Lesson 12: Multimeters
Multimeters are used to measure various components of electricity: voltage, resistance, and current. Multimeters can be used in many different applications; for instance, in industry, homes, laboratories, and for hobbies. The three main electrical components multimeters measure are voltage (volts), current (Amps), and resistance (Ohms). But before we get into measuring those components, let’s look at how to set up the multimeters.
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Nootte: This lesson is based off of the Knight Electronics Multimeters. If our class is using a e different brand of multimeters, the basic instructions are the same, but with slight
variations in numbers and specific placements.
The multimeter should come with a set of two test leads: a red lead and a black lead. The black lead is generally associated with ground or as labeled on the multimeter COM (common). You should connect the black lead to the COM jack on the multimeter. In the electrical world, ground is coined common because it is a “common return path” for electrical current. Ground allows for a potential that causes electrons in a circuit to begin moving. The lead plugged into the common jack is also called the negative lead, but more on that later. The other test lead packaged with the multimeter is a red test lead. The red test lead can be plugged into either of the three remaining jacks: V, A, or 20ADC. For most projects in the class, you will plug the red test lead into the V or A jacks. The V jack is used for measuring all voltages and resistances, hence the V for Volts and for Ohms. The A and 20A jacks are used when measuring current. The lead coming from any of these three jacks is considered the positive lead.
Once you have the test leads connected to the multimeter, you need to indicate to the multimeter what quantity you are trying to measure. The multimeters used in this class have capabilities to measure voltage, current, and resistance. A dial is located at the center of the multimeter. There are six portioned off sections on the multimeter around the dial. Starting in the top left moving clockwise: hFE, , (voltage from a DC source), V~, A~,
and (current from a DC source). In this class, you will not be concerned with using hFE.
The first section on the multimeter is . This section is used when measuring resistance. The numbers aligning the perimeter of the dial in the section are 200, 2k, 20k, 200k, 2M, 20M. The units of the resistance are Ohms (), where the k behind a number indicates that the measurement is a kiliOhm and M indicates a MegaOhm. The dial should be placed on the number closest to the value being measured, but not less than the value being
measured.
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Meeaassuurriinnggrreessiissttaanncce: Resistance is a measure of difference. Placing the test leads across e the component on the positive and negative sides will yield the desired output.
units are milliVolts. The same applies for the V~ section, however, the voltage source
originates from an alternating current source.
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Meeaassuurriinnggvvoollttaaggee: Much like resistance, voltage is measured as a difference, so placing the test leads over the component being measured on the negative and positive side will yield the desired value.
Next, we will discuss the section on the multimeter. The section is used when measuring direct current values. Similarly for the section, it is lined with the
numbers: 20, 2, 200m, 20m, 2m, 200. The base units for current are Amperes or Amps. The indicates that the measurement is microAmps and the m indicates the measurements are in milliAmps. The same applies for the A~ section, however, the current originates from
an alternating current source. When measuring values close to 20A, but not exceeding 20A, the red test lead should be plugged into the 20A jack. Turn the dial to the appropriate
, A~ and use accordingly.
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Meeaassuurriinnggccuurrrreennt: Unlike voltage, current is not a measure of difference. In order to t measure current, the test leads must be positions such that the multimeter is part of the circuit. Tell the students to picture current passing through the circuit. The test leads must be connected such that the current passes through the red lead into the multimeter and out of the black lead back to the circuit.
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Diiaallppllaacceemmeennt: Regardless of the component being measured, the numbers aligning the t perimeter of the dial all indicate the upper bound that can be measured. For instance, if you are measuring a resistor that should be 110 , you should place the dial at 200 in the section. This is because 200 is greater than 110 , but is also the closes to the 110 out of the available values greater than 110 . Another example is if yuou were measuring a circuit that should have a voltage source of 220 VDC. One may be inclined to place the dial on the 200 V location in the section. However, this would be incorrect because 220 V is greater than 200 V and thus, the reading will be erroneous. The dial would need to be placed at the 1000 V location in order to obtain a proper reading.
Troubleshooting
If you have properly set up your multimeter and no
readings are being shown in the display, be sure to check if the multimeter has been turned on. There is an on/off switch that must be flipped in order for the multimeter to take measurements. This is a simple solution that is often over looked.
sticker.) The fuse, shown in picture to the right, should be able to be pried out easily. Once removed, place a new fuze in the open spot and then replace the backing of the multimeter.
Suggestion: This may be a good section to discuss with students the concept of units and unit conversions. Since many of the units are read in the form of micro, milli, and kilo. Practice translating from base units to the kilo, milli, and micro forms and vice versa will be beneficial. Be sure to stress to the students that the prefixes indicate orders of magnitude to the base units not a completely different unit. Also, the prefixes can be interchanged with different base units (i.e. grams, ohms, volts, Amps, meters, etc.).
1 Mega (M) 1 kilo (k) 1 hecto (h) 1 deka (da) (base unit) 1 deci (d) 1 centi (c) 1 milli (m) 1 micro ()
106 103 102 101 1 10-1 10-2 10-3 10-6
Activity
Have the students create the following simple circuit on their Boe-Bot:
Given this circuit, have the students measure the resistance across the resistor, the current passing through the circuit, and the voltage drop across the resistor. (When measuring the resistor, do not turn the power on the Boe-Bot, but do turn the power on to measure the current and voltage.)
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Suuggggeessttiioonns: Have the students change the 220 s resistor to a 1 k resistor and repeat the measurements. Change Vin to Vdd and repeat. Add a second resistor into the circuit and measure the components using the multimeter.
In Class Practice
Have the students put the dial in the proper location, if they were measuring the hypothetical values:
1. 0.0015 A Ans. 2 in A (DC)
2. 9 V Ans. 20 in V (DC)
3. 0.000165 A Ans. 200 in A (DC)
4. 110 V Ans. 200 in V (DC)
5. 0.03 A Ans. 200 m in A (DC)
6. 220 Ans. 2k in
7. 30 Ans. 200 in
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220Ω Vin
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Soolluuttiioon: n
Resistance
Current
Notice the lead placement across the resistor.
Notice the dial placement is on 2kΩ
Notice the red test lead is in the VΩ jack.
Notice the test leads are included as a part of the circuit.
Voltage
Notice the lead placement across the resistor.
