Molly Ayala
Electrical measurements are crucial in various industries, including construction, manufacturing, and electrical engineering, to ensure the safety and efficiency of electrical systems. Electrical measuring instruments are used to monitor, measure, and analyse electrical circuits and systems. These instruments help measure electrical quantities such as voltage, current, resistance, and power. Common electrical measuring tools include multimeters, ammeters, voltmeters, ohmmeters, and oscilloscopes. These instruments provide valuable data for troubleshooting, maintenance, and ensuring the proper functioning of electrical components. Understanding how to use these tools correctly and interpreting their readings are essential skills for anyone working with electrical systems.
| Characteristics | Values |
|---|---|
| Electrical quantities | Voltage, current, resistance, power |
| Voltage measurement | Voltmeters; analog or digital; measures voltage or electrical potential difference |
| Current measurement | Ammeters; analog or digital; measures current in microamperes or milliamperes |
| Resistance measurement | Ohmmeter; measures resistance in ohms (Ω) |
| Power measurement | Wattmeter |
| Electrical energy consumption | Measured in Watt-Hour (Wh), Kilowatt-hour (kWh), Megawatt-hour (MWh) |
| Decibel | Represents gain in voltage, current or power; logarithmic unit |
| High-precision measurements | Used in experiments to determine fundamental physical properties |
| Less precise measurements | Used in everyday industrial practice |
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What You'll Learn
Using a multimeter to measure voltage, current and resistance
A multimeter is an essential tool for measuring electrical quantities of electrical components. It can measure voltage, current, and resistance. Here's a step-by-step guide on how to use a multimeter to measure these quantities:
Measuring Voltage
To measure voltage, you need to attach the multimeter in parallel with the circuit. This is because, in a parallel circuit, each circuit measurement has the same voltage. First, identify the voltage range. For household outlets, you're likely dealing with AC voltage, while for batteries or laptop power supplies, you're dealing with DC voltage. Set the multimeter to the appropriate range, which is usually labelled with "AC" or "DC" and a "V" for voltage. Plug the probes into the appropriate jacks, typically the common (COM) and VΩ jacks. The COM probe is usually black, while the other probe is red. Touch the probes to the positive and negative terminals of the voltage source, being careful to match the probe colours to the terminals. Note that you should not use a multimeter with high-voltage AC outlets unless you are experienced.
Measuring Current
Measuring current is one of the trickiest measurements in electronics. This is because you have to measure current in series. To measure current, you must attach the multimeter in series with the circuit. First, identify the current range. If it's above 200mA, you'll need to use the 10A port. Plug the probes into the appropriate jacks, with the black probe in the common port and the red probe in the mAVΩ port. Interrupt the circuit by removing the wire going to the resistor and inserting the multimeter in line with the circuit. This allows the multimeter to measure the current as it flows through it.
Measuring Resistance
Resistance is measured in ohms. On an analog multimeter, you'll see a scale labelled with ohms, resistance, or the ohm symbol (Ω). On a digital multimeter, look for similar tags next to a number. First, disconnect the component you're testing from any live circuitry. Set the multimeter to the resistance function. Touch the probes to either side of the component you're testing. On an analog multimeter, ensure the black probe touches the negative side. Move down the ohm range on the multimeter gradually until you get a reading.
Safety Considerations
When measuring AC voltage, exercise extreme caution as it can be dangerous. Only measure AC voltage in outlets if you know what you're doing. Respect DC voltage sources as well, as they can deliver a dangerous shock.
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Measuring voltage with a voltmeter
To measure voltage with a voltmeter, you will need a multimeter, which is an electrical instrument capable of measuring voltage, current, and resistance. The voltmeter is typically one of the functions of a multimeter. Analog multimeters tend to be less expensive than digital multimeters and may be more suitable for beginners. If your multimeter is digital, it will require a small battery to operate. Analog multimeters do not need a battery to measure voltage.
Manual-ranging meters have several different selector positions for each basic quantity: several for voltage, several for current, and several for resistance. For voltage, these ranges could be μV, mV, and V. Some digital multimeters are autoranging, which means they automatically shift gears to find the best measurement range.
Before you begin, insert the test probes into the voltmeter. By convention, black probes are used with the negative, or common, input of the voltmeter, while red probes are used with the positive input.
To measure the voltage coming off of a power supply stick, set the knob to "20V" in the DC range (the DC Voltage range has a V with a straight line next to it). Multimeters are generally not autoranging, so you will need to set the multimeter to a range that it can measure. For example, 2V measures voltages up to 2 volts, and 20V measures voltages up to 20 volts. If you are measuring a 12V battery, use the 20V setting.
To measure the voltage of an LED, touch the meter's test probes to the terminals (wire leads) of the LED. An LED uses a small amount of electricity to produce light, but it also generates DC voltage when exposed to light. You can then see how much voltage the LED is using, which is referred to as the voltage drop across the LED.
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Using an ammeter to measure current
To measure current using an ammeter, you will need to break the circuit open and insert the ammeter in series (in-line) with the circuit. This will allow the ammeter to intercept all the circuit's current without creating a short circuit.
Firstly, identify the wire or component terminal through which you want to measure the current. Pull that wire out of the breadboard hole and leave it hanging in mid-air. Insert a spare piece of wire into the hole and also leave the other end of this wire hanging in mid-air. Then, connect the ammeter between the two unconnected wire ends.
Next, you will need to set the ammeter current type and range. If your ammeter divides the range of current into settings, choose the highest setting to prevent blowing the meter's internal fuse if the amperage is too high. Select the type of current you will be measuring: AC (Alternating Current) or DC (Direct Current). Battery-powered circuits run on DC.
Now, break the circuit open at any point and connect the meter's test probes to the two points of the break to measure the current. If your meter is manually ranged, select the highest range for current, then move the selector switch to lower range positions until the strongest indication is obtained on the meter display without over-ranging it. If the meter indication is "backwards", reverse the test probe connections and try again.
Induction ammeters are different from in-series ammeters as they won't have leads or probes. Instead, they have a single clamp or ring through which a wire will pass. The induction sensor reads the current through the electromagnetic field given off by the electricity. Position the sensor around the power input wire and set the ammeter to auto to allow the meter to automatically adjust the range.
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Measuring resistance with an ohmmeter
Resistance is measured using analog or digital multimeters, also known as ohmmeters. Ohmmeters are electrical instruments capable of measuring voltage, current, and resistance. Resistance is measured in ohms, represented by the Greek letter omega (Ω).
To measure resistance with an ohmmeter, start by ensuring the circuit is powered off. Plug the black test lead into the common input jack, and the red or yellow lead into the resistance input jack. If using an analog ohmmeter, calibrate it for 0 Ω of resistance by touching the test probes together and adjusting the knob or wheel until the needle points to zero. For a digital ohmmeter, the display should automatically show OLΩ, indicating it is in Resistance mode. Touch the probes together and check for a display of zero or a very small amount of resistance. With the leads apart, the display should show OL or I.
Next, connect the probe tips across the component or portion of the circuit for which you want to determine resistance. If you get an OL (over limit), switch to the next highest setting. If the circuit includes a capacitor, discharge it before taking any resistance reading.
Finally, view the resistance reading on the display, noting the unit of measurement. The reading should be as expected based on the manufacturer's specifications, nameplate rating, Ohm's law, and Kirchhoff's law. Testing blindly can be dangerous and counterproductive.
It is important to avoid touching the metal parts of the test leads with your fingers during the measurement process, as the human body can become a parallel resistance path, lowering the total circuit resistance and leading to inaccurate readings.
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Understanding standard electrical units of measure
Electrical measurements refer to the methods, devices, and calculations used to measure electrical quantities. Electrical quantities can be measured to determine the electrical parameters of a system. Electrical units of measurement are based on the International System (SI) of units. The SI units to measure voltage, resistance, and current are Volt (V), Ohm (Ω), and Ampere (A), respectively. The unit of power in the SI system is the Watt, named after James Watt, the inventor of the steam engine.
Electricity is often measured in kilowatt-hours (kWh), which is the energy expended by a source of 1 kilowatt in one hour. Other standard units include the Watt-Hour (Wh), which is the amount of electrical energy consumed by a circuit over a given period, and the Decibel (dB), which is a one-tenth unit of Bel (B) used to represent gain in voltage, current, or power.
There are also several other electrical units derived from the SI unit of electricity. For example, 1 kV is equal to 1000 Volts, 1 mA is equal to one-thousandth of an Ampere, and 1 kW is equal to 1000 Watts. These multiples and sub-multiples of standard units are used to avoid writing multiple zeros when describing very large or very small values.
Angular Frequency (ω) is used in AC circuits to represent the phase relation between two waveforms. The Time Constant (τ) is a characteristic of an impedance circuit, representing the time it takes for the output to reach 63.7% of its maximum or minimum output value when subjected to a Step Response input.
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Frequently asked questions
The standard electrical quantities are voltage, current, resistance, and power.
The standard electrical units of measurement are volts, ohms, and amps. Volts measure voltage, ohms measure resistance, and amps measure current.
Electrical measuring instruments are essential tools for monitoring, measuring, and analyzing electrical circuits and systems. Some common instruments include multimeters, ammeters, voltmeters, wattmeters, ohmmeters, and oscilloscopes.
