A multimeter is an instrument used to check for AC or DC voltages, resistance or continuity of electrical components and small amounts of current in circuits. This instrument will let you check to see if there is voltage present on a circuit, etc. Here’s how to use an analog multimeter.
- Become familiar with the parts of a multimeter. Inspect the meter. Starting from the top and working to the bottom:
- The dial: This has the arc-shaped scales visible through the window. The pointer indicates values read from the scale.
- The pointer or needle: This is the thin black line at the left-most position in the dial face window in the image. The needle moves to the value measured.
- Arc shaped lines or scales on the meter dial face: These may be different colors for each scale, but will have different values. These determine the ranges of magnitude.
- A wider mirror-like surface shaped like the scales mentioned previously might also be present. The mirror is used to help reduce parallax viewing error by lining up the pointer with its reflection before reading the value the pointer is indicating. In the image, it appears as a wide gray strip between the red and black scales.
- A selector switch or knob: This allows changing the function (volts, ohms, amps) and scale (x1, x10, etc.) of the meter. Many functions have multiple ranges. It is important to have both set correctly, otherwise serious damage to the meter or harm to the operator may result. Most meters employ the knob type like the one shown in the image, but there are others. Regardless of the type, they work similarly. Some meters (like the one in the image above) have an “Off” position on this selector switch while others have a separate switch to turn the meter off. The meter should be set to “Off” when stored.
- Jacks or openings in the case to insert test leads. Most multimeters have several jacks. The one pictured has just two. One is usually labeled “COM” or (-) ,for common and negative. This is where the black test lead is connected. It will be used for nearly every measurement taken. The other jack(s) is labeled “V” (+) and the Omega symbol (an upside down horseshoe) for Volts and Ohms, respectively, and positive. The + and – symbols represent the polarity of probes when set for and testing DC volts. If the test leads were installed as suggested, the red lead would be positive as compared to the black test lead. This is nice to know when the circuit under test isn’t labeled + or -, as is usually the case. Many meters have additional jacks that are required for current or high-voltage tests. It is equally important to have the test leads connected to the proper jacks as it is to have the selector switch range and test type (volts, amps, ohms) set. All must be correct. Consult the meter manual if you’re unsure which jacks should be used.
- Test leads: There should be (2) test leads or probes. Generally, one is black and the other red.
- Battery and fuse compartment: Usually found on the reverse, but sometimes on the side. This holds the fuse (and possibly a spare), and the battery that supplies power to the meter for resistance tests. The meter may have more than one battery and they may be of different sizes. A fuse is provided to help protect the meter movement. Sometimes there is more than one fuse. A good fuse is required for the meter to function. Fully charged batteries will be required for resistance/continuity tests.
- Zero Adjustment: This is a small knob usually located near the dial that is labeled “Ohms Adjust”, “0 Adj”, or similar. This is used only in the ohms or resistance range, while the probes are shorted together (touching each other). Rotate the knob slowly to move the needle as close to the 0 position on the Ohms scale as possible. If new batteries are installed, this should be easy to do – a needle that will not go to zero indicates weak batteries that should be replaced.
Using the Ohm Function to Measure Resistance
- Set the multimeter to Ohms or Resistance (turn meter on if it has a separate power switch). Understand that resistance and continuity are opposites. When multimeter measures resistance in ohms, it can not measure continuity. When there is little resistance there is a great deal of continuity. Conversely, when there is a great deal of resistance, there is little continuity. With this in mind, when we measure resistance we can make assumptions about continuity based on the resistance values measured.
- Observe the meter indication. If the test leads are not in contact with anything, the needle or pointer of an analog meter will be resting at the left-most position. This is represents an infinite amount of resistance, or an “open circuit”; it is also safe to say there is the no continuity, or path between the black and red probes.
- Careful inspection of the dial should reveal the Ohm scale. It is usually the top-most scale and has values that are highest on the left of the dial (“∞” or a sideways “8″ for infinity) and gradually reduce to 0 on the right. This is opposite of the other scales; they have the lowest values on the left and increase going right.
- Connect the black test lead to the jack marked “Common” or “-”.
- Connect the red test lead to the jack marked with the Omega (Ohm symbol) or letter “R” near it.
- Set the range (if provided) to R x 100.
- Hold the probes at the end of the test leads together. The meter pointer should move fully to the right. Locate the “Zero Adjust” knob and rotate so that the the meter indicates “0″ (or as close to “0″ as possible).
- Note that this position is the “short circuit” or “zero ohms” indication for this R x 1 range of this meter.
- Always remember to “zero” the meter immediately after changing resistance ranges.
- Replace batteries (if needed). If you’re unable to obtain a zero ohm indication, this may mean the batteries are weak and should be replaced. Retry the zeroing step above again with fresh batteries.
- Measure resistance of something like a light bulb you know is good. Locate the two electrical contact points of the bulb. They will be the threaded base and the center of the bottom of the base. Have a helper hold the bulb by the glass only. Press the black probe against the threaded base and the red probe against the center tab on the bottom of the base. Watch the needle move from resting at the left and move quickly to 0 on the right.
- Try different ranges. Change the range of the meter to R x 1. Zero the meter again for this range. Repeat the step above. Observe how the meter did not go as far to the right as before. The scale of resistance has been changed so that each number on the R scale can be read directly. In the previous step, each number represented a value that was 100 times greater. Thus, 150 really was 15,000 before. Now, 150 is just 150. Had the R x 10 scale been selected, 150 would have been 1,500. The scale selected is very important for accurate measurements. With this understanding, study the R scale. It is not linear like the other scales. Values at the left side are harder to accurately read than those on the right. Trying to read 5 ohms on the meter while in the R x 100 range would look like 0. It would be much easier at the R x 1 scale instead. This is why when testing resistance, adjust the range so that the readings may be taken from the middle rather than the extreme left or right sides.
- Test resistance between hands. Set the meter to the highest R x value possible. Zero the meter.
- Loosely hold a probe in each hand and read the meter. Squeeze both probes tightly. Notice the resistance is reduced.
- Let go of the probes and wet your hands. Hold the probes again. Notice that the resistance is lower still.
- For these reasons, it is very important that the probes not touch anything other than the device under test. A device that has burned out will not show “open” on the meter when testing if your fingers provide an alternate path around the device, like when they are touching the probes. Testing round cartridge type and older style glass automotive fuses will indicate low values of resistance if the fuse is lying on a metal surface when under test. The meter indicates the resistance of the metal surface that the fuse is resting upon (providing an alternate path between the red and black probe around the fuse) instead of trying to determine resistance through the fuse. Every fuse, good or bad, will indicate “good”.
Using the Volts Function to Measure Voltage
- Set the meter for the highest range provided for AC Volts. Many times, the voltage to be measured has a value that is unknown. For this reason, the highest range possible is selected so that the meter circuitry and movement will not be damaged by voltage greater than expected. If the meter were set to the 50 volt range and a common U.S. electrical outlet were to be tested, the 120 volts present could irreparably damage the meter. Start high, and work downward to the lowest range that can be safely displayed.
- Insert the black probe in the “COM” or “-” jack.
- Insert the red probe in the “V” or “+” jack.
- Locate the voltage scales. There may be several Volt scales with different maximum values. The range chosen by the selector knob determines which voltage scale to read.
- The maximum value scale should coincide with selector knob ranges. The voltage scales, unlike the Ohm scales, are linear. The scale is accurate anywhere along its length. It will of course be much easier accurately reading 24 volts on a 50 volt scale than on a 250 volt scale, where it might look like it is anywhere between 20 and 30 volts.
- Test a common electrical outlet. In the U.S., you might expect 120 volts or even 240 volts. In other places, 240 or 380 volts might be expected.
- Press the black probe into one of the straight slots. It should be possible to let go of the black probe, as the contacts behind the face of the outlet should grip the probe, much like it does when a plug is inserted.
- Insert the red probe into the other straight slot. The meter should indicate a voltage very close to 120 or 240 volts (depending on type outlet tested).
- Remove the probes, and rotate the selector knob to the lowest range offered, that is greater than the voltage indicated (120 or 240).
- Reinsert the probes again as described earlier. The meter may indicate between 110 and as much as 125 volts this time. The range of the meter is important to obtain accurate measurements.
- If the pointer did not move, it is likely that DC was chosen instead of AC. The AC and DC modes are not compatible. The correct mode must be set. If not set correctly, the user would mistakenly believe there was no voltage present. This could be deadly.
- Be sure to try both modes if the pointer does not move. Set meter to AC volts mode, and try again.
- Whenever possible, try to connect at least one probe in such a way that it will not be required to hold both while making tests. Some meters have accessories that include alligator clips or other types of clamps that will assist doing this. Minimizing your contact with electrical circuits drastically reduces that chances of sustaining burns or injury.
Using the Amps Function to Measure Amperes
- Determine AC or DC by measuring the voltage of the circuit as outlined above.
- Set the meter to the highest AC or DC Amp range supported. If the circuit to be tested is AC but the meter will only measure DC amps (or vice-versa), stop. The meter must be able to measure the same mode (AC or DC) amps as the voltage in the circuit, otherwise it will indicate 0.
- Be aware that most multimeter s will only measure extremely small amounts of current, in the uA and mA ranges. 1 uA is .000001 amp and 1 mA is .001 amp. These are values of current that flow only in the most delicate electronic circuits, and are literally thousands (and even millions) of times smaller than values seen in the home and automotive circuits that most homeowners would be interested testing. Just for reference, a typical 100W / 120V light bulb will draw .833 Amps. This amount of current would likely damage the meter beyond repair.
- A “clamp-on” type ammeter would be ideal for the typical homeowner requirements, and does not require opening the circuit to take measurements (see below).
- Use a “clamp-on” ammeter. If this meter were to be used to measure current through a 4700 ohm resistor across 9 Volts DC, it would be done as outlined below:
- Insert the black probe into the “COM” or “-” jack.
- Insert the red probe into the “A” jack.
- Shut off power to the circuit.
- Open the portion of the circuit that is to be tested (one lead or the other of the resistor). Insert the meter in series with the circuit such that it completes the circuit. An ammeter is placed in series with the circuit to measure current. It cannot be placed “across” the circuit the way a voltmeter is used (otherwise the meter will probably be damaged). Polarity must be observed. Current flows from the positive side to the negative side. Set the range of current to the highest value.
- Apply power and adjust range of meter downward to allow accurate reading of pointer on the dial. Do not exceed the range of the meter, otherwise it may be damaged. A reading of about 2 milliamps should be indicated since from Ohm’s law I = V / R = (9 volts)/(4700 Ω) = .00191 amps = 1.91 mA.
- If you’re measuring the current consumed by the device itself, be aware of any filter capacitors or any element that requires an inrush (surge) current when switched on. Even if the operating current is low and within the range of the meter fuse, the surge can be MANY times higher than the operating current (as the empty filter capacitors are almost like a short circuit). Blowing the meter fuse is almost certain if the DUT’s (device under test) inrush current is many times higher than the fuses rating. In any case, always use the higher range measurement protected by the higher fuse rating (if your meter has two fuses), or just be careful.
- If the multimeter stops working, check the fuse. You can replace these at places like Radio Shack etc.
- When you are going to check any part for continuity, you must remove the power. Ohm meters supply their own power from an internal battery. Leaving power on while testing resistance will damage the meter.
- Respect electricity. If you don’t know something, ask questions and research the subject.
- Always check meters on known good voltage sources to verify operational status before using. A broken meter testing for volts will indicate 0 volts, regardless of the amount present.
- Never connect the meter across a battery or voltage source if it is set to measure current (amps). This is a common way to blow up a meter.
Edit Things You’ll Need
- Multimeter. Consider a digital meter instead of the older analog types. Digital meters usually offer automatic ranging and easy to read displays. Since they are electronic, the built-in software helps them withstand incorrect connection and ranges better than the mechanical meter movement in analog types.
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