LESSON – MULTIMETERS

 

 

LESSON NOTE

INTRO TO MULTIMETERS

A multimeter is a device that allows you to measure different characteristics such as voltage, current and resistance in circuits and devices.  It is a combination of a voltmeter and ammeter and an ohmmeter.

Below is an image of the most common model of multimeter that we have in class.  While it is simple, it is both effective and inexpensive making it accessible to everybody.

 

PARTS OF A MULTIMETER

 

Most multimeters have similar parts.  The digital display gives you your reading.  The setting dial allows you to specify the type of measurement you are wanting to make.  The terminals allow you to connect the probes to the multimeter.  And the probes allow you to connect the multimeter to the circuit or device that you are testing.


MULTIMETER’S POWER

 

Before using your multimeter, check if it turns on.  Some multimeters do not have an auto-off switch.  Forgetting them on will kill the battery inside it.  Please remember to turn off your multimeter when you are done using it.

 

PROBES


The black probe should always be connected to the black terminal (common).

The red probe should be connected in one of the red terminals.  There is usually one terminal for voltage, ohms and low currents and another terminal for larger currents.

SETTING THE DIAL

 

You must set the dial in order to measure a specific characteristic.  The common options on simple multimeters are:

  • DC voltage (symbol: straight lines above V)
  • AC voltage (symbol: wavy line above V)
  • Ohms (symbol: Omega)
  • DC current (symbol: straight lines above A)
  • AC current (symbol: wavy lines above A)

 

RANGES

 

You will notice that some multimeters have ranges for each setting.  For example, under DC voltage, a multimeter might have the following setting options:

  • 200 mV
  • 2000 mV
  • 20 V
  • 200 V
  • 1000 V

The value of the setting is the maximum value that can be read on that setting.  If you see a reading of 1, it means that you are exceeding that value. 

For example, if your multimeter is set to 20 V on DC voltage and the actual voltage of the device is 40 V, your reading will show 1 as you are reading above the 20 V maximum.  The solution is simply to move the dial over to the 200 V setting to get a good reading.


WARNING

It is important to set the dial correctly and connect the probes of the multimeters correctly to devices/circuits that we are measuring.  Not doing so could be dangerous and could cause damage to the multimeter.

 

SYMBOLS FOR VOLTMETER, AMMETER & OHMMETER

 

The multimeter is generally used as a voltmeter, an ammeter or an ohmmeter.  Each has its own symbol to use in circuit diagrams.

 

 

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MEASUREMENT

 

MEASURING VOLTAGE

 

TOOL: To measure voltage, we use a voltmeter. 

 

METHOD: Place one end on one side of a source or load and the other end on the other side to get the voltage drop.

ADVANTAGE: We do not need to disconnect the circuit to measure the voltage drop.

 

DISADVANTAGE: We need to have access to several points in the circuit to measure all the different voltage drops throughout.

 

EXAMPLES

 

To measure the voltage drop across the battery.

To measure the voltage drop across the light bulb.

 

MEASURING CURRENT

 

TOOL: To measure current, we use an ammeter.

 

METHOD: You must disconnect the circuit and place the ammeter in such a way that it becomes part of the circuit path.

 

ADVANTAGE: We can calculate the current for the entire circuit by having access to just one section.

 

DISADVANTAGE: We need to disconnect the circuit to do this.

 

 

EXAMPLE

 

In simple circuits, you can place the ammeter anywhere in the circuit because the current does not change in different locations.

 

MEASURING RESISTANCE

 

            TOOL: To measure resistance, we use an ohmmeter.

 

METHOD: You must disconnect the material (load) from the circuit and connect the ohmmeter to each side of the load.  Some more advanced ohmmeter provide four leads to better test for resistance.

 

EXAMPLES

 

This will measure the resistance in the resistor.

 

RESISTOR STRIPES

 

Most resistors have a pattern of colored stripes to indicate resistance.

 

 

Here is what each stripe does:

    • The last stripe is gold or silver.  This gives the precision of the resistor.  It also allows you to know which stripe is the first stripe (the one at the other end).

    • The first stripe gives you the first number of the resistance. (Brown is 1, Red is 2, …)

    • The second stripe gives you the second number of the resistance. (Brown is 1, Red is 2, …)

    • The third stripe gives you the multiplier by which you must multiply the first two numbers.

 

EXAMPLE

 

Here is the resistance of a resistor that has the following stripes: red, brown, orange, gold.

 

Digit #1 = 2 (red stripe)
Digit #2 = 1 (brown stripe)

Multiplier = times 1000 (orange stripe)

Precision = +/- 5% (gold stripe)

 

Therefore, the resistance is 21 x 1000 = 21000 Ohms.

 

SUMMARY OF MULTIMETER USE

 

Image source: https://slideplayer.com/slide/12275638/