ELECTRICAL CIRCUITS Multimeter

OUTLINE Purpose Measuring Voltage Measuring Current Measuring Resistance

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

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

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

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

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?

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

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”

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

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

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

Measuring Current =

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