How to Use a Volt Meter to Determine Positive & Negative Leads

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Finding positive and negative with a volt meter
Finding positive and negative with a volt meter (Image: voltmeter image by Roman Sigaev from Fotolia.com)

A volt meter is an instrument that measures electrical pressure. Usually it has several voltage ranges or scales that are accessible by a multiple position switch. The voltmeter can measure alternating current which has no polarity (positive or negative) or direct current which is normally marked positive (+) red lead or negative (-) black lead. Many times the volt meter function is performed with other electrical measurements by the same instrument. The multimeter or volt ohm meter will measure voltage, resistance and current. In order to identify the positive and negative leads of an unknown volt meter a simple test must be performed.

Things You'll Need

  • Volt meter (unknown polarity digital or analog readout)
  • 2 leads to the voltmeter
  • Red and black marking tape or paint
  • 9-V battery (new battery with + and - identified)

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Volt meter polarity identification

Set the volt meter on the 15 volt range as shown in the pictorial. The meter should be set to DC Volts or be designed to measure direct current (DC) power. Connect the leads to meter input jacks. Insert the known battery to the meter leads momentarily. Note which lead is attached to the positive (+) side of the battery and which is the negative (-). If the meter reads negative voltage on the digital readout or the needle goes down scale on the analog meter remove immediately. Stop the test to avoid damage to the meter.

Reverse the test battery. The analog meter will read upscale or a positive on the digital meter. It will register close to 9 volts. Note which lead is connected to the positive (+) side of the battery and which is connected to the negative (-) side.

Mark the unknown leads as follows. The lead on the positive (+) side of the battery is painted or taped in red. The lead on the negative (-) side of the battery is painted or taped in black. Follow the leads back to their respective jacks or meter inputs and color code or mark them the same color as the connecting leads. The volt meter is permanently identified as positive or negative along with the connecting leads.

Tips & Warnings

  • Use clear tape or sealer over the lead and meter markings of (+) red or (-) black to prevent wear off.
  • Always check the polarity of a DC power supply to protect your meter from burn out.

References

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