Insight - How Digital Multimeter Works

Fig. 1: Image of a Digital Multimeter

As the name suggests, multimeters are those measuring instruments which can be used to calculate multiple circuit characteristics. Making them digital gives highly precise outputs as unlike their analog counterparts, there is no needle whose pointer is to be figured out. How are the digital meters more advanced than their predecessor? What internal circuitry is there to power such quick and rapid calculations? Just simply hook it to the circuit and take readings on the fly? Multimeter does that for us. So let us explore the Nitty Gritties of the Multimeter that make it a jack of many (if not all) trades of electrical measurements.

Outer Casing

Fig. 2: Image Showing the Various Parts of Outer Structure of Multimeter

The image above depicts commonly used multimeter. Encased in a durable plastic casing, this measuring and testing instrument comes with an optional support to make it stand tilt for easy reading purposes.

Every multimeter comes with a few specifications that define the functions and range it can measure. For instance, the one in this insight can measure DC voltage in the range between 400mV to 1000V and resistance can be measured from 400Ohm to 400MegaOhm. Apart from measuring the conventional measurements of current, voltage and resistance, the instrument shown can also test logic, measure diode characteristics, and test transistor for small current gain and even measure frequency. To measure continuity, a buzzer is provided which makes a sound indicating the circuit is working.

Accuracy is one of the most critical aspects in the specifications. This degree of closeness of the measured result to the actual one should be as high as possible. Lesser the deviation margin, higher would be the accuracy. For instance, a multimeter measuring voltage with +/-0.6V accuracy would be more precise in its reading when compared to +/- 0.8V. Often quality of multimeters is judged on the basis of accuracy.

Input Ports and Battery

Fig. 3: Multimeter Ports

Most multimeters have a volt and a common port where the probes are attached. However, for measuring current, additional ports are provided. It is the inclusion of milliampere current port that requires a good protective circuitry in the multimeter, as accidental high current applications can damage the instrument and cause harm to the user as well.

Fig. 4: Battery and Fuse Encased in Multimeter Rear

Rear of the multimeter encases a 9V battery and a fuse. Placed between the battery and the input ports, the fuse acts as circuit protector cutting the measuring process off when inputs higher than bearable range are applied on the multimeter. The battery and fuse are closed by a flap using just a single screw so that they can be easily changed avoiding longer interruptions in measuring process. An extra fuse is provided for convenience purposes.

Internal Structure

Fig. 5: PCB and Circuitry of Multimeter

There are no screws involved in opening the multimeter box as the upper and lower sections are attached through the plastic latches. The PCB and all the circuitry are mounted upon the upper sectionwhile the lower section is a thin layer of anodized aluminum. This non conductive layer aids in uniform heat dispersalin cases of high current inputs to the multimeter.

PCB

Fig. 6: A Closer View of PCB and Circuit Arrangement

The PCB contains an assortment of various components including various types of resistors, capacitors, diodes and integrated circuits. Also, it hosts the battery, crystal oscillator, PTC, LCD and the buzzer which tests the continuity of device under test (DuT).

The ICs that are fixed on the PCB shown above are:

1. LM324DG: It is a low power operational amplifier IC which works as a comparator. This IC has quad input and outputs and requires only a single power supply. Thus it delivers optimized power at low voltage inputs.

Fig. 7: Operational Amplifier IC—LM324DG

2. HEF4070: Quadruple Exclusive OR Gates: This 14pin IC provides quadruple EX-OR functions with high noise immunity. This IC is mainly used as logical comparator and parity checkers.

Fig. 8: 14-Pin IC—HEF4070

3. HCF4069: It is a hex-function inverter 14pin IC. Working on a medium power requirement, this inverter IC takes 30ns to change its output from low to high and vice versa.

Fig. 9: Hex-function Inverter—HCF4069

4. TL062: An 8 pin JFET op-amp IC which designed for low power operations, it works in dual function mode which means it can do the work of two op-amps.

Fig. 10: JFET Op-Amp IC—TL062

Apart from all the ICs mentioned above, a cob IC which interfaces the LCD is also there which is mounted on the rear of the LCD screen.

Range Selector

Conducting Circular Rings and Range/Function Selection

Fig. 11: LCD, PCB, and Rotary Knob Switch

The PCB is bound to the top casing of the multimeter with the help of screws. A LCD and a rotary knob switch are sandwiched between the top casing and the other side of the PCB. Also, the contacts for switching the multimeter on and off can be seen. Some multimeters employ the rotary switch to handle the switching ON and OFF options while some require a slider switch, like the one in this insight.

The other side of the PCB has 11 concentric conducting rings among which connections are made and broken with the help of the rotary knob that functions as a switch. The pattern of the rings can vary depending on the multimeter manufacturer and the functions listed. None of the rings completes a complete circular pattern but are broken from some part or other. These lines are also greased so as to allow smooth run of the switch when it is rotated.

The rotation of the switch defines which part of the circuit on the PCB would be active and which would be not.

Fig. 12: Image of Rotary Switch (top) and Ring Allignment (bottom)

A better view of the how the rings are aligned according to the range/function selector can be seen above. As a matter of fact, the rotary switch does not necessarily make contact with the rings corresponding to the function they are placed near to.

For instance, when the multimeter is actuated to measure resistance in the range of 400K, the placement of the contacts of the switch can be seen in the images shown below:

Fig. 13: Figure Showing Position of Rotary Switch to Measure Resistance Range of 400K

Fig. 14: Rotary Switch Mechanism

Fig. 15: Indicator and Corresponding Placement of Pins

Fig. 16: Rotary Switch Positioning on PCB

Instead of being placed right beneath the range indicator, the contacts are placed at right angles to it. The metal leafs at the bottom of the dial act as jumper shorts which establish interconnections between different pairs of conducting rings at each position. The connection between the rings conveys an electric signal to PCB regarding the quantity and its respective range to be measured

Fig. 17: Track on Top Casing Where Switch is Positioned

In order to allow easy rotation of the switch, a track is provided on the inside of top casing along with two tiny metal balls. These tiny balls aid movement on the track and give a “click” sound whenever the knob is rotated to giving a confirmation that either range or function or both have been changed by the user. The use of tiny metal balls over a corrugated track also makes the dial and hence the multimeter mode stays in position even if it the setup shakes or the multimeter is dropped.

LCD

Fig. 18: 7-Segment LCD-Display of Multimeter

Giving a 7 segment output, LCD forms a critical configuration specification of the multimeter in terms of the digits that are displayed. Since LCD output is a direct measure of resolution of the multimeter, it is desired to have it show as many characters as possible. Display of the LCD is measured in number of digits it can show. Total numbers that can appear on the LCD are defined as counts. . The resolution of the LCD is defined by the number of counts along with the most significant digit. If the most significant digit is 0 or 1, a fraction of ½ accompanies the resolution and for other values less than 9, it is ¾. For instance, LCD that has a count of 3999, the resolution would be 3¾.

Fig. 19: LCD Resolution

Fig. 20: Plastic Covering of LCD (top) and Shock-Absorption Rubber Pads

The LCD is embedded on the PCB and is interfaced through pin-outs on the PCB itself. A transparent plastic casing is over the LCD protecting it from the scratches. Also, shock absorption is provided by the rubber pads closely attachedat the top and bottom of the LCD.

Working

After switching the instrument on, the user rotates the knob to the desired measuring function and its range. Corresponding to the function and range selection, concentric rings of the PCB get shorted. This in turn activates that section of the PCB which is responsible for carrying out measurements in that range. Since it is a digital measuring instrument, an Analog to Digital Converter is extensively used to convert the measurements into discrete values.

Fig. 21: Block Diagram of Mutimeter Functioning

Except current, most of the measurements are based on voltage. For instance, while measuring resistance, a small amount of current is sent across the terminals of the DuT. The voltage drop generated is taken as input and is divided by the current by the internal circuitry to determine the resistance.

The block diagram shown above, gives an overview of working of the multimeter. Taken through the probes, the input is analog and enters the internal circuitry in form of a wave. The input signal is first conditioned where-after it proceeds to its respective measurement circuitry. Further, it is optimized for its range selection and sent to an analog to digital converter. Analog to digital converter can be of various types depending upon capabilities of multimeter and the manufacturer involved. In order to convert the signal, the ADC takes samples of the analog wave. To ensure signal reconstruction, the rate of sampling should be at least twice the frequency of the analog signal.

Most of the ADC used in multimeters followsdual slope integration method in which the digital signal is compared to a reference. Their output goes to a successive approximation register (SAR) which sends the final output to the processing unit and balances the reference signal for optimized comparison. A clock input is needed for the SAR counter which is provided by a crystal oscillator.The processing involved in multimeters is usually limited to summing up the pulses and is an integrator circuit.

After analog to digital conversion, the resultant is sent to the processing unit which takes the values, decodes their magnitude and sends to the LCD.

Multimeters are into electronic measurement purposes since long and are expected to stay for long and get more modifications of measuring quantities. Analog multimeters were initially in trend but required calibration and human error often caused errors in measurements. With digital measurements, results are not only more accurate but can also be resolved to a high level. From voltage to currents, digital multimeters can now even measure temperature, capacitance and can now have RS232 connectors for communication to smarter machines. With newer designs rolling out every day and specialized ICs being made for each and every conceivable measurement, innovative developers continue to put more functionality into the cramped corners of the multimeter while operating at nominal power conditions and costs.