1. ELECTRICAL CIRCUITS
Multimeter

2. OUTLINE Purpose Measuring Voltage Measuring Current
Measuring Resistance

3. Purpose
The purpose of a Multimeter is to measure voltage, current and resistance
The proper selection for what you are measuring must be selected
V dc
V ac R
A dc
A ac
In this course we will only be measuring DC

4. Measuring Voltage Set the Multimeter to Vdc
Attach the two probes, black to “COM” or “ground”, and the red to the port labelled “V” or “Volts”
The number on the screen is the voltage reading for that power supply

5. Measuring Voltage
Reversing the probes will display a negative value on the screen
Notice that it is the same voltage, just a change in polarity

6. Measuring Voltage
Voltage can be measured whether the power supply is being used in a circuit or not
It can also be used to measure the voltage drop across any element of a circuit

7. Measuring Voltage Question
If the Multimeter displays 10.8 from the setup on the left, and 12.3 from the setup on the right, what would the Multimeter display if the probes were placed on the two legs of the LED?

8. Measuring Voltage Answer
Since the two elements are in series, and there is a total of 12.3 volts from the power supply, if the first element (a resistor) used up 10.8 volts, then the second element (an LED) would have to use up the remaining 1.5

9. Measuring Current
Depending on how much current we are measuring, we might use the “A dc” setting or the “mA dc” (milliamps) setting
If we are not sure how much current we will be measuring, always select the larger, “A”, setting
Connect the red probe to the appropriate current port, either “A” or “mA”

10. Measuring Current
When measuring current it is important to NEVER place the probes directly on the two battery terminals
This will cause a short circuit
Remember V = I * R
If our resistance goes down to zero, then our current, I, goes up to infinity

11. i.e. – break the circuit and use the Multimeter to complete it
Measuring Current
To measure current we always make the Multimeter “an element of the circuit”
i.e. – break the circuit and use the Multimeter to complete it

12. Measuring Current
Since we are measuring such a small current, we can get a more accurate reading by switching the Multimeter and probe to the “mA” settings

13. Measuring Current =

14. Measuring Resistance
To measure Resistance we simply select the resistance setting on the meter, and Ω port for the probe
Resistance is measured with no power supply in the circuit