What is A Multimeter?
A multimeter or a multitester, also known as a VOM (Volt-Ohm meter), is an Electronic measurin instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, curent and resistance. Multimeters may use analog or digital circuits—analog multimeters (AMM) and digital multimeters (often abbreviated DMM or DVOM.) Analog instruments are usually based on a microammeter whose pointer moves over a scale calibrated for all the different measurements that can be made; digital instruments usually display digits, but may display a bar of a length proportional to the quantity being measured.
A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as electronioc equipment, motor controls,domestic appliences, power suplies, and wiring systems.
Multimeters are available in a wide range of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5,000.
How do they Work?
A multimeter is a combination of a multirange DC voltmeter, multirange AC voltmeter, multirange ammeter, and multirange ohmmeter. An un-amplified analog multimeter combines a meter movement, range resistors and switches. For an analog meter movement, DC voltage is measured with a series resistor connected between the meter movement and the circuit under test. A set of switches allows greater resistance to be inserted for higher voltage ranges. The product of the basic full-scale deflection current of the movement, and the sum of the series resistance and the movement's own resistance, gives the full-scale voltage of the range. As an example, a meter movement that required 1 milliamp for full scale deflection, with an internal resistance of 500 ohms, would, on a 10-volt range of the multimeter, have 9,500 ohms of series resistance. For analog current ranges, low-resistance shunts are connected in parallel with the meter movement to divert most of the current around the coil. Again for the case of a hypothetical 1 mA, 500 ohm movement on a 1 Ampere range, the shunt resistance would be just over 0.5 ohms. Moving coil instruments respond only to the average value of the current through them. To measure alternating current, a rectifier diode is inserted in the circuit so that the average value of current is non-zero. Since the average value and the root-mean-square value of a waveform need not be the same, simple rectifier-type circuits may only be accurate for sinusoidal waveforms. Other wave shapes require a different calibration factor to relate RMS and average value. Since practical rectifiers have non-zero voltage drop, accuracy and sensitivity is poor at low values. To measure resistance, a small dry cell within the instrument passes a current through the device under test and the meter coil. Since the current available depends on the state of charge of the dry cell, a multimeter usually has an adjustment for the ohms scale to zero it. In the usual circuit found in analog multimeters, the meter deflection is inversely proportional to the resistance; so full-scale is 0 ohms, and high resistance corresponds to smaller deflections. The ohms scale is compressed, so resolution is better at lower resistance values. Amplified instruments simplify the design of the series and shunt resistor networks. The internal resistance of the coil is decoupled from the selection of the series and shunt range resistors; the series network becomes a voltage divider. Where AC measurements are required, the rectifier can be placed after the amplifier stage, improving precision at low range. Digital instruments, which necessarily incorporate amplifiers, use the same principles as analog instruments for range resistors. For resistance measurements, usually a small constant current is passed through the device under test and the digital multimeter reads the resultant voltage drop; this eliminates the scale compression found in analog meters, but requires a source of significant current. An autoranging digital multimeter can automatically adjust the scaling network so that the measurement uses the full precision of the A/D converter. In all types of multimeters, the quality of the switching elements is critical to stable and accurate measurements. Stability of the resistors is a limiting factor in the long-term accuracy and precision of the instrument.
How To Use a Multimeter?
- 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. 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 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
Use the ohm function to measure Resistance
1-Set the multimeter to Ohms or Resistance (turn meter on if it has a separate power switch). Understand that resistance and continuity are opposites. The 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. 2-Connect the black test lead to the jack marked "Common" or "-" 3-Connect the red test lead to the jack marked with the Omega (Ohm symbol) or letter "R" near it. 4-Set the range (if provided) to R x 100. 5-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. 6-Replace batteries if needed. If 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. 7-Measure resistance of something like a known-good lightbulb. 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. 8-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. 9-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".
- 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.
- 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.
- Respect electricity. If you don't know something, ask questions and research the subject.
Things you will 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.
Functions Of a Mulitimeter
The voltmeter function of a multimeter measures the electrical potential between two points and is measured in volts. For a 9-volt battery, the potential between the positive and negative terminals of the battery is nine volts and can be checked with a multimeter. This is often useful when a battery is suspected to be near the end of its life.
The ohmmeter function of a multimeter measures the resistance to current flow. This measurement is given in units called ohms. If an electrical circuit is "short-circuited," then its resistance is zero or near zero ohms. If a circuit is open, then it has infinite resistance and there is no current flow.
Modes of Current
Multimeters have the capacity to measure current and voltage in two different modes, alternating current, or AC, and direct current, or DC. The standard for household voltage outlets is alternating current. Different countries have different standards for household A.C. voltage, which is why many travelers find their 120 AC volt hair dryers malfunction when plugged into an outlet in a foreign country. If the voltage isn't known, it's helpful to check the voltage with a multimeter using the AC voltmeter function. Batteries supply DC current, while households supply AC current. When a current measurement is made, the multimeter must be set in the correct function, AC or DC, in order to obtain an accurate measurement.
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