How to use a multimeter or V-O-M (Volt-Ohm-Meter)

What is a multimeter anyway?

A multimeter normally consists of a voltage meter to measure AC or DC voltages…..

A multimeter normally consists of an ohm meter to measure resistance in a circuit, like a resistor or network…..

A multimeter normally has the ability to measure current thru a circuit or wire, some can do AC current and DC current from milliamps to 20 Amps.

Some multimeters now days can measure frequency as well as capacitance.

Some meters can check a diode and also measure the HFE or “gain” of a transistor, or whether it is a PNP or NPN type of transistor.

#1  Digital LCD or LED multimeter

Some LCD digital meters have a sensitivity of Megohms per volt, making them very attractive when measuring a high impedence circuit, because it won’t load down the circuit or measurement under test.

#2 Display driver circuit LED

#3  Analog Multimeter 

Quality analog meter movements sensitivity can be 20,000 ohms per volt.

To a circuit it could look like you just connected a 20K resistor in the circuit. Some sensitive circuits could see this as a load and effect the normal operation of that circuit.

Some inexpensive meters were so low in sensitivity that they were only good for measuring volts and resistance out of a circuit, because of the loading effect on the circuit under test.

#4 The meter movement

The D‘Arsonval movement is a DC moving coil-type movement in which an electromagnetic core is suspended between the poles of a permanent magnet. This was first called a galvanometer before the development of the D Arsonval movement. They were the first instruments used to detect and measure small amounts of electric currents.

Note…that when using an analog meter the Polarity or Positive and Negative leads MUST be hooked up correctly or the meter movement will deflect the needle in the opposite direction and can damage the movement.

#5 What are some of the uses of a typical multimeter?

* DC voltages when checking a battery or power supply output.

* Checking the charging voltage that your car’s alternator produces.

* Checking the AC voltage at an outlet in your home.

* Determining what AC voltages an unmarked transformer is producing on the wire leads.

* Measuring the ohms of a resistor or potentiometer.

* Checking the voice coils of a speaker.

* Checking a fuse or lamp to see if it is open.

* Measuring the current that a circuit is drawing from its power source.

* Measuring the charging current to a battery being charged.

* Use as a continuity checker with an audible tone.

#6  “VOM”  Volt Ohm Meter

 

#7  Digital meter face

#7A  Meter leads polarity

#7B  Observing correct polarity

#7C  Meter comparison

#8   Measuring vehicle alternator output

#9 Measuring Resistance (why we short the leads)

#9   Measuring Resistance

#9B  Brings back thoughts of …

#9C

#10 Measuring current demand

#11   Measuring current with leads reversed

#12 Continuity testing (low resistance range)

Used to track a short, if the circuit schematic indicates that the path between two points should not be 0 ohms, this a good use of your ohmmeter with good sharp points on your probes.

Checking for good ground through out a circuit.

For checking fuses that are sand filled (unable to see the fuse element). These are normally used in a high voltage circuit. The sand filled fuse prevents the fuse from arcing inside when it opens.

Checking connections or solder joints that look suspicious.

My multimeter is probably the most used piece of test equipment I own.

As the price keeps coming down on relatively decent quality meters, everyone should have one in their tool bag or on the bench.

It will come in handy, if only to check that battery in your kitchen timer or smoke alarm.

73 Earl