position AC or DC.
- Place the black meter lead on the low side or the ground side of the component or circuit that is being measured.
- Place the red meter lead on the high side or the positive side of the component or circuit that is being measured.
Observe the circuit in Illustration 6. The tests leads are connected in parallel across the circuit load. With a 12 volt power source connected to the load, the meter should read a voltage drop equal to the source voltage or to 12 volts.
If the meter reads a voltage drop less than 12 volts, it would indicate that an unwanted resistance was present in the circuit. A logical process would be to measure the voltage drop across the closed switch contacts. If a voltage reading was present, it would indicate that the switch contacts were corroded. This will require the switch to be replaced.
Note: In actual measurements, the meter reading will not exactly equal the power source voltage, because the individual wires will offer some small resistance. In most practical applications, a voltage drop of 0.1 volts is acceptable for normal circuit wiring conditions.
The 9U7330 digital multimeter is a high impedance meter. This means the meter will not significantly increase the current flow in the circuit that is being measured. Voltage measurements should always be made with the circuit under power. The 9U7330 Digital Multimeter is ideal for use in circuits controlled by solid state devices such as, electronic components, computers, and microprocessors.
Measuring AC/DC Current
When you use the multimeter to make current measurements, it is necessary that the meter probes must be connected in series with the load or circuit under test. To toggle between alternating and direct current measurements, use the "Blue" push button.
When you measure current, the meter's internal shunt resistors develop a voltage across the meter's terminals. This is called burden voltage. The burden voltage is very low, but the voltage could possibly affect precision measurements.
When you measure current flow, the Fluke 87 multimeter is designed with low resistance to not affect the current flow in the circuit. When you measure current in a circuit, always start with the red lead of the multimeter in the Amp input (10 A fused) of the meter. Only move the red lead into the mA/µA input after you have determined the current is below the mA/µA input maximum current rating (400 mA).
The meter has a buffer which allows it to momentarily measure current flows higher than 10A. This buffer is designed to handle the surge current when a circuit is first turned on. The meter is capable of reading 20 amps for a period not to exceed 30 seconds.
To measure current, perform the following tasks:
- Place the black multimeter input lead in the "Com" port and the red input lead in the "A" (amp) port.
- Create an open in the circuit, by pulling the fuse, or by opening the switch.
- Place the leads in series with the circuit, so that the circuit amperage is flowing through the meter.
- Apply power to the circuit.
If the current flow exceeds the rating of the fuse in the meter, the fuse will open.
When you use the multimeter to make resistance measurements, it is necessary to turn off the circuit power and discharge all capacitors before attempting circuit measurements. If an external voltage is present across the component being tested, it will be impossible to record an accurate measurement.
The digital multimeter measures resistance by passing a known current through the external circuit or the component. The digital multimeter measures the respective voltage drop. The meter then internally calculates the resistance using the Ohm's Law equation R = E/I. It is important to remember that the resistance displayed by the meter is the total resistance through all possible paths between the two meter probes. To accurately measure most circuits or most components, it is necessary to isolate the circuit or the component from other paths.
The resistance of the test leads can affect the accuracy when the meter is in the lowest (400 ohm) range. The expected error is approximately 0.1 to 0.2 ohms for a standard pair of test leads. To determine the actual error, short the test leads together and reads the value displayed on the meter. Use the "REL" mode on the 9U7330 to automatically subtract the lead resistance from the actual measurements.
To accurately measure resistance, perform the following tasks:
- Make sure that the circuit or the component power is turned OFF.
- Place the red lead in the jack marked Volt/Ohms and the black lead in the jack marked "Com".
- Place the rotary selector in the Ohms position.
- Place the meter leads across the component or the circuit that is being measured.
Note: It is important that your fingers are not touching the tips of the meter leads when you are performing resistance measurements. Internal body resistance can affect the measurement.
Note: In the circuit under test in Illustration 9, the power source is isolated from the circuit by opening the switch. The switch isolates the resistor from any other path that may affect the accuracy of the measurement.