A multimeter is a device used to measure AC or DC voltage, resistance, continuity of electrical components, and small amounts of current in circuits. This instrument will let you know if a certain circuit has voltage. By doing so, the multimeter will help you accomplish a wide variety of useful tasks, such as measuring ohms, volts, and amps.
Part 1 of 4: Get Familiar with the Device
Step 1. Locate the dial on the multimeter
The dial has arc-shaped scales visible through the window and a needle will mark the reading values on the scales.
- The arc-shaped markings on the meter dial may have different colors for each measurement scale, so they will have different values.
- A wide mirror-like surface similar to scales could also be found. This is to help reduce error in reading by aligning the pointer with its reflection before reading the measurement. In the image, it appears as a broad gray band between the red and black scale.
- Many of the newer multimeters have digital readings instead of the analog scale. The function is essentially the same, the only difference is the numerical reading.
Step 2. Find the selector or knob
This will allow you to change the function (voltage, resistance, amperage) and scale (x1, x10, etc.) of the meter. Many of the functions have different ranges. It is important to have both selected correctly, otherwise it could cause serious damage to the device or to the one who is using it.
Some meters have an “Off” position while others have a separate switch to turn it off. When the multimeter is not in use it should be turned off
Step 3. Locate the connector holes in the housing to insert the test leads
Most multimeters have several "terminals" (these types of holes, also called "jack") for this purpose.
- One is usually marked with the letters "COM" or (-), for common. The black wire should be plugged into this. It should be used for almost any measurement that is taken.
- The other terminal is marked "V" (+) and the Omega symbol (a horseshoe with the opening facing down) for volts and ohms respectively.
- The symbols "+" and "-" represent the polarity of the test points when preparing to measure direct current (DC) voltage. If the cables are installed as suggested, red will be positive and black will be negative. It is good to know the polarity when the circuit we are reviewing is not indicated with "+" and "-", as is often the case.
- Many multimeters have additional terminals that are used for current or high voltage tests. It is as important to correctly connect the test points to the terminals as it is to position the selector appropriately according to the range and the test to be performed (volts, amps or ohms). Everything should be correct. Consult the device manual if you are not sure which terminal to use.
Step 4. Locate the test points
There should be two test points. Usually one is black. They are used to connect to any device you plan to check and measure.
Step 5. Find the compartment for the battery and fuse
It is usually located in the rear, but sometimes it is on the side. This houses the fuse (and possibly a spare) and the battery that provides the power for the endurance tests.
The device could have more than one battery and they can be of different sizes. The fuse serves to protect the movement of the meter. Sometimes there will also be more than one fuse. It is necessary to have a good fuse for it to work properly. Batteries must be fully charged in order to perform resistance and continuity tests
Step 6. Find the zero adjustment knob
This is a small knob that will usually be near the dial or at the base and is labeled “Ohms Adjust,” “0 Adj,” or something similar. Used only in the resistance or ohm range when the test leads are close together (touching each other).
Turn the knob slowly to move the indicator as close as possible to the 0 position on the ohm scale. If the batteries in the device are new, it will be easier to do. If the needle does not want to reach the 0 position, it means that the batteries must be changed now
Part 2 of 4: Measure resistance
Step 1. Set the selector to the "Ohm" or "Resistance" position
Turn on the appliance if it has a separate on-off switch. When the multimeter measures resistance in ohms, it will not be able to measure continuity, since resistance or continuity are opposite. When there is little resistance, there will be greater continuity and vice versa. With this in mind, you can make assumptions about continuity based on the resistance measurement obtained.
Inspect the dial to find the ohm scale. It is usually the top scale and its values go from the highest to the left ("∞" or an "8" horizontally which means infinity) and gradually goes down to 0 on the right. This is opposite to the other scales, which have lower values to the left and increase to the right
Step 2. Observe the measurement indicator
If the test leads are not in contact with anything, the needle or pointer of an analog multimeter will not move from the leftmost home position. This represents an infinite amount of resistance or an "open circuit"; we can say without fear of being wrong that there will be no continuity, or passage between the red and black wires.
Step 3. Connect the test leads
Connect the black test lead to the terminal marked "Common" or "-". Then, connect the red test lead to the terminal marked with the omega sign (ohm symbol) or letter “R” nearby.
Put the range in the Rx100 position (if it has one)
Step 4. Gather the test leads at the end of the cables and keep them in contact
The meter pointer should move all the way to the right. Locate the "Zero Adjust" knob and turn it until the needle reads "0" (or as close to 0 as possible).
- Note that this position is the “short circuit” or “zero ohm” indication for this Rx1 range on this meter.
- Always remember to zero the meter immediately after changing the resistance range, otherwise it will give an erroneous reading.
- If you can't get a zero ohm reading, it could mean the batteries are dead and need to be replaced. Redo the previous step of zeroing with fresh batteries
Step 5. Measure the resistance of something like a light bulb that you know works
Locate the two electrical contact points of the bulb, which are the threaded area of the base and the center of the bottom of the base.
- Have someone hold the bulb only by the glass.
- Press the tip of the black wire over the threaded area of the base and the tip of the red wire over the center of the bottom of the base.
- Watch the needle move rapidly from rest on the left to zero on the right.
Step 6. Try different ranges
Change the range of the multimeter to Rx1. Set it to zero again for this range and repeat the previous step. Note that the needle will not go as far to the right as before. The resistance scale has changed, so the numbers on the R scale are read directly.
- In the previous step, each number represented a value that was 100 times larger. Before, 150 was actually 15,000. Now 150 is 150. If the Rx10 range had been chosen, a reading of 150 would be 1,500. The selected scale is very important to take exact measurements.
- Knowing this, study the R scale. It is not linear like the other scales. Values on the left are more difficult to read accurately than those on the right. Trying to read 5 ohms on the multimeter in the Rx100 range will make the measurement appear to be 0. Instead of doing so, it will be much easier to measure in the Rx1 range. So when measuring resistors, you should adjust the range so that the readings appear in the middle rather than either extreme.
Step 7. Test resistance between coats
Set the meter to the highest Rx value possible and also zero it.
- Loosely hold the test leads in each hand and read the measurement. Now squeeze the tips of the cables tightly. Notice that the resistance decreases.
- Leave the wires and wet your hands. Clamp the cables again. Notice that the resistance is lower than before.
Step 8. Make sure the reading is accurate
It is very important that the test leads do not touch anything other than the device you want to measure. A burned device will not appear "open" on the reading if your fingers provide an alternate step around it, for example if you touch the tips.
If when measuring the resistance of a round cartridge or antique automotive glass type fuse it is resting on a metal surface, the multimeter will indicate the resistance of the metal it is resting on (which provides an alternate step between the red test lead and the black one around the fuse) instead of measuring the resistance across the fuse. Each fuse in this case, good or bad, will indicate "good", that is, a wrong reading
Part 3 of 4: Measure voltage
Step 1. Set the multimeter's selector switch to its highest range for volts in alternating current (AC)
Many times the voltage of the circuit to be measured is unknown. For this reason, the highest possible range should be selected so that the circuits and the movement of the apparatus are not damaged by a higher voltage than expected.
If the multimeter were set to a 50 volt range and a common (US) voltage of 120 were to be measured, the multimeter could be irreparably damaged. Start high and work your way down to the lowest range that can be safely displayed
Step 2. Connect the test leads
Connect the black test lead to the "COM" or "-" terminal. Then, connect the red test lead to the “V” or “+” terminal.
Step 3. Locate the voltage scales
There could be several with different maximum values. The range chosen on the selector will determine which voltage scale to read.
The maximum values of the scales must coincide with the different ranges of the selector. Voltage scales, unlike ohms, are linear and exact anywhere from start to finish. Of course, it will be easier to read 24 volts more accurately on a 50 scale than on a 250 volt scale, where you could indicate between 20 and 30 but not 24 accurately
Step 4. Try measuring a common outlet
In America it will be 120 volts or even 240; elsewhere, 240 or 380 volts.
- Put the black test lead into one of the holes in the outlet. You could loosen the black tip, as the contacts behind the surface of the outlet will hold the tips, just like a plug.
- Insert the red test lead into the other hole in the outlet. The meter should indicate a voltage very close to 120 or 240 volts (depending on the type of outlet).
Step 5. Remove the test leads
Turn the dial to the lowest possible range that is higher than the voltage indicated in the previous reading (120 or 240).
Step 6. Insert the tips again as described above
This time, the meter could read between 110 and 125 volts at most. The range of the multimeter will be important in obtaining accurate measurements.
- If the pointer does not move, you have probably chosen DC instead of AC. AC and DC modes are not supported. The correct type of current must be chosen. If you don't set it correctly, you might think there is no voltage, which could be a very dangerous mistake.
- Make sure to try both modes if the needle doesn't move. Put the selector in volts AC mode and try again.
Step 7. Try not to hold both
If possible, try to connect at least one probe so that you don't have to hold both while taking the measurement. Some multimeters come with accessories that include clamps or clamps of some kind to help you do this. Minimizing your contact with electrical circuits will dramatically reduce the chances of burns or injuries from electrocution.
Part 4 of 4: Measure Amps
Step 1. Make sure you have measured the voltage first
You will need to determine if the circuit is direct current (DC) or alternating current (AC) by measuring its voltage as explained above.
Step 2. Set the multimeter to the highest AC or DC amp rating it has
If the circuit to be measured is alternating current, but the multimeter only measures direct current (or vice versa), stop. The meter must have the ability to measure amps of the same type of current (AC or DC) as the circuit voltage, otherwise it will indicate 0.
- Keep in mind that most multimeters will only measure extremely small currents, in the uA and mA ranges. 1 uA equals 0.000001 amps and 1 mA equals 0.001 amps. These current values flow only in the most delicate electronic circuits and are literally thousands (and even millions) of times less than the values that are handled in the circuits of homes or cars, so most people will rarely be interested in measure them.
- Just for reference, a common 100W / 120V bulb runs 0.833 amps. This amount of current could easily permanently damage the multimeter.
Step 3. Consider using a clamp-on ammeter
This device is ideal for home use and for measuring current through a 4700 ohm resistor in a 9 volt DC (direct current) circuit:
- Insert the black test lead into the "COM" or "-" terminal. Then insert the red tip into terminal "A".
- Cut off the power supply to the circuit.
- It opens the part of the circuit to be measured (one wire or the other of the resistor). Insert the meter in series so that it completes the circuit. The ammeter should be placed in series with the circuit to measure current. It cannot be placed on the circuit as you can with a voltmeter (otherwise the meter is likely to be damaged).
- Observe the polarity. The current flows from the positive side to the negative side. Set the current range to the highest value.
- Supplies power and adjusts the range by turning it down to get an accurate dial needle reading. Do not exceed the range, or you could damage the meter. You should get a reading of about 2 milliamps, since according to Ohm's law: I = V / R = (9 volts) / (4700 Ω) =.00191 amps = 1.91 mA.
Step 4. Be careful with a filter capacitor or other element that requires a current flow when it is turned on
Even if the operating current is low and within the tolerance of the multimeter's fuse, the discharge could be many times greater than the operating current, because empty filter capacitors are almost like a short circuit. The multimeter's fuse will most certainly blow if the input current of the device you are measuring is many times greater than the tolerance of the fuses. In any case, always use a higher range measurement protected by a higher fuse rating and be careful.
- If the multimeter stops working, check the fuse. You can buy a replacement at a hardware store like Radio Shack, etc.
- When you go to check continuity in any appliance or circuit, cut off the power. The multimeter has its own internal battery power source. Leaving the power on while you test the resistance will damage the meter.
- Respect electricity. If you don't know something in particular, ask someone with more experience.
- Always Check the multimeter for proper operation on a verified power source. A failed meter will indicate 0 volts regardless of the actual voltage.
- Never connect the meter to the terminals of a battery or a voltage source if the selector is set to measure current (amps). This is the safest way to melt a meter.