How to use a Cen Tech digital multimeter

Written by john demerceau
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How to use a Cen Tech digital multimeter
You need to remove a transistor from its circuit board to test its efficiency with the Cen Tech Digital Multimeter. (Jupiterimages/Photos.com/Getty Images)

The Cen-Tech Seven Function Digital Multimeter Model 90899 is an inexpensive, yet fully-featured device that you can use to measure the strength and efficiency of electrical circuits. This model features the ability to test for AC voltage, DC voltage, DC amperage and resistance (ohms), as well as functions for testing the strength and condition of diodes, transistors and dry cell batteries. It is sold with the two leads that you need to test your circuits and all you need to do to change functions is to adjust the dial.

Skill level:
Easy

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Instructions

    Testing AC voltage

  1. 1

    Turn the main dial so the dot on the knob points to "750" in the "ACV" section of the selector menu at the centre of the unit if you do not know at all what the voltage of your circuit is, or set it to "250" slightly to the right if you know that your circuit carries less than 200 volts.

  2. 2

    Plug the red lead into the centre jack, which is marked "VΩmA" and the black lead into the bottom jack, which is marked "COM." Switch your multimeter "On" by sliding the switch under the main dial to the right.

  3. 3

    Touch the exposed wires that supply the current you are testing simultaneously and carefully with the leads, or insert the leads into your wall outlet so that they touch the metal contacts.

  4. 4

    Check the reading that appears on the LCD screen above the selector dial and note it, or turn the dial one stop to the right to the "250" setting to get a more accurate reading if the initial voltage reading is less than 200.

  5. 5

    Turn your Cen-Tech 90899 off. Remove the test leads and store the device and leads together in a convenient place for future use.

    DC voltage measurements

  1. 1

    Turn the main selector so that the dot points to the "1000" setting in the DCV section, slightly to the left of the centre of the selection menu.

  2. 2

    Plug the red lead into the centre jack, which is marked "VΩmA" and the black lead into the bottom jack, which is marked "COM." Switch your multimeter "On" by sliding the switch under the main dial to the right.

  3. 3

    Touch each of the wires that supply the current you are testing with each of the leads.

  4. 4

    Note your reading as it appears on the LCD meter screen and if it is less than 200, turn the dial to the left so that the dot points to 200 volts DC. Take another reading and if it is less than 20 repeat the reading process after you turn the dial one more stop to the left to the 20 volts DC setting. Follow this reading with another reading after turning the dial yet another step if the reading is less than 2 volts -- 200m, 2,000 millivolts, as per the setting -- and take a final reading after turning the dial one more stop to the left, to the 200m setting, if the reading is less than 200 millivolts.

  5. 5

    Turn the multimeter off and remove the leads for storage; storing the leads and meter together is recommended for convenience.

    DC current (amperage) measurements

  1. 1

    Turn the main dial so the dot on the selector switch points to the "10 A" setting, which is located at the lower right of the selector menu, below the DCA selection list.

  2. 2

    Plug the red lead into the top jack, which is labelled "10 ADC." Connect the black lead to the bottom jack marked "COM." Slide the switch below the selector dial to the right to turn the meter on.

  3. 3

    Touch each of the exposed wires or conductors with one of the probes and note the reading that appears on the LCD meter screen.

  4. 4

    Turn off the unit, remove the red lead from the 10 ADC jack and insert it into the centre VΩmA jack if your reading is below 0.2 amperes. Turn the selector dial to 200 ma and take another reading. Repeat the process, turning the dial to 20 ma, 200 picoamps or 20 picoamps depending on the readings which you see.

  5. 5

    Turn off the multimeter and remove the leads before storing the meter and leads together for future use.

    Resistance (ohms) measurements

  1. 1

    Turn off any voltage source that is supplying AC or DC to the circuit or conductor you are testing. Take a voltage reading as per section 1 or 2 to make sure that there is no voltage whatsoever passing through the circuit.

  2. 2

    Turn the main dial to the 200 Ω position at the left side of the selection menu.

  3. 3

    Connect the red test lead to the jack marked "VΩmA." Insert the black test lead into the bottom jack that is marked "COM." Slide the switch below the selector dial to the "On" (right) position.

  4. 4

    Touch the test leads to each other and make sure the LCD meter screen reads "0."

  5. 5

    Touch each of the exposed wires or conductor pins that you need to test with each of the test probes and note the reading from the main LCD screen.

  6. 6

    Turn the dial to the left one stop to the 2,000-ohm setting and repeat Step 5 if the reading is 1 ohm. Keep turning the dial to the left one stop for the 20,000, 200,000 and 2,000,000 (2,000 kiloohm) settings and repeating the test procedure until you get an accurate reading -- a reading other than 1 -- as necessary.

  7. 7

    Turn the multimeter off and remove the leads before storing the meter and leads together for future use.

    Transistor (hFE or forward current gain) measurements

  1. 1

    Turn the selector dial to the "hFE" position at the bottom right of the selection menu.

  2. 2

    Slide the power switch below the main dial to "On."

  3. 3

    Insert the pins of the transistor you want to test into the appropriate multi-pin "hFE" jack at the bottom left of the unit (NPN or PNP) according to the EBC (Emitter, Base, Collector) sequence for your transistor. Check the transistor manual or information to make sure you have the correct position.

  4. 4

    Note the reading for the hFE value; if it does not correspond to the value that you need for your application replace the transistor with one in better condition or with a transistor of a different type.

  5. 5

    Store the multimeter for future use; keep the leads together with it for convenience.

    Diode voltage drop measurement

  1. 1

    Turn the main selector dial one stop past the bottom centre so that the indicator dot points to the "Diode" position that is marked with an arrow and a line similar to this: ->|-.

  2. 2

    Connect the red test lead to the jack marked "VΩmA." Insert the black test lead into the bottom jack that is marked "COM." Slide the switch below the selector dial to the "On" (right) position.

  3. 3

    Connect the red probe to the anode (input connector) and the black probe to the cathode (output terminal) of your diode.

  4. 4

    Note the approximate voltage drop in millivolts that appears on the main LCD screen. Switch the position of the red and black probes if you get a reading of "1," which indicates reverse polarity.

  5. 5

    Turn your multimeter off, remove the leads and store the meter together with the leads for future use.

    Testing dry-cell batteries

  1. 1

    Turn the selector dial so that the indicator dot points to the battery setting at the right of the selector menu; the battery setting is marked with a horizontal line, followed by two vertical lines, followed by another horizontal line similar to this: -| |-

  2. 2

    Plug the red test lead into the jack marked "VΩmA." Insert the black test lead into the bottom jack that is marked "COM." Slide the switch below the selector dial to the "On" (right) position.

  3. 3

    Touch the red probe to the positive terminal of the battery -- top terminal for 1.5 volt AA, AAA, C and D cells or crimped terminal on 9-volt cells -- and the black to the negative (bottom or smooth) terminal of your battery.

  4. 4

    Note the reading, which corresponds to amperage under a load of 370 milliohms; for a 1.5 volt cell it should be 25 milliamps and for a 9-volt cell the reading is 4 milliamps.

  5. 5

    Turn your multimeter off, remove the leads and store the meter together with the leads for future use.

Tips and warnings

  • In the event of poor performance or malfunction, change the factory-installed fuse and/or battery as follows:
  • Remove the test leads from your multimeter and turn your unit over so that the dial faces away from you.
  • Open and take out the two cover screws with a Phillips head screwdriver and remove the back cover.
  • Pull out the battery and/or fuse and replace the battery with a 9-volt battery and the fuse with a 500mA/250V fast-acting fuse as necessary.
  • Replace the cover and reposition and tighten the screws.
  • A dead fuse will give you no readings whatsoever; a weak battery may give you incorrect readings or show only parts of the LCD display.
  • Never use the battery tester function to test automotive or motorcycle batteries; use the DC voltage setting for this purpose.
  • Never touch the metal part of the probes with your hands. Wear protective electrician's gloves when testing AC circuitry.
  • Do not change the type of test you perform without turning off the unit first.

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