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Voltage and Current: Motors (2019)
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Voltage and Current: Motors (2019)

Failed Motor Circuit

Mini Project Current Investigation

Let’s take a deeper look into what current is, and what it has to do with voltage. To do this, we will use the multimeter to measure the current in our motor circuit.

Block diagram: Multi-meter connected to circuit
Step 1: Disconnect the alligator clip

Disconnect an alligator clip from a motor lead so that the battery doesn’t get used up. Go through the following section to learn how to use multimeter to see the amount of current is going through the motor.

Module: Measuring current
Testing Current with a Multimeter: Required items and tools

To begin testing your circuit, you will need the following:

  • Digital Multimeter Digital Multimeter
Measuring Voltage with Multimeters

Multimeters measure current differently for Direct Current (DC) Circuits and Alternating Current (AC) Circuits. On a multimeter these setting are shown with different symbols. Some multimeters have a setting that measures both AC and DC current. Current is measured in a unit called Amperes, more commonly called Amps. Measuring amperage can damage a multimeter. Check the bottom ports of your multimeter to see the maximum input the multimeter can handle.

  • AC Amperage
  • DC Amperage
  • AC/DC Amperage
Safety Notice

AC current can be very dangerous (especially the main voltage out of outlets on a building’s wall) and great care must be taken when measuring it with a multimeter. If you feel like you must test or measure AC, it is recommend you get a non-contact tester rather than use a digital multimeter. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current. While any amount of current over 10 milliamps (0.01 amp) is capable of producing painful to severe shock, currents between 100 and 200 mA (0.1 to 0.2 amp) can be lethal with a high voltage (40+ Volts).

Step 1: Set Multimeter to DC Amperage Mode
picture of both manual and auto ranging voltage selection

Depending on the multimeter you have you may have one setting or many different settings for measuring Current.

auto ranging multimeter
  • An auto ranging multimeter will automatically change the the range of current it is measuring in to accurately measure the amperage.
  • Some auto ranging multimeters, such as the orange one shown, have separate settings for measuring currents with vastly different magnitudes.
  • This multimeter has a setting for measuring micro amps (?A) or 0.000001 Amps, milli amps (mA) or 0.001 Amps, and A (Amps). Depending on the current provided by your power source you may need to change this setting. If your multimeter shows a value of “1” on the left side of the screen you have Overloaded the measurement setting and should choose a larger setting to measure the current.
manual ranging multimeter
  • With a manual selection multimeter the correct resolution must be selected to correctly measure the current. The settings shown on the multimeter show the maximum current that setting can successfully read.
Overloaded current measurement
  • If you select a setting that is too small for the Current you are measuring, the multimeter will display a 1 on the left-most side of the screen.
  • This means that the multimeter is overloaded or out-of-range of the current being measured. Having the multimeter setting too low may prevent your circuit from working. Make sure your circuit is working (LED lighting up or motor spinning) before recording the measurement on a multimeter.
Measuring a coin cell with different current settings
  • If you select a Current setting that is too high, you will not have the same accuracy as if you were to use a setting that is close to the amount of measured Current. Depending on your circuit, having the multimeter setting too high may prevent your circuit from working. Make sure your circuit is working (LED lighting up or motor spinning) before recording the measurement on a multimeter.
Step 2: Insert lead probes into the terminals
Probes in the correct ports

When measuring Current, make sure your probes are plugged into the correct ports. The Black Probe should be in “COM” or common and the Red Probe should be in the Port with the Current symbol (A).The following example will use an LED, LEDs do not draw high amount of current so the measurement will be taken using the mA port.

Step 3: Measure the Amperage

The multimeter can be used to test current going through parts of the circuit between components. Measuring the current between components requires the circuit to be open, or disconnected. The multimeter will act as a "wire" in your compleated circuit.

probing LED circuit
  1. Set the multimeter to measure mA or 200mA if you have a manual selection multimeter.
  2. Take the positive (red) multimeter test lead and connect it to the positive side of the battery.
  3. Connect the black lead to the LED.
  4. This circuit shows that it has 5.26 milliAmps of current going through it.
  • What happens if you switch the the red and black leads?
    Nothing bad happens! The reading on the multimeter is just negative. The multimeter measures current with its direction. If the multimeter has a negative number the current is flowing from the black lead and out the black lead.

Check Your Understanding

What setting should the dial be turned to to measure DC amperage in the milli amp range?
Hover over parts of the image and click to select
  • Voltage Setting 1
  • Voltage Setting 2
  • Resistance Setting
  • Diode / Continuity Setting
  • Hertz Setting
  • Current Setting 1
  • Current Setting 2
  • Current Setting 3
Which terminal should the RED lead be plugged into for this multimeter, if we are measuring a circuit in the that has a cuurent around 30 mA?
Hover over parts of the image and click to select
  • 200mA to 10A current terminal
  • The selected terminal only receives the red lead, to measure capacitance, temperature, hFE and 0.001mA to 200mA current.
  • Return terminal for all measurements
  • Voltage, resistance, frequency, diode, and continuity test terminal
Step 2: Measure the current in the circuit

Measure the current that is going through the alligator clip and the motor lead using the multimeter.

It should read somewhere around 70 μA (nano Amps) or 0.07mA (mili Amps). This circuit has a brand new battery and the current is 0.08mA. Record this value in your log or keep this in mind for the next section.

Explanation What is Amperage?

Amperage is a measure of electrical current flow, also called amps for short. An Amp is a measurement of the number of electrons in a conductor flowing past a certain point in a given amount of time. Think of a highway and you want to measure all the cars passing exit 4 past exit 5 in one hour. In a circuit, this measurement would be looking over a small portion of a wire for a second.

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