Electronics Primer: Using a Multimeter
Measuring DC Voltage, DC Current, and Resistance
Multimeters are very handy tools for measuring what is going on in a circuit. You can buy one at any hardware or auto supply store. They range in price from less than $10 to hundreds of dollars. For the purpose of measuring most currents, voltages, and resistances, you can get by with an inexpensive model. Most new multimeters are digital.
|Figure 1. Three multimeters. Note that there are 3–4 sockets for the probes, depending on the model. The red probe may need to be inserted in different sockets to measure voltage and current.|
Multimeters that are "auto-ranging" are useful because you don't need to change the dial to measure different levels. If you think you might be using it in low light (for example, in a project that involves measuring light produced in an experiment), consider getting one with a "back-light."
The first rule for getting the most out of your multimeter is to read the manual. The manual will have instructions for basic operation of the instrument and safety information about potential dangers. Once you have read the manual, added the batteries, and attached the probes (the wires, which are usually red and black), try some of the simple tests below.
- Set the multimeter to read "resistance." Check that the two probes are inserted in the right holes.
- What does the readout say when the probes are not touching anything? When the two probes are separated, there is an infinite resistance separating them, since air does not conduct electricity. Make a mental note of your multimeter's readout for infinite resistance, because it varies with the manufacturer.
- Touch the two probes together. Now what does the readout say? When you touch the two probes together, the resistance is close to zero, since the metal tips are excellent conductors.
- Measure the resistance of some resistors that are not attached to a circuit. For example, test resistors of 100 Ω (ohms), 10,000 Ω, and 1 MΩ (megaohm, or 1 million ohms). You can buy these online at www.radioshack.com. Touch the probes to the wires on either side of the central cylinder. Watch the units: a "k" means kilo-ohms (thousands of ohms), and an "M" means megaohms. Look online for a chart that tells you how to read the value of the resistance based on the colored bands.
- Never measure resistance in a circuit when power is applied. You must also discharge capacitors in a circuit before measuring resistance, because if there is any source of current other than the multimeter itself, you will get erroneous readings. If the circuit you are working with has large capacitors, you should test them to see if they are carrying a charge. Test the capacitor for charge using a voltmeter, set to high DC voltage. If there is a charge on the capacitor, use a high-wattage resistor to discharge it. Carefully touch the two leads of the resistor to the leads of the capacitor. It may take several seconds for the capacitor to discharge. For more details about how to safely discharge capacitors, visit the following website: Capacitor Testing, Safe Discharging, and Other Related Information.
- Set the multimeter to read "direct current (DC) voltage." DC voltage is the kind produced by a battery—it does not alternate from positive to negative. Check that the two probes are inserted into the correct holes to measure DC voltage.
- Touch the probes to the terminal ends of a fresh 9-V battery. You should get a reading of approximately 9 V. The battery has a positive and a negative pole. Note that your multimeter also has a positive and a negative probe. If you attach the positive probe to the negative side of the battery, it will still read out 9 V, but it will have a negative sign in front of it.
- Make the circuit, as described in the Science Buddies page Build and Test a Simple Circuit. See Figure 2, below.
- With the circuit closed and the LED "on," measure the voltage across the resistor by touching the probes to the metal wires on either side of the cylinder (at positions "2" and "3" in Figure 2).
- Measure the voltage across the LED (positions "4" and "5" in Figure 2).
|Figure 2. An LED circuit. Current flows from the positive terminal (1), through the resistor (2 and 3), the LED (4 and 5), the white wire (6 and 7), and then to the negative terminal (8).|
- Compare the voltage of the battery to the sum of the voltage drops across the resistor and the LED.
- Set the multimeter to read "direct current (DC)." Important: Check your multimeter to see where the probe should be plugged in so it reads "current."
- In order to measure current, you have to open up the circuit and attach the leads from the multimeter so that the current flows through the multimeter. To do this, use jumper wires and wires with alligator clips to add the multimeter to the circuit, as shown in Figure 3, below.
|Figure 3. To measure current, the multimeter is inserted into the circuit.|
- The white wire is unplugged from the breadboard and attached to the red probe from the multimeter ("1").
- A jumper wire ("2") is connected to the negative bus.
- The other end of the jumper wire is attached to the negative lead from the multimeter.
- The current now flows through the resistor, the LED, the multimeter, and then to the negative terminal of the battery.
- Depending on your resistor and LED, the current will be measured at around 23 mA (milliamps).