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Application Note

Forward Bias - Diode Conducts Correct reading: Meter will read about 0.5 - 0.8 volt.

Unlike its predecessor, the Analog Ohmmeter, Digital Ohmmeters require a special Diode Check Function because the current circulated by the normal Ohms Function of a digital meter is too low to adequately check a diode.

In the Diode Check Position, the reading given by a digital meter in the forward bias direction (meter positive to diode anode and meter negative to diode cathode) is actually the voltage required to overcome the internal diode junction potential. For a silicon diode this will be about 0.5 - 0.8 volt; a germanium diode will read slightly lower, about 0.3 - 0.5 volt. Symbol Notation K (or C) = Cathode, A = Anode.

TPI 183 Digital Multimeter

Reverse Bias - Diode Blocks Correct reading: TPI Meter will read OUCH (open circuit).

Typical Axial-Lead Diode

A K

Band Identifies Cathode

Diode Test Procedure

WARNING! Ohms and Diode Check measurements can be made only on de-energized circuits! The Ohmmeter battery provides power to make this measurement. You may need to remove the diode from the circuit to get a reliable test. See Note below.

• Plug in the meter leads as shown: Black lead - COM (Common), Red lead - Ω (Ohms). • Select the (Diode Test) function.

• Connect the leads to the Diode-Under-Test as shown in the drawing above and verify the readings are correct for both a forward and reverse bias. (This is sometimes referred to as checking the front-to-back ratio.) Note: Large Stud-Mounted Diodes are bolted to a heat sink and Hockey Puck Units are compressed between the heat sinks; removing them from the circuit can be time-consuming and may be unnecessary. In these situations, test the entire assembly first, then, if the assembly tests shorted, remove and test the diodes individually. Hockey Puck Diodes must be compressed in a heat sink assembly or test fixture to be tested as they require compression to make-up the internal connections.

Stud-Mounted Rectifiers may be either Standard Polarity (Stud Cathode - Upper Left Illustration) or Reverse Polarity (Stud Anode - Lower Right ). If unmarked, you can test the diode to determine its polarity. With the meter connected as above, when the meter indicates the diode is conducting (about 0.5 - 0.8 volt) the red lead is connected to the diode anode and the black lead to the cathode.

Incorrect readings: If diode reads 0 in both directions, it is shorted. If it reads OUCH (open circuit) both directions, it is open. Select

K _A

K A

AN0002 Copyright 2003 Kilowatt Classroom, LLC.

DC Voltage Measurements

Application Note

DCEMEAS

Measuring DC Volts

CAUTION! Do not attempt to make a voltage measurement if a test lead is plugged in the A or µmA input

jack. Instrument damage and/or personal injury may result.

Note: Voltage is always measured across two circuit points ( in parallel with circuit element under test). On the voltage range with leads plugged-in to the meter as shown the meter has a very high input impedance and draws almost no current from the circuit under test.

WARNING! Do not attempt to make a voltage measurements of more than 1000 VDC or of a voltage level that is unknown.

CAUTION! Always check meter test leads before use to be certain they are in good condition and use test leads with an insulating rating acceptable for the maximum system voltage.

Example: Checking Double-Ended DC Power Supply Output Voltage ---

• Set Meter Selector Switch on V (DC Volts - Steady or Pulsing)

• Plug in the meter leads as shown: Black lead - Meter COM (Common), Red lead - Meter V (Volts). • Clip black test lead to power supply COM (common).

• Clip red test lead to power supply POS V (PS DC Pos Out). Meter will display the positive DC voltage. • To check PS NEG Out, move red lead to power supply NEG V. Meter will indicate the negative DC voltage with negative sign on the display.

The TPI 183 is auto-ranging (selects appropriate decimal point) and will display the voltage to the greatest degree of accuracy possible.

--- Select V Minus sign displayed when V

input (red lead) is negative with respect to COM POS V NEG V COM OUTPUT ADJ TPI 183 Multimeter

AN0003

DC Current Measurement

Application Note

PLC Analog Input Module _{24 VDC Loop} Power Supply POS + _{_} COM NEG POS Other Inputs Measuring DC Milliamps

CAUTION! Do not attempt to make a current measurement with the test leads connect in parallel with the circuit to be tested. Test leads must be connected in series with the circuit.

Note: Current is always measured with the meter placed in series with the circuit. On the current range with leads plugged-in to the meter as shown the meter has a very low input impedance and the current flow through the meter is limited by the circuit elements in series with the meter.

WARNING! Do not attempt to make a current measurements if more than 600 volts is present. Instrument damage and/or personal injury may result.

CAUTION! Always check meter test leads before use to be certain they are in good condition and use test leads with an insulating rating acceptable for the system voltage.

Example: Process Control 4 -20 mA Loop Current Measurement

• Set Meter Selector Switch on mA (AC or DC Milliamps).

• Plug in the meter leads as shown: Black lead - COM (Common), Red lead - µmA (micro or milliamps). • Open 4 - 20 milliamp loop and connect the meter in series with the loop. Note: This loop can be opened

at any one of three points. Convenience usually dictates the location. Caution - Be sure loop can be opened safely without causing a system operating problem!

• Connect meter red lead clip to the Transmitter Negative terminal.

• Close the loop by connecting the meter black lead clip to the conductor which was removed from the Transmitter Negative terminal. (This results in a current flow through the meter in a positive to negative direction.) Ch Input TPI 183 Multimeter Loop-Powered 2-Wire Transmitter 4 - 20 mA output Process Variable Optional Test Points

+ _

+ _

Typical 4-20 mA Control Loop