How to use electrical testing equipment

Written by rusty noble
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How to use electrical testing equipment
Digital multimeter (digital multimeter 3 image by dinostock from Fotolia.com)

Most basic electrical troubleshooting can be done with a multimeter. There are concepts that can be applied to more sophisticated electrical testing equipment as the user gains experience.

Skill level:
Moderately Easy

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Things you need

  • Analogue or a digital multimeter (both optional)

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Instructions

    Measuring Resistance

  1. 1

    Switch the multimeter to the resistance scale.The resistance scale on your multimeter may say "Resistance", or "Ohms" or simply have an omega symbol, which is the symbol for ohms. If multiple scales are available, start with the highest scale and then move down to the lower scales as needed to get a proper reading. If your multimeter has multiple terminals, make sure the leads are connected to the resistance terminals.

  2. 2

    Remove all power from the item or circuit you are measuring. Turn it off, unplug it from the wall and remove any batteries. Any power applied to the circuit can damage or ruin the multimeter.

  3. 3

    Touch the two leads of the multimeter together and verify the resistance reads zero. Move the two leads apart and verify the resistance reads infinity. If these tests fail, there is a fault in the multimeter.

  4. 4

    Touch the two leads to the two points in the circuit to measure the resistance between the two points. It does not matter which lead is used in which location. If the resistance measures zero, it means the two points are connected to each other (you have a short circuit). If the resistance reads infinity, it means the two points are not connected (you have an open circuit). If you are expecting a short circuit, and there is some resistance measured, it is often due to a faulty connector or a faulty solder joint.

    Measuring Voltage

  1. 1

    Determine whether you are measuring direct current (DC) or alternating current (AC) by checking the documentation of the item you are measuring.

  2. 2

    Use extreme caution to avoid electrical shock. The Northwestern University Mechatronics Design Laboratory notes that voltages above 60 V DC and 30 V AC rms pose a shock hazard. Never attempt to measure high voltages such as those associated with a cathode ray tube (CRT) in older television sets and computer monitors. High voltage measurements require special equipment.

  3. 3

    Switch the multimeter to the proper voltage scale. Volts AC may be designated as V AC and volts DC may be designated as V DC. If multiple scales are available, choose the highest scale and then move down scales until a proper reading is obtained. Plug the leads into the proper terminals. For DC measurements the red lead should be connected to the positive or + terminal and the black lead should be connected to the negative or - terminal.

  4. 4

    Apply power to the unit being tested and touch the two leads to the two points being tested as noted in Step 5 (DC) and Step 6 (AC).

  5. 5

    Apply the negative lead to ground and the positive lead to the point being checked for most DC measurements. Ground is usually where the negative lead of the battery is connected. If the needle on an analogue meter moves to the left, immediately switch the leads as the polarity is wrong.

  6. 6

    Apply the leads to the two points being tested for AC measurements. The polarity of the leads does not matter for AC measurements.

    Measuring Current

  1. 1

    Switch the multimeter to the proper current scale. For most multimeters this will be indicated as DC ma. If multiple scales are available, select the highest and then move down scales until you get a satisfactory reading. Connect the leads to the current or DC ma terminals of your meter.

  2. 2

    Remove power from the circuit. Open the circuit being tested by unsoldering a lead or disconnecting a connector.

  3. 3

    Connect the leads to both sides of the open circuit and apply power to the circuit. If the needle on an analogue meter moves to the left, remove power and reverse the leads before testing again.

  4. 4

    Remove power, remove the multimeter leads, and restore the circuit to normal.

Tips and warnings

  • Be sure to check that the leads are in the proper terminals on the multimeter. Some multimeters have different terminals for resistance, AC voltage, DC voltage and current.
  • When using an analogue meter, the needle may move left for DC voltage and resistance readings. Simply reverse the leads to correct the situation.
  • Whenever you are measuring voltage or current, power must be applied to the circuit, presenting the danger of electrical shock. A good precaution is to remove metal jewelery such as watches and rings, and work on the circuit with one hand at a time. Voltages above 60 V DC and 30 V AC rms pose a shock hazard.
  • When you are measuring resistance, remove all power from the circuit to prevent severe damage to your multimeter.

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