# i have to explain how a multimeter works and i don’t know how it works, how does a multimeter work?

• There are two kinds of multimeters, digital and analog. Both measure at least DC (direct current) voltage, current, and resistance and most measure at least AC (alternating current) voltage. The digital multimeters often include added features not found in analog meters, for example measuring capacitance and frequency.

All measurements are made by connecting two probes (test leads) to the circuit being measured, after selecting the meter function and range, usually with a multi-position switch.

For an analog meter, a D’Arsonval meter movement consisting of a small coil, suspended by bearings in the poles of a permanent magnet, is used to measure DC current. The current flowing in the coil creates a magnetic field that interacts with the permanent magnet to cause a torque about the bearing axles. This torque is restrained by spiral springs so the coil rotates an amount that is linearly proportional to the current.

Typical full-scale (maximum deflection) sensitivites are 50 microamperes. Sometimes more sensitive movements are used, but that also makes the meter movement more susceptable to damage from shock… like being dropped or knocked off a work bench.

Higher DC currents are measured by switching in precision low-resistance shunts in parallel with the meter coil, diverting most of the measured current through the shunt.

DC voltages are measured by inserting precision resistors in series with the meter coil, limiting the current through the meter so that a full-scale voltage of, say, 3 V, causes a full-scale current of, say, 50 microamperes through the meter. The reciprocal of the full-scale current gives the meter sensitivity in “ohms per volt” which in this example is 20,000 ohms per volt.

That was considered very good sensitivity for an analog meter until vacuum tube voltmeters (VTVMs) came along. These placed a sensitive vacuum tube amplifier ahead of the meter and raised the input impedance of the meter to several million ohms, even on the most sensitive range. Later, field-effect transistors (FETs) replaced the vacuum tubes and soon after that digital meters came along and replaced both of them.

The simple analog meter with the D’Arsonval meter movement survived, even to this day, for a number of reasons. First, it is easy to interpret the movement of the needle pointer, even out the corner of your eye. Second, they are still fairly inexpensive despite the labor-intensive meter construction. Third, they don’t require any batteries to measure voltage and current.

Measuring resistance is the third function of virtually all multimeters. In an analog meter this is accomplished by using an internal battery, usually a 1.5 volt dry cell, to apply a “calibrated” full-scale current through the meter when the test leads are shorted. Full-scale current represents zero ohms. Any resistance greater than zero will cause less current up to an open circuit or infinite resistance which causes no current. The resistance scale of an analog multimeter is very non-linear and the “zero” will shift as the battery ages, requiring constant re-calibration before use.

Digital multimeters actually only measure voltage. Current is measured as the voltage across a shunt and resistance is measured by measuring the voltage across the resistance with a constant current flowing through the resistance.

All meters, analog and digital, must somehow convert AC voltages and currents to their DC equivalents for measuring purposes. Analog meters use simple rectifier diodes; as a result the conversion is only calibrated for sinusoid waveforms, like power line voltages. Digital meters can be a bit more sophisticated and calculate “true RMS” (root-mean-square) from almost arbitrary waveform inputs. This might be helpful to someone measuring audio waveforms, or power line waveforms distorted by harmonic content.

And finally, some digital multimeters also provide a moving “analog bar” to represent the digital reading. This can be very useful because the eye is sensitive to movement (it is “hardwired” to do so) but requires the brain to interpret changing digital numbers.

How a digital voltmeter works internally depends on the manufacturer. The early meters used a dual-slope integrating digital conversion technique that was reasonably accurate and good at surpressing noise. It is slow however, about ten readings per second or less. With modern digital signal processing, over-sampling 1-bit delta-sigma converters can perform with better accuracy, and usually more rapidly.

A side benefit to digital signal processing is the ability to measure time intervals, and hence frequency of an AC signal, or the time to charge a capacitor with a constant current to a specific voltage, and hence measure capacitance. Virtually all digital meters, and some analog meters, feature a “beeper” to allow continuity (low resistance, open or closed circuit) measurements without viewing the meter. Some “multimeters” now even include low-bandwidth waveform display and recording functions, much like an oscilloscope.

• You must differentiate between needle (analog) and digital (electronic)l multimeters. Needle ones have a d’arsonval galvanometer which measures DC microamps. Voltage dividers are used at different scales. In AC, a single wave rectifier is used. Because the rectifier is a nonlinear element, it is a lot less accurate. Digital multimeters are similar, except that they are much more accurate and their input impedance is much higher, thereby not burdening the circuit to be measured unnecessarily. There is a lot more to be said. This is just the basic basics. =======================================… Regular – cheap – multimeters do not measure RMS, but average AC, while the scale reading is corrected assuming a sinusoidal RMS. The ones that measure true sinusoidal RMS are more expensive and are “special order”. Then you have multimeters which measure true harmonic RMS, up to a certain frecuency. Those are the really expensive ones. . .

• How Does A Multimeter Work

• You might just want to stick to how to use a multimeter.

Two easy things:

1) Put it on the 20k or 200k resistance range and hold the electrodes. You should get a resistance reading depending on how tight you hold the probes. This shows your skin resistance.

2) Put it on the 20V DC setting. Check a collection of batteries. See how close you get to 1.5V and 9V. A low reading means the battery is discharged.

How it works? You probably don’t want to know that the digital ones use dual slope integration, right?