Figure 1. A typical multimeter. Technical Note The symbol that is used for a unit is usually different than the symbol for a variable in an equation. For example, voltage, current, and resistance are related by Ohm's law see the References tab to learn more about Ohm's law : [Please enable JavaScript to view equation] which is usually expressed as [Please enable JavaScript to view equation] In this equation, V represents voltage, I represents current, and R represents resistance.
Figure 2. In a basic series circuit left , each element has the same current but not necessarily the same voltage; that will only happen if their resistances are all the same.
In a basic parallel circuit right , each element has the same voltage but not necessarily the same current; that will only happen if their resistances are all the same. Figure 3. The multimeter on the left is manual-ranging, with many different options indicated by metric prefixes for measuring different amounts of voltage, current, and resistance. The multimeter on the right is auto-ranging note how it has fewer options for the selection knob , meaning it will automatically select the appropriate range.
This stands for alternating current AC. Note that the voltage in an AC circuit is usually referred to as "AC voltage" even though it sounds strange to say "alternating current voltage". You use these settings when you are measuring a circuit with alternating current or voltage. The straight lines stand for direct current.
You use these settings when you are measuring a circuit with direct current e. Continuity check series of parallel arcs : This is a setting used to check if two things are electrically connected. The multimeter will beep if there is a conductive path between the two probe tips meaning, if the resistance is very close to zero , and will not make any noise if there is no conductive path.
Note that sometimes the continuity check can be combined with other functions on a single setting. Diode check triangle with some lines through it : This function is used to test a diode , which is like a one-way valve for electricity; it only lets current flow in one direction.
The exact function of the diode check can be different on different multimeters. Check your multimeter's manual to learn about how the diode check function works for your model. Table 1. Some symbol examples from different multimeters. Check out the gallery for more examples. Figure 4. A typical pair of multimeter probes. Figure 5. This section includes answers to the following questions: How do I measure voltage?
How do I measure current? How do I measure resistance? How do I do a continuity check? How do I do a diode check? How do I know which scale to pick for voltage, current, or resistance, and how do I read the numbers at different scales?
My multimeter isn't working! What's wrong? How do I know if I need to change the fuse? How do I change the fuse? How do I measure voltage? To measure voltage, follow these steps: Plug your black and red probes into the appropriate sockets also referred to as "ports" on your multimeter. For most multimeters, the black probe should be plugged into the socket labeled "COM," and the red probe into the socket labeled with a "V" it might also have some other symbols.
Remember to check out our image gallery, the Multimeter Overview tab, or your multimeter's manual if you have trouble identifying the right socket. Choose the appropriate voltage setting on your multimeter's dial.
Remember that most battery-powered circuits will have direct current, but the setting you select will depend on the science project you are doing. If you are working with a manual-ranging multimeter, you can estimate the range you need based on the battery or batteries powering your circuit. For example, if your circuit is powered by a single 9V battery, it probably doesn't make sense to select the setting for V, and 2V would be too low.
If available, you would want to select 20V. Touch the probe tips to your circuit in parallel with the element you want to measure voltage across refer to the Multimeter Overview tab for an explanation of series and parallel circuits. For example, Figure 6 shows how to measure the voltage drop across a lightbulb powered by the battery.
Be sure to use the red probe on the side connected to the positive battery terminal, and the black probe on the side connected to the negative battery terminal nothing will be harmed if you get this backwards, but your voltage reading will be negative.
Figure 6. Measuring voltage across a lightbulb by attaching the multimeter probes in parallel. Current flow is represented by the yellow arrows. In voltage-measurement mode, the multimeter's resistance is very high , so almost all of the current flows through the lightbulb, and the multimeter does not have a big impact on the circuit. Figure 7. Measuring the current through a lightbulb by attaching a multimeter in series. In current-measurement mode, the multimeter's resistance is very low , so the current can easily flow through the multimeter without affecting the rest of the circuit.
Notice how the knob has been set to measure direct current DCA and the red probe is plugged into the port for measuring current, labeled with an "A. Figure 8. Measuring the resistance of a lightbulb using a multimeter. Notice how the lightbulb has been disconnected from the circuit. The multimeter supplies its own small amount of current, which allows it to measure the resistance.
Figure 9. Using a multimeter to do a continuity test. If a conductive path is formed between the probe tips, the multimeter will beep. If the conductive path is broken possibly due to a wire that has come loose in your circuit, or a bad solder connection , the multimeter will not beep.
Readings when measuring voltage of a single AA battery using different dial settings on a manual-ranging multimeter. Figure A typical fuse. This section answers the questions: What can I do with a multimeter around the house?
What shouldn't I do with my multimeter? Which Science Buddies projects require a multimeter? What can I do with a multimeter around the house? Test batteries! Have you ever wondered if a device or toy stopped working because the batteries were dead? With a multimeter, you can make sure batteries are actually dead before recycling them by testing their voltage. However, batteries will effectively be "dead" long before they reach 0V.
For example, if a battery is supplying 0. Do you have rechargeable batteries? If you're messing with AC, we recommend you get a non-contact tester rather than use a digital multimeter. Use the V with a straight line to measure DC Voltage. What happens if you switch the red and black probes? The reading on the multimeter is simply negative. Nothing bad happens! The multimeter measures voltage in relation to the common probe. Now let's construct a simple circuit to demonstrate how to measure voltage in a real world scenario.
To begin, let's make sure the circuit you are working on is powered up correctly. If your project should be at 5V but is less than 4. Measuring the voltage coming off of a Power Supply Stick. Multimeters are generally not autoranging. You have 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.
So if you've measuring a 12V battery, use the 20V setting. Use the 20V setting. If you set it incorrectly, you will probably see the meter screen change and then read '1'. With some force imagine poking a fork into a piece of cooked meat , push the probes onto two exposed pieces of metal. One probe should contact a GND connection. One probe to the VCC or 5V connection. We can test different parts of the circuit as well. This practice is called nodal analysis , and it is a basic building block in circuit analysis.
By measuring the voltage across the circuit we can see how much voltage each component requires. Let's measure the whole circuit first. Measuring from where the voltage is going in to the resistor and then where ground is on the LED, we should see the full voltage of the circuit, expected to be around 5V. We can then see how much voltage the LED is using. This is what is referred to as the voltage drop across the LED.
If that doesn't make sense now, fear not. It will as you explore the world of electronics more. The important thing to take away is that different parts of a circuit can be measured to analyze the circuit as a whole. This LED is using 2. This is lower than the forward voltage stated in the datasheet on account of the circuit only having small amount of current running though it, but more on that in a bit. What happens if you select a voltage setting that is too low for the voltage you're trying to measure?
Nothing bad. The meter will simply display a 1. This is the meter trying to tell you that it is overloaded or out-of-range. Whatever you're trying to read is too much for that particular setting. Try changing the multimeter knob to a the next highest setting. Reading the 5V across this circuit is too much for the 2V setting on the multimeter.
Why does the meter knob read 20V and not 10V? If you're looking to measure a voltage less than 20V, you turn to the 20V setting. This will allow you to read from 2. The first digit on many multimeters is only able to display a '1' so the ranges are limited to 1 9. Hence the 20V max range instead of 99V max range.
Normal resistors have color codes on them. If you don't know what they mean, that's ok! There are plenty of online calculators that are easy to use. However, if you ever find yourself without internet access, a multimeter is very handy at measuring resistance. Then hold the probes against the resistor legs with the same amount of pressure you when pressing a key on a keyboard. The meter will read one of three things, 0. In this case, the meter reads 0. If the multimeter reads 1 or displays OL , it's overloaded.
There is no harm if this happen, it simply means the range knob needs to be adjusted. If the multimeter reads 0. Don't worry, it'll work just fine as a pull-up or general resistor. Not a whole lot changed. However, you'll notice that there is one more digit after the decimal point giving us a slightly higher resolution in our reading. What about the next lowest setting? As a rule of thumb, it's rare to see a resistor less than 1 Ohm.
Remember that measuring resistance is not perfect. Temperature can affect the reading a lot. Also, measuring resistance of a device while it is physically installed in a circuit can be very tricky. The surrounding components on a circuit board can greatly affect the reading. Reading current is one of the trickiest and most insightful readings in the world of embedded electronics.
It's tricky because you have to measure current in series. Where voltage is measure by poking at VCC and GND in parallel , to measure current you have to physically interrupt the flow of current and put the meter in-line. To demonstrate this, we'll use the same circuit we used in the measuring voltage section. The first thing we'll need is an extra piece of wire. As mentioned, we'll need to physically interrupt the circuit to measure the current. Said another way, pull out the VCC wire going to the resistor, add a wire where that wire was connected, and then probe from the power pin on the power supply to the resistor.
This effectively "breaks" power to the circuit. We then insert the multimeter in-line so that it can measure the current as it "flows" through to the multimeter into the bread board.
For these pictures, we cheated and used alligator clips. Note: Some stubborn leads will only show an open circuit after bending the lead severely at either end.
This suggests the lead is broken but still making intermittent connection. It also should be cut shorter and re-joined. Exercise 6 : Check a light bulb to see if it is OK. Note: the resistance of a light bulb increases greatly when power is applied. This is because, as with most resistances, the resistance increases with temperature.
Exercise 7 : Check if a fuse is blown or not. If you think a fuse is blown, the best way to know for sure is to remove the fuse with the power off! Most multimeters have the facility to measure small amounts of DC current. Some meters also allow the measurement of AC current.
Although the measurement of DC current is explained here, the procedure for AC current follows similar principles. When we measure voltage, we are measuring the difference in voltage from one probe to the other. That is, we measure the voltage across a particular resistance.
Example : Here we have two 1. By placing the probes as shown we can measure the voltage 3 volts across the light bulb. To measure the current in a circuit we need to measure the current flowing through the resistance. So what is the secret?
In the article on The Dreaded Ohms Law , we learned that the current flowing through a series circuit is the same throughout the whole of the circuit. Therefore if we can measure the current flowing through any part of the circuit, we are effectively measuring the current flowing through the resistance. That is, the current flowing through the resistance is the same as the current flowing through the wire, which is the same as that flowing through the batteries to use our example.
So how do we do all this? We could cut the wire between the battery and the light bulb. Then connect one probe to each of the cut ends, with the multimeter selected to measure DC current. This would work as we are measuring the current flowing through the wire and the multimeter. Because it is a series circuit, we are also measuring the current flowing through the light bulb and the batteries. However it is not always wise to cut wires unnecessarily.
In our example, an obvious place to break the circuit and insert our probes would be at the end of one of the batteries. Most battery holders have a spring to help make a good contact. It is normally possible to separate the batteries and insert a small piece of cardboard to isolate the batteries from each other. Then it is a matter of simply placing a probe on either side of the cardboard.
Whichever way it is possible to break the circuit, it is at that point that the probes need to be inserted. On DC it does matter. The black negative probe should go on the positive side of the break. That is, on analog meters, the pointer will quickly move off the left-hand side of the scale.
When measuring AC current if your meter has that selection , be very careful not to touch the metal points of the probes. This is because most often when measuring AC, it is at dangerous high voltages. Many meters only allow the measurement of very small DC currents. Often 25 milli-amps mA is the maximum. Many meters also have the capacity to measure 10 Amps.
To do this you normally need to move the red probe to a different socket on the meter. This is often 10 amps AC only, not DC. Be sure to read the manual closely to know what you meter can and can not do.
Many digital meters allow a maximum of mA. If this limit is exceeded, a fuse in the meter will probably need replacing. It is wise to have a good stock of replacement fuses on hand. Inserting the probes between two batteries is an easy way of checking the charging current being delivered to Ni-Cad batteries. When measuring DC voltages, the red positive probe goes to the positive side of whatever is being tested.
When measuring resistance, ensure there is no power applied to the resistance being tested. When measuring current, it is necessary to break the circuit in an appropriate place and insert the probes in series with the circuit under test. The black probe goes to the positive side of the break.
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But first, some Understanding Audio Understanding Audio Levels. A basic understanding of the general audio levels mentioned in this article will help you avoid the common mistakes often made when connecting audio Viewed 45k times. Jeffsbach Jeffsbach 91 1 1 gold badge 1 1 silver badge 2 2 bronze badges. He said I should know I went to school for electronics Add a comment.
Active Oldest Votes. If the meter is analog, you can damage the needle with negative voltages in some cases. Also, the sockets where the leads plug into the voltmeter are usually well labeled. In case of bannana plugs, its really how you decided to plug in the leads. Of course the only dignified way to do so is to plug the red into positive and black into negative.
If you connect the lead "incorrectly" to a battery or other DC source, the meter should still work, but will show a "-" sign. It should work with either polarity when measuring AC.
Spehro Pefhany Spehro Pefhany k 12 12 gold badges silver badges bronze badges. A plus may mean positive input or positive output and so in negative also. Saying a casual plus or minus causes a lots of misunderstanding on basics.
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