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How Do I Use a Multimeter?

Knowing a little about Ohm's Law helps understand what you're looking at because a multimeter is used to measure voltage, current, or resistance in an electrical or electronic circuit.

How Do I Use a Multimeter?


Greetings Douglas,

How do I use a Multimeter Correctly?

Thank you, A R

A multimeter doesn't "do" anything; a multimeter shows you what is happening. But to use a multimeter correctly, you first have to know what it does. "Multi" means many, and "meter" is an indicator - it shows you something. Overall, a multi-meter shows you many things. A multimeter, though, can only show you one thing at a time.

Each of the types of readings have to be switched to on the meter in order to use it for one type of reading or another type.

Also, whether the meter is to read AC or DC also has to be chosen by you. Depending on the meter, AC can say "AC" or could have a squiggly line, DC can say "DC" or have a solid line over a dashed line. Read the manual that came with the meter to find out, and if the manual isn't available, look up the manual on the web for the make and model of the meter.

Voltage - Current - Resistance

Using a multimeter, you can look at the amount of voltage, or you can look at the amount of resistance, or you can look at the amount of current.
  • More or less Voltage will push more or less electrical Current through a Resistance

  • More or less Resistance allows less or more Current when pushed through by a certain Voltage

As you can see, the three types of readings, Voltage, Current, and Resistance, doesn't stand alone. The three types of readings relate to each other, and this relationship is shown in a mathematical formula - Ohm's Law.

Ohm's Law - "E = I x R"

  • "E" is electrical push/pull, it's the Electromotive Force. "E" is measured in voltage. Voltage is one of the measurements on the multimeter.

  • "I" is the intensity of the electrical current, it's a quantity of electrons going past a single point. "I" is measured in Amps or Amperage. A single Amp is a very, very, very large number of electrons going past a point in a wire in a second. Amps is one of the measurements on the multimeter.

  • "R" is resistance to current, it determines how much current can be pushed through a wire or electronic component by the Electromotive Force. "R" or Resistance is measured in Ohms. Ohms is one of the measurements on the multimeter.

Ohm's Law can be used to find the value of "E", "I", or "R".
  • E = I x R - To find the Voltage value of "E", multiply the Amperage value of "I" by the Resistance value (Ohms) of "R".

  • I = E / R - To find the Amperage value of I", divide the Voltage value of "E" by the Resistance value (Ohms) of "R".

  • R = E / I - To find the Resistance value (Ohms) of "R", divide the Voltage value of "E" by the Amperage value of "I".

Voltage

Voltage comes in two forms: DC Voltage and AC Voltage.

DC Voltage is what comes out of a battery. DC voltage is constant, never changing. AC Voltage is the same thing - except it keeps reversing, similar the battery turned over and over dozens of time a second. Sometimes the AC voltage reversals can be faster like radio waves reversing millions to billions of times a second.

DC Voltage Polarity

The red probe and the black probe tell you the polarity of the meter, not the polarity of what is being measured. You have to look at the multimeter's display to find out the polarity of what the multimeter is measuring.

If you are measuring the voltage of a pair of wires, and the display doesn't show a "-" sign, then the red probe is on the positive wire and the black probe is on the negative wire. If you are measuring the voltage of a pair of wires, and the display does show a "-" sign, then the red probe is on the negative wire and the black probe is on the positive wire.

An easy way to think about it is the display shows the polarity of what the red probe is measuring. The black probe is always showing the opposite voltage.

Check It Out

A "AA" or "AAA" battery has two poles or contacts. You can think of as very short wires.

Set the meter to read DC Voltage. Put the red probe on one of the battery contacts and the black probe on the other contact. The meter's display shows you two things.
  • It shows you what the actual voltage is on the battery (batteries are chemically based and will almost never be the exact voltage shown on the name of the battery).

  • It shows you the polarity of the contacts or wires. If the red probe is on the negative contact, the multimeter's display will show the "-" sign, if the red probe is on the positive contact, there will be no sign, or a "+" sign.

Try reversing the probes and look at the display again. The "+" or "-" sign should have changed, but the voltage should be the same.

AC Voltage

Because AC Voltage Polarity is continually reversing, AC Voltage Polarity can't be measured with a multimeter. The multimeter, when measuring AC voltage, shows an Equivalent-to-DC voltage. The display on the multimeter just won't show a polarity sign.

Resistance

When measuring resistance, the multimeter actually applies a small voltage and measures the current that the wire or component allows through it. The multimeter then uses Ohm's Law (see above) to figure out the resistance in Ohms.

The display often shows a letter with the resistance.
  • 110 Ohms is 110 Ohms
  • 4.7K Ohms is 4.7 Kilo Ohms, or 4,700 Ohms
  • 33M Ohms is 33 Meg Ohms, or 33,000,000 Ohms

Current

Like gallons or liters, current is volume. Think of electrical current as the equivalent gallons or liters of water going over a dam, per second.

DC Current Polarity

Normally, the way to measure current is to put the meter directly in the path of the current. In essence, the pathway for the current has to be broken for the meter to measure the current through the pathway.
  • If the meter doesn't have a "-" sign on the display, that means that the electrons are moving through the meter is the normal direction - into the black lead and out of the red lead.

  • If the meter has a "-" sign on the display, that means that the electrons are moving the opposite direction than normal - into the red lead and out the black lead.

Be careful. The multimeter manufacturers cannot properly protect a multimeter without using a fuse. If you measure the current that a sealed lead acid battery can put out, for instance, the sealed lead acid battery can easily put out 50 to 500 amps and burn out the fuse.

AC Current

Because AC Current Polarity is continually reversing, AC Current Polarity can't be measured with a multimeter. The multimeter, when measuring AC current, shows an Equivalent-to-DC current. The display on the multimeter just won't show a polarity sign.

Be careful. The multimeter manufacturers cannot properly protect a multimeter without using a fuse. The circuit breaker for the outlet may say "20 Amps", but it takes time for the circuit breaker to trip. If you measure the current that an electrical outlet can put out, for instance, during that very short time before the breaker trips, the electrical outlet can easily put out 50 to 500 amps and burn out the fuse.

Douglas Krantz
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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